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Populations of the Sea
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Protozoa are single-celled organisms microscopic or minute in size. The sea bottom harbors many creeping and attached protozoa of the ameboid or ciliate types, but we shall be concerned mainly with the pelagic forms inhabiting the plankton.


Order Dinoflagellata. In the broadest sense, this group contains both animals and plants, it being a borderline group.

Foremost among the protozoa in the economy of the sea are the dinoflagellates, chiefly because of the capacity of many types to carry on photosynthesis. These holophytic members are considered more fully in the discussion on plants, and for oceanographic studies are properly included in the phytoplankton. It will suffice to mention here only Noctiluca (fig. 225g) as an important representative of the holozoic members, none of which have chromatophores. The soft spherical body of Noctiluca is pale pink in color and bears a conspicuous flexible tentacle. The maximum size is only about 1.5 mm, but, when reproducing in profusion by simple cell division, the countless numbers produced may, by their accumulation, impart a pinkish-red color to considerable areas of surface coastal water, and the masses may be blown into conspicuous windrows or patches resembling “tomato soup.” Noctiluca are voracious feeders, engulfing particulate food such as diatoms and other small organisms. This form is also important as a contributor to the luminescence of the sea.


Order Foraminifera. The oceanographic interest of this order (and also, to some extent, of the following order) lies in the skeletal structures produced by its members. In the foraminifera the shells are variously formed, with one or more chambers arranged in a straight line or in a spiral (fig. 225a). Some are provided with many pores for the projection of protoplasmic pseudopodia used in capturing food. The shells are constructed typically of calcium carbonate, but silica and chitin are also used, and in some

benthic arenaceous forms they consist of an agglomeration of foreign materials cemented together. The greatest numbers of individuals are planktonic in life, but upon sinking to the bottom the shells form an important constituent of globigerina ooze, named for the abundant pelagic genus Globigerina (p. 816). Fossil foraminiferal shells are much used in the study of geological strata, being indices of past geological conditions, and are useful in the field of oil geology. A few foraminifera occur in fresh water, but the vast majority occur in the sea, either living on the bottom even at depths of 6000 m or floating freely in the water, preferring the warmer seas. There are over 1200 species, of which about 26 are pelagic. A recent catalogue of foraminifera (Ellis and Messina, 1940) includes some 18,000 living and extinct species.

Order Radiolaria. These are planktonic organisms whose skeletons are composed mainly of silica, but the Acantharia contain acanthin (strontium sulphate), and all types possess an inner capsule of chitin. The siliceous skeletons are formed in the most intricate and widely divergent patterns in the different species and are the most beautiful of all objects found in the sea (fig. 225e,f). Upon sinking and mingling with the bottom sediments, the skeletons become the type constituents of the siliceous radiolarian oozes found most abundantly covering the ocean floor in the deep tropical waters of the Pacific Ocean (fig. 253). There are about 4400 species, all marine.


Suborder Tintinnoinea. These protozoans, commonly called tintinnids, are mostly of extremely small size, varying from 20 μ for Tintinnopsis nana to 640 μ for Cymatocylis robusta. Swimming is accomplished by the beating of a whorl of hairlike cilia at the anterior end. Their loricae, or shells, range in shape from tubular to urn-shaped structures that are secreted in a stereotyped fashion by the animal and may or may not include agglomerated foreign material such as bits of sand, diatom shells, and coccoliths (fig. 225c,d). The tintinnids at times are found in vast numbers, especially in coastal water, where they are important feeders on the smallest plankton, the nannoplankton. Their sensitivity to small changes in environmental conditions makes them fluctuate in numbers with seasonal or other changes. There are 692 known species,

mainly marine. (Kofoid and Campbell, 1929.) Examples: Favella, Tintinnopsis, Tintinnus.


The sponges are multicellular animals, though of simple and loose organization, either with spicules of silica or calcium carbonate imbedded in their bodies for support or with fibrous skeletons made of the horny substance spongin, as in the common commercial sponge. Sponges are all benthic and nearly all marine, only one family occurring in fresh water. In the sea they are to be found in all parts and at all depths, the siliceous forms living largely in the deep sea. Sponges grow attached to the substratum and obtain their food by propelling water through tiny pores in the body wall and filtering out the microorganisms and detritus that may be present. There are about 2500 species, mostly marine.


Coelenterata are tubelike primitive forms with a continuous body wall surrounding a simple digestive cavity with but one opening encircled by tentacles used in capturing food. The group shows a remarkable degree of polymorphism; that is, a single species may present a variety of forms reducible either to the sessile polyp or the swimming medusoid type.

Class Hydrozoa. To this class belong the hydroids commonly found growing in little tufts on rocks and sea weeds along the coast. From these branching polyps are budded the small jellyfish or medusae such as Obelia (fig. 79). The Siphonophora, an order of this class, are characteristic of the open sea and are represented by the beautiful blue Velella (“by-the-wind sailer”) (fig. 226b) and Physalia (the “Portuguese man-of-war”), neither of which possesses a sessile stage. They are planktonic colonial medusae, exhibiting the maximum development of polymorphism of all animals. There are about 2700 species of hydrozoa.

Class Scyphozoa. To this class belong the larger medusae with eight notches in the margin of the bell. Here are included the giant jellyfishes, some of which may become 2 m in diameter. A much-suppressed sessile polyp stage is present in the group. The 200 species are entirely marine. Examples: Aurelia, Cyanea.

Class Anthozoa. To this class belong the sea anemones, corals, and alcyonarians. There is no medusoid stage, and many of the polyps are colonial; some, especially the corals, are notable for their precipitation of calcareous skeletal structures, which, through long periods of accumulation, are important in the

building up of coral reefs and similar formations. All 6100 known species of anthozoa are marine.


Ctenophora are small globular or flattened forms of jellylike consistency and with eight meridional rows of fused cilia used in swimming. Some possess a pair of trailing tentacles used in the capturing of food. The abundant globular species are commonly known as “comb jellies” or “sea walnuts” (fig. 226a). There are 80 species, all marine. Numerically important genera are Pleurobrachia and Beroë.


Platyhelminthes are flatworms, a large number of which are found in the sea, either free-living or parasitic.

Class Turbellaria. Nearly all of this class are free-living on the bottom under stones and in crevices, where they move about by means of cilia covering the body.

Class Nemertinea. These are ribbonlike worms sometimes considered as a separate phylum. The benthic species live among rocks, algae, mussels, and so on, or burrow in the bottom, where they capture small organisms by means of a long eversible proboscis. Extraordinary size variations occur, some species being only 5 mm long, while one, Lineus longissimus, may become 25 m in length when extended, and therefore is the longest of the invertebrates; however, its threadlike form contains but little bulk. Fifty-two planktonic species of nemerteans are known, some living at great depths—for example, Pelagonemertes. (Coe, 1926). The planktonic forms are modified, some with caudal and horizontal fins for swimming (fig. 228c). There are about 550 species of nemerteans, of which nearly all are marine.


The thread or round worms occur largely as parasites, but some are found in the plankton, and very large numbers occur in decaying organic detritus on the bottom. There are about 1500 species, many of which are nonmarine.


Class Rotatoria (Rotifera). These are tiny benthic or planktonic organisms provided with rings of cilia for swimming and for gathering food. Vast numbers may occur in the neritic plankton during the warmer seasons. There are about 1200 species of rotifers, of which most are fresh-water inhabitants.



These colonial animals, known as “sea mats” or “moss animals,” form flexible tufts or thin incrustations over the surface of solid objects both in intertidal and deep waters. Below low tide, many species form rigid, erect, latticed or branched colonies. The individual minute animals have calcareous protective skeletons and possess a ring of ciliated tentacles for gathering microscopic food. There are over 3000 species, about 35 of which are nonmarine.


Brachiopoda are ancient sessile animals superficially resembling bivalve molluscs, but the hinged calcareous or horny shells are dorsoventrally situated instead of laterally, as in the molluscs, and the animals gather their food by means of delicate ciliated arms attached within the shell. They grow permanently attached to rocks and shells, usually in the littoral zone below low tide. A few live in burrows. All are marine and all are very abundant as fossils in the Paleozoic and Mesozoic rocks. About 120 living and 3500 fossil species are known.


Phoronidea are wormlike animals, living in membranous tubes in the sand and collecting food by means of ciliated tentacles. There are about 12 marine species.


Chaetognatha include numerous but small (maxima about 75 mm long) holoplanktonic wormlike animals known as “arrow worms” or “glass worms.” They are highly transparent and provided with eyespots, a caudal fin and one or two pairs of lateral fins, and with strong chitinous jaws and teeth for capture of prey. They occur from the surface to great depths and are distributed far to sea in all latitudes. All 30 known species are marine. Sagitta (fig. 228a) is the most abundant genus.


Annelida are true worms with elongated bodies composed of a series of similar segments.

Order Polychaeta. These are marine worms of great abundance provided with many setae and typically with a variety of well-defined head structures such as eyes, tentacles, chitinous jaws, ciliated cirri, and so forth, which are modified in keeping with their habits of life and mode of feeding. They have a wide distribution horizontally and bathymetrically.

For the most part they are benthic, either creeping or burrowing, as in Nereis, Glycera, and Arenicola, or sedentary in calcareous or fibrous tubes, as in Spirorbis and Sebella. Tomopteris is pelagic (fig. 228b). There are in all over 3500 species, nearly all marine.

Order Oligochaeta. These are earthworms, of which only a very few are marine, living near shore.

Class Echiuroidea. These are fleshy marine worms with only one or two pairs of setae. They are unsegmented or indistinctly segmented in the adult. They live in burrows in the mud and sand of the littoral zone. There are about 20 species.


Arthropoda include animals with a segmented, chitinous exoskeleton and with jointed appendages, variously modified for locomotion, feeding, and other activities.

Class Crustacea. Entomostraca. This group, formerly considered a subclass, is of convenience in designating a large assemblage of small, primitive crustacea belonging to several subclasses and orders distinguished from the higher crustacea, or Malacostraca.

Suborder Cladocera. Only a few occur in the sea. Examples: Podon, Evadne, sometimes important in neritic plankton. Very numerous in fresh water.

Order Ostracoda. This order includes more than 2000 species, mostly marine, living in the plankton and on the bottom (fig. 227b).

Order Cirripedia. These are the barnacles which as adults have calcareous shells and live sessilely in all benthic habitats, especially coastal. Some grow attached to drifting objects or upon whales and other animals, or they may form special floats for suspension. There are about 500 species, all marine.

Order Copepoda. Though small in size (about 0.3 mm to 8 mm in length), the copepods bulk large in the animal substance of the sea, for they are by far the most abundant of all crustaceans and usually constitute about 70 per cent of the zooplankton. There are over 6000 species of copedods, found mostly in the sea, where some 750 species are planktonic and extremely numerous. Many others are benthic or parasitic. The three main suborders of free-living forms are Calanoida (fig. 227c), Cyclopoida (fig. 229d), and Harpacticoida (fig. 229a). The first two are mainly pelagic,

the last benthic. Like other Entomostraca and some Malacostraca, they gather food by means of fine bristles on certain appendages (p. 887).

subclass malacostraca. These are the large crustacea, mostly benthic, many with strong claws and biting mouth appendages.

Order Mysidacea. There are about 300 species, mostly marine, living on or near the bottom.

Order Cumacea. About 400 species of this order are known; nearly all are marine, benthic.

Order Euphausiacea. These are commonly known as “krill,” and in some regions are very abundant in the plankton and near or on the bottom. Some attain a length of about 50 mm, and may at times be the major constituent of the zooplankton. There are 85 known species, all marine. Examples: Euphausia, Meganyctiphanes (fig. 227a).

Order Amphipoda. There are about 3000 species, nearly all marine, in various habitats.

Order Isopoda. Over 3000 species are known; they are mostly marine, living on the bottom and on vegetation or burrowing in wood. Examples: Limnoria, Munnopsis (figs. 77 and 221).

Order Stomatopoda. This order contains about 200 species, all marine, benthic, most common in shallow water of lower latitudes.

Order Decapoda. Decapoda include crabs, lobsters, shrimps. They are widely distributed in both the pelagic and benthic regions. Most of the over 8000 species are marine.

Class Arachnoida. This class is well represented in the sea by a number of marine mites, over 400 species of sea spiders or pycnogonids, and 5 species of Limulus, the king crab. All are benthic.

Class Insecta. Only one insect is submarine during its whole life; a few others live on the foreshore or skip over the surface in search of food. Example: Halobates.


The molluscs are noted particularly for their construction of an infinite variety of calcareous shells encasing the body and for the structural modifications that have taken place in the soft parts known as the foot and the mantle. These modifications are associated with the method of locomotion and capture of food.


Class Amphineura. The chitons are all flat, benthic animals creeping with the aid of a broad, flat foot. There are about 630 species, all marine.

Class Scaphopoda. Tusk shells live in the bottom mud from shallow water to depths of over 5000 m. All 200 known species are marine.

Class Gastropoda. In most types there is a spiral shell, and the foot is used in creeping. In this and the preceding classes a rasplike radula is a characteristic food-gathering organ. Some gastropods are holoplanktonic and may be without shells. These are the marine pteropods and heteropods (about 90 species of each) with the foot modified for swimming (fig. 228d,f). The latter are especially characteristic of the oceanic waters of the lower latitudes. There are about 49,000 species in the class, mostly marine.

Class Pelecypoda. The clams, oysters, and mussels have a hatchet-shaped foot which in many is used for digging. All are benthic, usually sessile or burrowing in mud, rock, or wood. The soft parts are enclosed within hinged shells and the food is conveyed to the mouth by means of ciliary action setting up water currents, sometimes through long siphons. There are about 11,000 species, of which about four fifths are marine.

Class Cephalopoda. In the squids, devilfish, and so forth, the foot is divided to form arms used in capture of prey. In keeping with their active, predacious habits, the eyes are usually well developed, but blind deep-sea forms occur. In Nautilus and related forms there is a well-developed shell. Cephalopods are either benthic or pelagic, some living at great depths. The giant squid, Architeuthis princeps, having a body girth of nearly 1 m and attaining a total length of about 16 m, is the largest of all invertebrates. There are about 400 species, all marine.


Echinodermata are animals with calcareous plates forming a more or less rigid skeleton, or with scattered plates and spicules embedded in the body wall. Many are provided with spines. All are marine, and all but a few sea cucumbers are benthic.

Class Holothuroidea. The sea cucumbers are mainly benthic. only members of the order Pelagiothurida being planktonic. There are over 650 species, some living in abyssal regions.

Class Asteroidea. The sea stars are among the most conspicuous of shore animals, but they live also at very great depths. About 1100 species are known.


Class Ophuroidea. There are more than 1600 species of brittle stars, with a wide horizontal and bathymetric distribution.

Class Echinoidea. There are about 600 species of sea urchins and sand dollars, a few of which live in deep water.

Class Crinoidea. About 800 species of sea lilies and sea feathers are known, with the center of distribution in the East Indian waters, but they also occur in many other waters. The former live mainly in the deep sea and are anchored by long stalks. The latter occur mainly at shallower depths and are without stalks. The class is a vanishing remnant of a formerly abundant group that has left more than 2000 fossil species.


Chordata are animals which in some stage of their life have gill slits and a skeletal axis known as a notochord.

Subphylum Tunicata. These are primitive chordates; of about 700 species, all are marine.

Class Larvacea (Appendicularia). These are small planktonic forms, sometimes abundant. Examples: Oikopleura (fig. 228e), Fritillaria.

Class Ascidiacea. These are sessile ascidians such as Ciona and Culeolus.

Class Thaliacea. This class is made up of pelagic tunicates that float singly or in chains; they may be very abundant at the surface in the warmer waters. Examples: Salpa, Doliolum.

Other protochordates are the wormlike Enteropneusta and the fishlike Cephalochorda, both of which are found burrowing in mud and sand.

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Populations of the Sea
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