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The study of world distribution of marine animals is the function of marine zoogeography. Its chief aim is the characterization of the animal world of the sea. In causal zoogeography we seek the causes that are or have been operative to bring about the type of animal distribution actually found in present-day faunas, whether benthos, nekton, or plankton.

In the earliest studies attempts were made to establish arbitrary zoogeographic boundaries along lines of latitude, and so forth, but since these have no ecological significance rational faunal boundaries could not result, and it was not until boundaries were drawn to follow certain isotherms that the faunal divisions were also more or less clearly circumscribed. Even this does not necessarily result in rational zoogeographic boundaries because, after all, it is the actual distribution of animals themselves that forms the basis for zoogeographic divisions, since the thermal relations or other factors are not always clear. However, animal distribution (as well as other biological phenomena in the sea) is not haphazard but has resulted from orderly events some of which, although historical, may have lost their continuity, while others are continually operative as single or multiple factors conditioning dispersal, survival, and abundance. Any apparent absence of order results from lack of sufficient information with which to decipher and trace the complicated patterns that result from the inherent nature of the organisms and the factors in the environment. Marine ecology, which is concerned with the organisms in relation to their present-day environmental conditions, is a vital link in the study of zoogeography and many other biological problems of the sea.

The ecological groups to be considered in this division of the present chapter are (a) benthos, the animals of the sea floor; (b) nekton, the swimming animals; (c) zooplankton, the floating animals.


Benthos, Animals of the Sea Floor

Wherever successful dredging operations have been conducted, the ocean floors have been found to be inhabited by benthic animals from the Arctic to the Antarctic and from shore to the greatest depth. However, the number of animals (that is, the concentration per unit area) varies greatly; moreover, the kinds of animals that make up the major portion of the population differ, especially the species, genera and families. Such differences are apparent in the populations of such small topographic units as biotopes, as well as in the larger environmental divisions, and they are the biological criteria for establishing the vertical zones, littoral, archibenthic, and abyssal-benthic, as well as the horizontal faunal areas to be discussed later. It is mainly with the fauna of these larger divisions that we shall deal. For greater detail on the zoogeography of the seas the reader is referred especially to Ekman's text (1935), in which an extensive bibliography is also included.

Animals of the Littoral Zone. The outstanding feature of the littoral zone, especially of the upper or eulittoral zone extending to depths of about 40 to 60 m, is the great diversity and variability of the physical-chemical conditions of habitats. The substratum varies from clean firm rocks to shifting sands and soft muds. Marked salinity gradients sometimes exist, and seasonal and diurnal fluctuations add variety to the life of animals of this zone. Wave and tide actions are highly important, particularly in the shallower portions. Morphologically, the animals are variously modified along special lines associated, for instance, with the type of bottom, degree of exposure, depth, feeding habits. Many of the sessile forms, such as the limpets and chitons of the intertidal zones, are flattened and streamlined the better to withstand the wash and impact of rushing waters. Mussels are securely attached by strong and flexible byssus threads, while adult barnacles, corals, tube worms, and encrusting Bryozoa are rigidly and permanently cemented to rocks, shells, or other solid objects. Less rigidly attached are the hydroids, sponges, and anemones. The sessile or immobile habit so conspicuous in vast numbers of adult marine organisms is highly characteristic of life in the sea. This mode of life is made possible only by the continuous supply of floating microscopic food and the water movements necessary for its production and dispersal (see following chapters).

Among the free-moving bottom forms we find adaptations and habits so varied that all conceivable habitat facies are used. The sea urchin, Strongylocentrotus purpuratus, for example, is able to bore into rocks for protection on exposed coasts. The shells of molluscs are frequently of more sturdy structure when grown on exposed, wave-washed shores. Corals on exposed reefs are massive and compact in comparison to the more fragile and branched types found in lagoons (Vaughan, 1919).

Narrow rock crevices and the under sides of rocks offer protection from enemies and surf for many dorsoventrally compressed forms, among which the small flat-topped crab Petrolisthes is an excellent example of the larger types, but numerous small flatworms, nemerteans, annelids, brittle stars, and others mingle in these common retreats. Many active forms, well supplied with prehensile appendages or sucking discs, live upon the rocks, hydroids, or seaweeds along the shore. The fishes and crustaceans swimming temporarily or skipping over the bottom in search of food must also be considered a part of the benthos. In some fishes (Leparidae) the ventral fins are transformed into sucking discs for attachment, but with increased depth of water and fewer solid objects for attachment there is an accompanying reduction in the disc.

On muddy or soft bottom environments other adaptive modifications result. The shelled animals of these environments build relatively thin, fragile shells as compared with those of animals in exposed or rocky situations. Burrowing bivalves of muddy, sandy bottoms commonly possess an enlarged “foot” useful in digging, and the siphons are elongated to extend above the substratum for intake of water providing food and oxygen. In contrast, the bivalves of hard bottoms may have these structures much reduced, and in more active forms like the scallops, tactile organs and even eyes are developed on the mantle edge. Creeping snails possess a broad foot to aid in gliding over soft mud. Burrowing worms are able to maintain permanent or temporary tubes by means of a mucous or fibrous lining secreted by the animals. Many mud-inhabiting animals are detritus feeders, eating the mud for the organic material it contains or sucking up the detritus that has settled at the mud-and-water interface.

In the littoral zone there is an abundant supply of food for animals. This results directly from favorable conditions for the production of plants, both attached and floating, and from the availability of these plants directly or indirectly to the benthic animals. An appreciable amount of organic material of terrigenous or fresh-water origin must supplement the great quantities produced in this zone. Due to this ready supply of food, the littoral zone produces benthic animals in abundance. The actual concentration is variable, of course, depending upon such local conditions as type of bottom, rate of flow of overlying water, river outflow, and upon meteorological conditions. The last is especially pronounced in intertidal situations, where seasonal rains and freshets may dilute tide pools and exposed flats with devastating results to the more sensitive species. Unusual temperatures during exposure on the foreshore result in great losses (p. 844). Most animals of this zone have a wide range of tolerance to changing conditions, but the selective action of the environment in certain areas may produce a great concentration of the species most suited to the conditions. For example,

Thoracophelia mucronata, a polychaetous worm living in the sand of exposed beaches of southern California, may reach a concentration of 2183 individuals under .1 m2 of sand. The narrow strip of beach inhabited by these worms is seasonally subjected to marked erosion and deposition of sand. During the winter months repeated storms may wash away the sand to a vertical depth of 1 to 1½ m. This makes for a precarious habitat for forms which are not adapted instinctively or physically to vertical migration and which could not keep pace with the diminishing or increasing depth of sand. Another example is the horn shell, Cerithidea californica, which occurs in vast numbers on the surface of isolated mud flats, where few other animals competing for its food can survive.

Most littoral areas are provided with good circulation, owing to irregular bottom configuration, to the effect of tidal actions and winds, and to seasonal or diurnal convection. However, in some bays the exchange of waters may be sluggish, with the result that the free oxygen is used up by decomposing organic matter—usually abundant in such bays—and that hydrogen sulphide is produced, making for precarious if not fatal living conditions. Extreme cases of this nature are found in certain threshold fjords of Norway, where the mouth of the fjord is partially cut off from the sea by a sill of shallower depth than the inner portion of the fjord and where fresh water from adjacent land drainage may form a thin top layer. These situations offer an excellent example of the effect of physical-chemical circumstances on the success of animal life. Normally in these pools there can be no exchange of water between the ocean and the deeper water of the fjord below the sill depth because the water flowing over the sill is of lower density than the deep water of the fjord. Much of the organic material resulting from the plankton and its dependent life in the upper layers sinks to the bottom and, in decomposing, depletes the oxygen supply of the bottom water. Highly toxic conditions, resulting particularly from production of hydrogen sulphide, make it impossible for benthic aerobic life to inhabit the bottoms of these fjords. In some fjords oysters are cultivated, but they must be kept suspended in racks above the hydrogen-sulphide-charged bottom water. The layer of fresh water insulates the lower water and may make for tropical submarine climate with temperatures rising to 30°C (p. 871). Occasionally unusual circumstances, such as continuous offshore winds, build up an offshore gradient that forces upward the heavier deeper oceanic water outside the sill to a height sufficient for it to flow over the sill into the pool, where it lifts the lighter toxic hydrogen-sulphide-laden waters toward the surface. This water is lethal to the fjord animals, fishes, and invertebrates, which upon death sink to the already organically rich muds of the bottom. These or similar catastrophic circumstances lead to mass fossilization of littoral marine life; and,

where a fresh-water layer has been sufficiently developed, there may be a mingling of fresh-water forms giving an abnormal assemblage of fossil faunas.

Horizontal Distribution of the Littoral Fauna. As a result of faunal studies and compilation of the work of many specialists, Ekman in his Tiergeographie des Meeres (1935) has divided the seas into faunal zones characterized by the species, genera, and families of animals found within the littoral zone of these regions (fig. 220). The animals thus employed include not only the littoral benthos but also pelagic forms that are bound by their life histories to the coastal zones. In such an analysis, the assemblage of species and genera of different animal groups that are confined to or are characteristic of the population of an area is the criterion for establishing the faunal regions. Obviously the number of endemic genera or of higher taxonomic orders is often of greater significance than the number of endemic species in distinguishing a faunal region, for the species are biologically of more recent origin. For example, the Pacific and the Atlantic tropical faunas of America, though now separated by the isthmus of Panama and having only relatively few species in common, show by numerous common genera (33 for certain crabs) and by geminate (closely related) species that these faunas were a continuous fauna in past geologic ages when the two great oceans were connected in this tropical region.

The faunal areas are not sharply defined, of course, and the boundaries are to be considered transition zones the width of which is determined largely by hydrographic features, water temperature being a cardinal determining factor. But other factors also determine the geographical extent of a faunal area. These are especially continental land barriers or broad expanses of deep water such as the East Pacific oceanic barrier (fig. 220). This, owing to its depth, precludes spreading of adult littoral forms through the abyssal zone, and, because of the vast horizontal extent of water between the American shores and the easternmost Polynesian Islands, prevents transport of pelagic larval stages of littoral forms except when these stages are of specially long duration.

Broadly speaking, the littoral fauna may be divided clearly into arctic, tropical, and antarctic. Between these there are gradations giving rise to such divisions as boreal, temperate, or antiboreal Kerguelen fauna. Some of the faunal divisions may also be subdivided into east-west regions; for instance, the tropical fauna, though homogenous in many characteristics—for example, in the formation of coral reefs—may be recognized as consisting of four parts, namely Indo-West Pacific, Pacific Tropical American, Atlantic Tropical American, and the Tropical West African.

Other faunal areas of the sea are given in fig. 220. These areas are again subdivided as the classification is made narrower to meet more local conditions. The Atlantic Boreal is, for example, divided into east and west sections and the Arctic into High and Low Arctic. For additional details and reference to original data, the student is referred to Ekman (1935). Many animals of eurythermic nature may be quite cosmopolitan and the sublittoral fauna may extend beyond the boundaries shown by the eulittoral animals. See also p. 845 for comparison of thermic boundaries to those of certain faunal regions.


Faunal areas of littoral animals (based on Ekman).

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Deep-sea Benthos. In 1843 Edward Forbes, pioneer in marine biology, observed the diminution of the number of animals with increasing depth of water beyond the littoral zone and he established with some hesitation what he called an “azoic zone” covering the deep ocean floor from depths below 300 to 700 m. Before this time, however, in 1819, Sir John Ross reported having found worms in mud brought up from a depth of 1800 m in Baffin Bay, but the idea of life existing in great depths was then so untenable that these and other findings were discredited. It was not until 1860 that positive proof of the existence of animal life in the deep sea was first provided by a broken submarine cable that was brought up for repair from a depth of over 2000 m in the Mediterranean with various bivalve molluscs, gastropods, hydroids, alcyonarians, and worms attached. The dredging operations of the Challenger and other expeditions have definitely shown that benthic animals do live in very great depths, probably in smaller numbers even at the greatest depths, since pressure and cold seem not to be excluding factors. During the Challenger Expedition more than 1500 animal species were discovered below 1000 m and a dredge haul at 6250 m yielded 20 specimens belonging to 10 species. Bottom deposits brought up in sounding tubes from the great deeps contained remains of foraminifera and sponges that probably live at these depths.

The summary in table 94, compiled from Murray (1895) and based on the Challenger observations, will serve to illustrate the vertical distribution of bottom fauna of size sufficiently large to be taken by the apparatus used. The specimens were collected with both dredge and bottom trawl and may therefore include a few not strictly benthic.

All of the animal world below the littoral zone may be spoken of collectively as the deep-sea fauna. Although not many deep-sea collections have been made, the endemic fauna appears to be divisible vertically into two parts, an upper archibenthic fauna (continental deep-sea fauna) and a lower-abyssal fauna, the dividing line between these two being placed at about 1000 m depth.

There is no well-defined border line, of course, between littoral and deep-sea fauna; the border is even less clearly defined between the archibenthic and the abyssal. The natural boundary between faunas is the region of most distinct faunal change. This boundary, however, is influenced by outside factors such as temperature and light, with their

attendant influence on food supply, as well as by depth of water. The border line thus lies at different depths in polar and equatorial regions. Also, as indicated below, many species are eurybathic, that is, they endure great ranges of depth, thereby causing a great deal of overlapping where characteristically littoral forms extend far downward and endemic deep-sea forms rise unusually high. Ekman considers that, in general, in most regions the boundary between the littoral and the deep-sea faunas may be located between 200 and 400 m. Eurybathic species add but little to a zoogeographic characterization, but they are nevertheless of great biological interest because of their adaptability to conditions of depth.


(After Murray)

Zone Number of stations Average yield of species at each station Average yield of individuals at each station
                180 m 70 62.8
  180 to 900 m 40 51.2 150
  900 to 1800 m 23 30.9   87
1800 to 2700 m 25 24.0   80
2700 to 3600 m 32 15.6   39
3600 to 4500 m 32 10.6     25.6
            4500 m 25   9.4   24

Outstanding among the eurybathic forms given by Ekman are:

Pennatularia Kophobelemnon setelliferum 36 to 3600 m
Polychaetes Amphictera gunneri littoral to 5000 m
Cirripeds Verruca stroemia littoral to 3000 m
Cumacea Diastylis laevis 9 to 3980 m
Bivalves Scrobicularia longicallus 36 to 4400 m
Snails Neptunea islandica 30 to 3000 m
Starfish Henricia sanguinolenta 0 to 2450 m
Brittle stars Ophiocten sericeum 5 to 4500 m
Sea urchins Echinocardium australe 0 to 4900 m
Sea cucumber Mesothuria intestinalis 20 to 2000 m

That much of the animal life in the deep sea is truly endemic, not merely a downward extension of eurybathic forms, is shown by the presence of vast numbers of species and many genera and higher taxonomic orders that are found consistently only in these deeper zones. Important among the characteristically deep-sea forms are the glass sponges, Hexactinellida, with 15 families, 80 genera, and about 400 species; seven families of Pennatularia; the deep-sea Holothuroidea, of the order Elasipoda with four families, over 20 genera, and many species.


Tables 95 and 96, based on summaries of Ekman, depict further the arrangement of certain elements of the faunas of the three major bathymetric zones.

A striking characteristic of the deep-sea fauna is the relatively smaller number of species in proportion to the number of genera. On the basis of the Challenger data Murray (1913) concluded that the ratio of species to genera decreases regularly from coastal to offshore deep water, so that in the deepest zone the ratio of species to genera is 5 to 4, whereas in the shallow coastal water it is 3 to 1. It is a significant fact that whole orders and numerous families of various taxonomic groups are confined to the deep sea or are characteristic of its population.


(Ekman, 1935)

Animals Zones Number of species
Littoral, or littoral and archibenthic 31
Crinoidea Littoral to abyssal 10
(North Atlantic) Archibenthic 11
Archibenthic and abyssal, or purely abyssal 35
Littoral or mostly so 13
Starfish Littoral to abyssal 28
(North Atlantic) Archibenthic or abyssal 97

(Ekman, 1935)

Animals Zones Number of genera
Purely littoral 62
Littoral and archibenthic 14
Tunicata Littoral to abyssal 11
Purely archibenthic.   3
Archibenthic and abyssal   4
Purely abyssal 13

Deep-sea animals of the benthic region are in the main mud-dwelling forms adapted in various ways to this mode of life. A considerable number, typified by the isopod genus Munnopsis (fig. 221) and the shrimp Nematocarcinus, are adapted by elongated appendages to the quiet water and the softest of muds; sponges and hydroids are provided with

projections, spines, or rootlike structures for anchorage and support. In order to reach above the soft ooze the crinoids, or sea lilies, of the deep sea are usually long-stalked as compared with their short-stalked relatives of shallower water, the sea feathers. Many other forms also are long-stalked, for instance the tunicate Culeolus murrayi which floats off the bottom like a bulb anchored by a thread-like stalk that may be about three quarters of a meter long. Most deep-sea animals lack, or have exceedingly weak, calcareous skeletons. The sea urchins have frail discontinuous plates as compared with their heavy-shelled littoral relatives. The slowly water does not necessitate strong or streamline adaptation. Many forms show adaptations associated with abyssal darkness, but these are best discussed with the pelagic life of the deep sea.


Munnopsis typica, a deep-water isopod (redrawn from Sars).

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As previously stated, the population of the benthic region of the deep sea is relatively sparse, the animals decreasing numerically with increasing depth, also with distance from shore. The one factor most operative in limiting the abundance of animal life in the deep sea is undoubtedly food. Abyssal animals do indeed live a precarious life with regard to food. It is not surprising that only relatively few animals have been found on bottoms covered with red clay, for most deep-sea animals depend upon the nourishment obtainable directly or indirectly from the bottom oozes and tests show that the red clay is of all bottom deposits the poorest in organic material. Yet even it is not without its quota of marine metazoan life. It has been pointed out that with the intensified adversity of living conditions in abyssal regions, the animals adapted to survive the physical conditions there would be numerous were it not for a shortage of food. Since plants can live only in the lighted upper strata of the sea, it follows that deep-sea animals are either carnivores or detritus feeders, a population utterly dependent upon plant and animal production in the upper water layers and upon the ultimate sinking of dead bodies of these plants and animals to greater depths. Much of the surface production is broken down, however, by bacterial or autolytic action within the upper or intermediate layers and is thus lost to the dependent abyssal life. The direct food of abyssal forms

undoubtedly results largely from pelagic animals of deep or intermediate depth, animals which have themselves in turn been nourished in main by organisms produced above them.

It is instructive to note that the poverty of animals in deep-sea zones is apparently not a direct result of depth of water but is intimately connected with distance from continental shores. Murray reports that collections made in depths between 1800 and 3650 m near shore yielded per haul an average of 121 specimens belonging to 39 species, whereas collections made in comparable depths further than about 500 km from shore yielded an average of only 21 specimens and 10 species per haul. This strongly indicates a relative shortage of food for benthic animals in the offshore localities. In the preceding chapter it was emphasized that conditions for phytoplankton production are enhanced in coastal waters where the supply of mineral nutrients in the lighted zone can be enriched through vertical circulation extending to a depth sufficient to tap the store of nutrients that accumulate there through the sinking of organisms produced in surface layers. Owing to the movements of such nutrient-enriched surface water, much of its plankton load becomes deposited in the deep inshore waters and thus supplies more food to the deep-sea benthic animals living there than is possible far from shore. Any offshore hydrographic condition that leads to enrichment of the plant nutrients in the surface layers will produce similar results, reflected in the abundance of benthic fauna that can be supported under such waters. This is illustrated by the surprisingly rich deep-sea benthos encountered by the Challenger Expedition in the deep offshore waters of the Antarctic, especially in the Kerguelen region. Murray believed that this exceptional abundance of deep-sea life resulted from offshore extension of coastal conditions owing to floating ice or to greater destruction of plankton life at the junction of waters of separate origin, but study of the now better-known hydrographic features of the Antarctic reveals that these rich accumulations lie under the region of the great Antarctic Convergence (fig. 158, p. 606) and, according to the investigations carried out by the Discovery, the surface waters of this region show possibilities of a rich supply of nutrients and of a great production of phyto- and zooplankton.

From this dependent relationship it becomes obvious that the deep-sea and abyssal fauna could have come into being only after the pelagic life of the sea had become established, or simultaneously with it. Both the pelagic animals and the deep-sea forms are believed to have been derived from the littoral fauna. The many structural modifications and the numerous endemic genera give evidence that the fauna of great depths is, indeed, an ancient one. But in generalizing, we may say that although the deep-sea fauna possesses many bizarre and unusual forms, nevertheless the structural adaptations found in these animals are only

modifications of better-known forms found in shallower water. Several archaic forms are found that were originally known only from fossil beds. Among these “living fossils” are several glass sponges; decapods, including Polycheles and Willemoesia; stalked crinoids, for example Rhizocrinus lofotensis; and a number of other forms either originally known only as fossils or later shown to have existed since early geological times.

Horizontal Distribution of Deep-sea Benthic Animals. As a rule deep-sea animals in general are widely distributed, but not to such an extent as was formerly believed would be the case owing to the uniform conditions of the deep-sea environment. The abyssal fauna, however, is the most widely distributed of benthic life, the archibenthic being next, and the littoral fauna the least widely distributed. In other words, the horizontal distribution of marine benthic animals increases directly with increasing depth. Benthic deep-sea genera are usually cosmopolitan, although the species may belong within the limits of certain oceans. Geographical submarine barriers are also influential in limiting distribution. A classical example is the Wyville Thomson Ridge, which forms a barrier between the deep-sea faunas of the Atlantic and the Norwegian Sea, only 12 per cent of the faunas being common to both seas. The reason for this will be discussed further on p. 849.

Nekton, the Swimming Animals

The assemblage of animals comprising this group are provided with efficient locomotive organs enabling them to swim against currents and waves. The locomotor efforts are not only capable of being sustained for considerable length of time, but the movement is also effectively directed towards pursuit of prey, escape from enemies, and instinctive migratory journeys. Structurally, most nektonic animals are well adapted to these ends. They are typically streamlined in shape and frequently covered with slime to decrease resistance in passing through the water. The musculature, nervous system, and sense of vision are notably well developed. Among the members are adult fishes, squids, whales, dolphins, seals, and a few crustaceans. All parts of the pelagic region of the sea contain representatives of the group which, together with the plankton, make up the pelagic life of the sea. It has been noted by Hjort (1912) that the idea of a pelagic mode of life was originally associated with animal life of the ocean surface, but it applies also to the drifting and swimming life of deeper waters, since its main characteristic is its independence of the bottom. Deep-living pelagic animals are called bathypelagic. Pelagic animals are either neritic or oceanic, depending upon whether they belong to the neritic or the oceanic province. There is no well-defined line between the two.

In the nekton we find the great migratory animals which make journeys of hundreds of miles to and from their breeding grounds or roam

over vast ocean areas in search of food. The great migrations of many species as a rule are associated with breeding and feeding requirements and with alternations between the two, the breeding habitat suggesting the more primitive home of the species involved. Striking examples of the migratory journeys of the fishes are offered by the anadromous salmon, Oncorhynchus tschawytscha, of which young stages marked in the Columbia River have been recovered later in the Gulf of Alaska, and grown fish marked in Alaskan waters later recovered in the Columbia. Other species of salmon also may journey over 650 km, presumably returning to the stream in which they were born (Am. Assoc. Adv. Sci., 1939).

The spawning migrations of the European eel, Anguilla vulgaris, are perhaps of the most remarkable among fishes. The journeys involve the swimming of the adult eel over a distance of about 5000 km from European coasts to the spawning area near Bermuda and the subsequent return of the larvae to the fresh-water habitats on the coast of Europe. This will be more fully discussed in a later chapter.

The guiding instinct in these journeys appears more remarkable than that in migratory birds, for recognizable landmarks do not exist in the open sea, where the horizontal chemical-physical gradients are too weak and variable to render anything but negligible directing aids during the oceanic portion of the journey.

Among the marine mammals whales are known to travel great distances. Recovery of American harpoons embedded in blue whales killed in Barents Sea, where harpoons of such make were never used, indicate migrations from the coast of North America (Hjort, 1912). These and other whales alternately travel from low-latitude breeding grounds to the rich feeding grounds of higher latitudes.

The only invertebrates that are clearly nektonic are some of the cephalopods, especially the squids. These animals are powerful swimmers which effect rapid locomotion by spasmodically ejecting jets of water from the mantle cavity through the swimming organ known as the funnel, a tubelike structure representing a modification of the molluscan foot. Associated with this rapid locomotion the cephalopods possess highly developed eyes, and also a highly developed brain as compared with other invertebrates.

A few of the pelagic prawns may be included as nekton, although they are usually on the borderline, approaching either benthic or planktonic life. Such special groups as seals, otters, and marine snakes may for convenience be placed also in this category, although the life habits of some require a period on the immediate foreshore for the care of the young.

Biologically the nekton includes only a very few of the major animal groups, as indicated above; but the large size of individual members, their tendency to form into schools, and their commercial value as food,

oil, and so forth, make the group a conspicuous one and of outstanding importance to man. From the standpoint of the economy of the sea, the group is dependent and must therefore have developed as an ecological group after the plankton and benthos were established. The relatively high phylogenetic ranking of the members is also in keeping with this.

Zooplankton, the Floating Animals

The benthic and nektonic groups were the first to be observed or studied by man. But as systematic investigations of the sea progressed, with the improvement of collecting methods and the use of the microscope, it became obvious that another ecological group existed, which for convenience of study should be considered distinct from the bottom-dwelling and the fast-swimming forms to which it holds such vital relationships. This group is now called the plankton, a term first used by Victor Hensen in 1887 to distinguish the vast assemblage of feebly swimming or floating organisms, both plants and animals, that drift about with little or no resistance to water movements. To the plankton belong not only by far the greatest number of marine organisms, but also those of widest dispersal.


Photomicrograph of a plankton community containing copepods (Calanus and Metridia), young euphausiids, and fish eggs.

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The general plankton is divided into two large groups: Phytoplankton and Zooplankton. To the phytoplankton belong most of the diatoms, dinoflagellates, and other unicellular plants or animal-like plants that are capable of synthesizing food. These, in contrast to the consumers making up the zooplankton, are the chief producers of the primary food of the sea. They are more fully discussed in chapters XVI and XIX.


In the zooplankton are included many of the protozoa, especially tintinnids, radiolarians, and foraminifera, a large number of the small crustaceans such as copepods, ostracods, euphausiids, amphiopods; the jellyfishes and siphonophores; many worms; a number of molluscs, such as the pteropods and heteropods; and the eggs and larval stages of most of the benthic and nektonic animals of all kinds. Figures 222 and 223 are photomicrographs of typical zooplankton catches from coastal waters.


Photomicrograph of plankton dominated by the arrow worm, Sagitta, and including copepods, young euphausiids, and fish larvae.

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Undoubtedly the first notice taken of the general microscopic plankton was not of the individual organisms themselves, but rather of such phenomena as discoloration of the water, associated with their swarming. These observations, of course, could only increase the mysteries of the sea, for the causes were in most instances entirely unknown. Probably one of the earliest references applying to general plankton phenomena was made by Pytheas in the fourth century B.C. During a voyage in the North Atlantic he reported that the sea became sluggish and thick like a jellyfish (Herdman, 1923). There are records where the occurrence of the color phenomenon was accompanied by considerable destruction of marine life along the coasts, such as has been witnessed on the coasts of California, Japan, and elsewhere. The terms “red-water,” “red-tide,” and “sliming” have been applied to some of these displays. In other instances, the discoloring became associated with good or bad fishing conditions, and recent investigations (Hardy et al, 1936) have, indeed, shown that there is a correlation between herring catches and color of water resulting from microorganisms (p. 907). Perhaps the most

spectacular and mystifying of displays resulting from plankton life was that of bioluminescence (“phosphorescence”) of the sea.

Among the first important typically planktonic organisms to be studied in detail were the boreal copepod Calanus finmarchicus (fig. 227-c), described by Gunnerus under the name Monaculus finmarchicus in 1765, and Ceratium tripos (fig. 74-a), a dinoflagellate described by O. F. Müller in 1777. Little was accomplished, however, until about 1846, when Johannes Müller introduced the plankton net for use in extensive studies. Since then the plankton has been the subject of a vast amount of investigation.

It will be noted that the three categories, benthos, nekton, and plankton, are not sharply separated. There are not only transitory stages but also many organisms of borderline habits. Most members of the nekton and benthos are for a period properly plankton. The swimming powers of many animals put them midway between the plankton and the nekton, and many forms, for example some mysids, amphipods, cumacids, and so forth, live both on or near the bottom and are sometimes called hypoplankton. Many of the planktonic animals do swim freely and may quickly move many times their body length, although, owing to their small size, little distance is covered. The direction of swimming is frequently haphazard and intermittent, resulting in little progress. However, under certain directing stimuli such as light and gravity, the resultant movements do bring about consistent, although restricted, migrations.

Categories of animal plankton are defined according to duration of the life cycle in the pelagic state, according to size, or according to habitat.

Temporary Plankton. The planktonic eggs and larvae of the benthos and nekton make up what is known collectively as the temporary plankton (fig. 224). This temporary element, or meroplankton as it is sometimes called, is especially abundant in the neritic waters and is composed mainly of developmental stages of the invertebrates, but includes also the young of the fishes.

The following examples give the computed maximum concentrations of various invertebrate larvae taken at separate stations by means of a No. 20 plankton net towed vertically from 25 to 0 m in neritic waters of the Bering Sea during August (Johnson, 1937):

Larvae Number per cubic meter of water
Ophiopluteus.   1,235
Echinopluteus 12,195
Bipinnarian       837
Annelid   8,130
Barnacle nauplius       694
Pelecypod veliger 17,073
Gastropod veliger   7,883


Characteristic larvae of the meroplankton: (a) chaetate larva of the annelid Platynereis agassizi; (b) zoea of sand crab, Emerita analoga; (c) cyphonautes larva of bryozoa; (d) tadpole larva of sessile tunicate; (e) pilidium larva of nemertean worm; (f) advanced pluteus larva of sea urchin; (g) fish egg with embryo; (h) trochophore larva of scaleworm; (i) veliger larva of snail; (j) pluteus larva of brittle star; (k) nauplius larva of barnacle; (l) cypris larva of barnacle; (m) planula larva of coelenterate; (n) medusa of hydroid.

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The temporary plankton is characteristically seasonal in occurrence since it is dependent upon the spawning habits of the parental stock. But there is sufficient variation in spawning time of different species, or even continuous spawning of a single species, to provide a greater or smaller amount of temporary plankton at all seasons, even in high boreal waters where the temporary element is usually much suppressed. The length of larval period is also important in this respect, and may vary

from a few hours, as in the tube worm Spirorbis, to a period of perhaps four or five months, as in Emerita, the sand crab.


Characteristic holoplankton protozoa: (a) foraminifera (Globigerina); (d) dinoflagellate (Gymnodinium); (c) tintinnid (Stenosomella); (d) tintinnid (Favella); (e) radiolarian (Protocystis); (f) radiolarian; (g) dinoflagellate (Noctiluca).

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Characteristic holoplankton coelenterates and ctenophores: (a) comb-jelly (Pleurobrachia); (b) siphonophore (Velella); (c) jellyfish (Aglantha); (d) siphonophore (Diphyes).

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Permanent Plankton. The remaining part of the plankton is made up of animals living their complete life cycle in the floating state and is called the permanent plankton or holoplankton (figs. 225228).

The holoplankton is composed of forms representing nearly every phylum of the animal kingdom with the exception of the sponges, bryozoans,

and phoronids. Certain of the parasitic nematodes and flatworms are not in the strict sense planktonic though they are associated with planktonic hosts. The free-living Platyhelminthes are predominately benthic although their larvae are conspicuous members of the temporary plankton. There are a number of pelagic nemerteans such as Planktonemertes and Nectonemertes. The rotifers of the Trochelminthes are intermediate members, since many, if not all, at times possess resting stages in the form of special eggs which sink to the bottom and await return of favorable living conditions in the plankton. Among the echinoderms only the sea cucumbers have members, Pelagothuria (fig. 229-e) with two species and Planktothuria with one species, which live their whole life cycle in the plankton.


Characteristic holoplankton crustaceans: (a) euphausiid (Euphausia); (b) ostracod (Conchoecia); copepod (Calanus); (d) amphipod (Phronemia) in empty mantle of the pelagic tunicate Salpa.

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Characteristic holoplankton, miscellaneous: (a) arrow worm (Sagitta); (b) annelid (Tomopteris); (c) nemertean (Nectonemertes); (d) pteropod mollusc (Limacina); (e) tunicate (Oikopleura); (f) pteropod mollusc (Clione).

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Some floating adaptations of plankton animals: (a) copepod (Aegisthus); (b) decapod (Lucifer); (c) barnacle nauplius; (d) copepod (Oithona); (e) holothurian (Pelagiothuria); (f) pelagic egg of copepod (Tortanus); (g) phyllosome larva of lobster; (h, i) copepod (Sapphirina), side and dorsal views.

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All other phyla are abundantly represented in the holoplankton. However, no plankton animals play so vital a role in the economy of the sea as do the Crustacea of the phylum Arthropoda, and among these the copepods rank first in most parts of the ocean although in many instances euphausiids are of equal or greater importance as food for the larger plankton-feeding animals (p. 905). Among the higher crustaceans we should mention the decapod crustacean, Lucifer, which, though of small importance numerically, illustrates a remarkable divergence from others of the group in adjusting itself to a floating life (fig. 229-b). It is of interest to note here that in the sea, as on land, it is the arthropods that have gained the greatest diversity and numbers. In the sea the Crustacea are a counterpart of the Insecta on land.

Macro-, Micro-, and Nannoplankton. Most of the planktonic organisms are microscopic or semimicroscopic in size but there are notable exceptions, for instance, among the scyphozoan jelly fishes, some of which may attain 1 m or more in diameter, with tentacles up to 25 m long. This wide range in size has, for convenience of study, led to yet further subdivision of the plankton on the basis of relative size. Thus we may distinguish broadly between three (or more) convenient size groups. No sharp lines can be drawn between these, but usually the macroplankton is that taken with a coarse net. It includes the large forms and many small ones that can be readily seen with the unaided eye, that is, animals of about 1 mm or more in length that would be normally caught with a net of No. 00 or 000 bolting cloth. (The phytoplankton does not usually form a part of this division.) The forms between about 1 mm and 1 cm are sometimes called mesoplankton, but the term should be avoided in this sense because it is also used to designate the general plankton living in mid-depth waters below the epiplankton. The largest of the plankton forms are sometimes called megaloplankton. The microplankton, also in part called net plankton, is that which is composed of individuals below about 1 mm in size, but yet large enough to be retained by a net of No. 20 bolting cloth with a mesh aperture of about 0.076 mm. The nannoplankton (dwarf plankton) comprises many of the very small forms (about 5 to 60 μ) such as the smaller diatoms, dinoflagellates, coccolithophores, protozoans, and bacteria, which readily pass through the meshes of a new No. 20 net (fig. 90, p. 377) and must therefore be collected by centrifuging the water. Sizes below these are the ultraplankton. Lohmann (1903) demonstrated the presence of nannoplankton by using hard filter paper, and also by a unique method in which he examined the tiny forms caught in the filtering apparatus of the “house” of the appendicularian Oikopleura. The appendicularian “house” is a temporary gelatinous structure that serves as a complete investing case which protects the animal and also acts as a filtering apparatus for catching food (fig. 239). In the “house” there is a set of

two filters, the coarseness of which depends upon the size of the animal. The first or outer filter removes organisms of size greater than about 127 μ by 34.5 μ. The inner filter retains organisms above 30 μ in diameter. With these fine filters the animals gather the nannoplankton for food.

Neritic and Oceanic Plankton. Many pelagic organisms, both plants and animals, are bound strictly to coastal waters while others occur normally only in offshore waters. These requirements give rise to two divisions of the plankton, based on their relative dependence on the coast.

To the neritic ecological division belong the forms which normally inhabit the waters in the coastal areas and extend only a short distance seaward, depending upon the depth and type of circulation. The neritic forms on the whole prefer relatively warm water during the growing season, with some reduction of salinity. They are usually seasonal in occurrence, especially in the northern latitudes. Vast swarms of pelagic larvae of benthic invertebrates and many fish eggs and fish larvae are characteristic of the neritic plankton. Resting stages occur in the life histories of many of the species, such as the cladocerans Podon and Evadne, and the rotifers. It must not be understood, however, that the neritic plankton is composed chiefly of temporary plankton. On the contrary, the dominant element is often the holoplankton, adults and young, which are bound to the coastal regions for some unknown reason—perhaps by nutritive, physical, or chemical bonds. The copepods loom large among these permanent plankton forms.

In discussing the plankton of the Gulf of Maine, Bigelow (1926) cites certain of the medusae as conspicuous inhabitants of neritic nature. These are the moon jelly Aurelia; the large red jellyfish Cyanea; and the small hydromedusæ Melicertum companula and Sarsia, all of which are budded from sessile stages in the shallow coastal zone. Many other medusae and also pelagic larvae of such animals as shore crabs and molluscs could be added to this list of strictly neritic forms. It is significant to note, as Bigelow has pointed out for Melicertum companula, that some of these pelagic stages are confined mainly to coastal zones because they have originated there from parental stock dependent upon the immediate coast and not because of any inability to withstand oceanic conditions. Some neritic animals may be found in oceanic waters into which they have been carried, but they are relatively or totally sterile, being unable to complete successfully all stages of the life cycle (p. 859).

The neritic forms are the greatest producers of the sea, for in the coastal waters the food materials are most readily available for plants and through these, successively, for small and large animals. Practical evidence of this relation is seen in the fact that most commercial fish are taken in coastal waters.


The oceanic division, as the name implies, includes forms that are contrasted with the coastal population, since they occur typically at some distance from the coast and over profound depths in the open sea. They constitute the so-called “blue-water” or high-oceanic population. The animals of the oceanic plankton are characteristically holoplankton types that are not dependent on the sea bottom or on the proximity of land; indeed, as a rule, they do not survive long if blown by winds or swept by currents into really neritic situations. Characteristic animals of the oceanic plankton are: Velella, which floats in vast swarms at the surface with its oblique sail projecting to catch the surface breeze; the salps; the violet snail, Janthina exigua, which in time of storm drifts to the beaches in company with Velella; many copepods of different genera, Sapphirina, Rhincalanus, Eucalanus, and others. It is not possible to distinguish sharply between the neritic and the oceanic populations, for there is much overlapping at the border line; there are also forms of relatively great ecological valence, that is, forms tolerant of a wide range of ecological conditions, and such forms are to be found indifferently in both coastal or offshore situations. These are the panthalassic types.

Special Adaptations of Animals to Planktonic Existence. In discussing the adaptations of the phytoplankton, it was pointed out that the rate of sinking of a body heavier than water depends upon the ratio of surplus weight to friction, and friction in turn is determined mainly by the surface area. The simplest way to obtain a relatively large area is to reduce the absolute size. Therefore the small size of most planktonic animals is of extreme importance in keeping them afloat; but, as indicated for the plants, special structural adaptations are also provided.

In contrast to most members of the phytoplankton, the planktonic animals usually have some power to swim. That this is highly important, especially for the larger crustacean forms, is shown in the following summary from Gardiner (1933), which gives the average time required for anesthetized Calanus finmarchicus of different lengths to sink through a column of water 250 mm deep having a temperature of 18.5°C and a salinity of 35.01 ‰:

Length of specimen (mm) Mean time for sinking (seconds) Length of specimen (mm) Mean time for sinking (seconds)
2.1 181.3 3.2 70.3
2.3 110.6 3.4 58.9
2.6 114.9 3.6 57.9
2.8 106.8 3.9 43.9
3.0   74.3 4.0 31.0


The experimental data included additional lengths, and the correlation between sinking time and length of specimen, expressed as coefficient of correlation, was

The ability to swim varies greatly with different animals, but even very feeble swimming, when favorably directed as a response to gravity or to light, must work to great advantage in keeping the organism within the water stratum having tolerable living conditions. The ciliates depend largely on ciliary action as do also the ctenophores, the trochophores, and other similar larvae. It is significant to note that the pelagic young of bottom-dwelling forms almost invariabley possess a great development of cilia or a greater length of spines and bristles in greater profusion than occurs in the adults. Euphausiids and at least the larger copepods keep their locomotive organs (pleopods, maxillae, second antennae, and so forth) almost incessantly in motion for feeding, respiration, and swimming, and appear to depend rather little on special suspensory organs. The chaetognaths, or arrow worms, all species but one of which are planktonic, depend mainly upon swimming, for which they are rather well adapted by development of fins and longitudinal muscles. But many of the plankton animals that can swim are also especially adapted to resist sinking when they are in a state of rest. Here the means of adaptation are mostly along the lines of increased length of appendages, of spines or bristles, or of dorsoventral flattening of the body (fig. 229, p. 818).

The radiolaria and pelagic foraminifera develop long spines and irregular shapes, a method not unlike that employed by some diatoms, and the size of some species of radiolaria may be much decreased in the warmer surface water. In Challengeria, for example, certain of the smaller species averaging about 0.11 to 0.16 mm live in the less viscous surface waters of 50 to 400 m depth, whereas the larger species averaging from 0.35 to 0.58 mm live in the colder viscous waters of 1500 to 5000 m depth. Between these depths the average size of different species ranges from 0.26 to 0.28 mm. The chief method of flotation for many of the copepods is found in the increased length of appendages and the spines or bristles that grow upon them. The tropical forms show a greater development of plumose structures than do the northern species, and this is just what we should expect because of the reduced viscosity of the warmer water (see p. 857). The warm-water copepod, Sapphirina (fig. 229h and i), has no extensive development of spines of plumose structure, but it is flattened dorsoventrally in a manner that enlarges the ventral surface and gives greater water resistance while sinking. The lobster phyllosoma larva is similarly constructed and adapted to its pelagic period. The long spines, antennae, and so forth, common in

bilateral animals, usually project laterally, thus giving resistance at right angles to the pull of gravity, and therefore function as does the flattened body of Sapphirina. The development of webs, as in Pelagiothuria, is another form of adaptation.

The presence of oil is a common and most effective method of reducing specific weight in pelagic eggs; it is also found in the copepods, euphausiids, radiolarians, Noctiluca, and others. In the copepod Calanus finmarchicus there is a well-defined hydrostatic organ filled with oil; this consists of a very thin sinus lying in the mid-dorsal axis of the body. The oil also apparently functions as a food reserve since it varies greatly in quantity, and its quantity is perhaps indicative of past living conditions.

Among the siphonophores, pneumatic floats serve as a hydrostatic apparatus. Physalia and Velella are good examples of this type of adjustment and, in addition to the floats, possess sails that extend above the water surface to enlist the aid of the wind in giving more rapid transportation.

Finally, it should be noted that the water content of many animals of the plankton community is very high, being in excess of 95 per cent in some jellyfish. The jelly-like transparency of many plankton animals, for instance the salps, the leptocephalus eel larvae, the pelagic annelid Tomopteris, the ctenophores, the heteropods, the chaetognaths, the copepods Eucalanus and Haloptilus, and many others attest to the high water content of their bodies. The maintenance of this high water content is facilitated because the body fluids are isotonic with sea water. The skeletons (when present) of planktonic animals are remarkably thin in contrast to many related benthic forms. Some of the typical means of flotation are illustrated in fig. 229.

For a fuller discussion of the planktonic adaptations, the reader is referred especially to Steuer (1910).

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