Plankton and Filter Feeders

Under this heading are included the forms that feed upon microscopic or semimicroscopic organisms and suspended detritus floating or swimming freely in the water. It is here that the uniqueness of the food cycle of the aquatic environment is most clearly manifest. It is not practicable to segregate strictly the true plankton feeders as a group from the feeders on finely divided, suspended organic detritus because most plankton feeders include detritus in their diet, owing in part to the method of gathering food. They may also be designated as “suspension feeders,” after Hirsch. Many of the plankton feeders may be called filter feeders because of the method in which they collect their food. Most of them are provided with some type of screening device through which the water is passed while the small organisms are retained as food. A few examples will serve as illustrations.

In filter-feeding copepods, Calanus finmarchicus for example, the head appendages known as the second maxillae are provided with a number of curved setae or spines each covered by numerous fine hairlike processes. The appendages are paired and together form the main part of a filtering net or chamber just posterior to the mouth. The head appendages lying between the first antennae and the second maxillae,

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that is, the second antennae, the mandibular plaps, and the first maxillae, are also richly supplied with plumose setae and vibrate regularly at a rapid speed (600 times per minute for C. finmarchicus). By stroboscopic analysis it has been shown (Cannon, 1928) that these appendages, together with the maxillipeds lying just anterior to the second maxillae, set up swirls of water which result in two vortices (fig. 238). A major “swimming vortex” moves the animal slowly forward through the water, while a smaller countervortex, the “feeding vortex,” forces a stream of water forward into the filter net formed by the setae of the second maxillae. The minute food particles are screened out and passed forward to the mandibles and the mouth. The second maxillae do not move rhythmically as do the other head appendages, but remain still except when being flexed vertically to reject unwanted food into the swimming vortex. A system of feeding currents is also set up by the mysids or possum shrimps. They may either filter the food directly from the

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water or, assuming a vertical position head downwards, they may gather food from the bottom deposits.

The great abundance of euphausiid crustaceans makes them highly important plankton feeders because they share largely with the copepods the distinction of being grazers upon diatoms and, owing to their habit of living in large swarms below the euphotic zone, they must be of great significance in intercepting and utilizing the slowly sinking plant material produced above in the lighted zone. Euphausiids are known to be quite omnivorous, feeding on a wide variety of floating material, plants, animals, and detritus. This they comb out of the water with their long thoracic limbs which together form a basket through which water is pumped by the swimming legs (Bigelow, 1926, Lebour, 1924b).

The pelagic tunicate Oikopleura is a most remarkable type of filter feeder. Its food consists of the minutest of drifting organisms, the nannoplankton such as coccolithophores, bacteria, small diatoms and dinoflagellates, and other minute forms, which are filtered from the water by means of gratings in the animal's temporary vestments or “houses.” This portion of the plankton may constitute as much as a third or more of the total mass of plankton at some seasons and localities. The house of Oikopleura, in which it lives while drifting about in the plankton (fig. 239), is a gelationous investing structure secreted by the animal. Water is drawn into the house through funnel-like structures guarded by a set of fine mesh gratings (outer filter) capable of excluding organisms of size greater than about 0.127 mm × 0.0345 mm. In the house the water circulates through another set of filters (inner filter) that retain organisms about 0.030 mm in diameter. The water is circulated by undulatory movements of the animal's “tail” and is expelled through a second opening in the house, thus propelling the house through the water. The extremely fine material collected on the inner filter is drawn into the animal's mouth by means of ciliar action. After a few hours the screening devices become clogged and the animal then escapes from the structure through a third separate opening (exit). Having freed itself from the old useless vestment, it secretes a new one with all the complicated structures for gathering the type of food upon which it is dependent.

There are also many plankton filter feeders among the sedentary or burrowing animals. Indeed, the permanently attached forms so characteristic of the marine fauna (and by comparison so conspicuously wanting in land fauna) can exist as such only because the water carries to them sufficient nourishment in the form of suspended particulate food,

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and it remains only for them to develop means of utilizing the supply. Typical are the adult barnacles, which gather food blindly through rhythmic motion of modified appendages covered with plumose setae which screen out small particles of food carried within reach by occasional waves or water currents. Mussels and clams also filter plankton and detritus from the water, passing the food together with mucus down the sides of the ciliated gills into the ciliated food groove extending along the length of the gills to the labial palps which sort the food prior to carrying it into the mouth. That a great deal of selection takes place is indicated by Fox et al (1936), who report that for a seven-month period, the aggregate stomach content of the California mussel was over 97 per cent dinoflagellates while for the same period the phytoplankton of the water was over 97 per cent diatoms. Other animals combining the habit of filter and mucus feeding are the sea squirts such as Ciona. In these animals the water is first filtered of its large particles by a crown of tentacles guarding the oral opening and is then passed through a sort of grating which forms the branchial basket and which is supplied with an estimated 200,000 openings and is heavily ciliated for propulsion of water and for spreading of sheets of mucus over the inside surface of the branchial basket. In the passage of water from the oral opening through the grating and out of the atriopore the minute particles of food become entangled in the mucus and pass with it as a thread into the esophagus (MacGinitie, 1939).

A familiar example of the filter-feeding habit is that of the simple sponge, wherein flagellated cells lining internal cavities propel the water into the sponge by way of the numerous incurrent pores covering the surface of the body. After passing through the more or less complicated canal system, the water is then expelled through a common opening, the osculum, but enroute the individual flagellated cells select out the fine particles of food carried by the water.

Many plankton feeders may be better classified as preying animals, although in some respects they combine this habit with filter feeding. Any attempt to distinguish between such categories is based largely on the relative degree of selectivity exercised in feeding. Few animals are wholly indiscriminate in feeding, and even filter feeders exercise some degree of selection, either by a mechanical segregation of size dependent upon apertures of the screening apparatus, as in Oikopleura or sponges, or by rejecting through ciliary or other action certain particles unpalatable for chemical or physical reasons.

We have mentioned copepods chiefly as herbivorous plankton filter feeders but not all copepods feed upon diatoms and other phytoplankton organisms. The free-living types like Tortanus and Candace that may be considered largely carnivorous and rapacious have very strongly built mouth appendages for catching and holding their prey.

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Jellyfishes and ctenophores are highly predaceous in habit, feeding voraciously upon other plankton animals that drift within their reach. The former paralyzes its prey by means of batteries of nettle cells which cover the tentacles. The latter (Pleurobrachia) when in swarms are very destructive to large numbers of other small organisms, of which they sweep the waters quite clean. The prey is entangled in the trailing tentacles which are provided with sticky adhesive cells. In his study of food relationships in the Gulf of Maine, Bigelow states that “of all the members of the plankton, the most destructive to smaller or weaker animals are the several coelenterates, and especially the ctenophore genus Pleurobrachia, a pirate to which no living creature small enough for it to capture and swallow comes amiss.”

The formidable jaws of the arrow worm Sagitta attest that it is also a highly rapacious plankton feeder; being able to snatch individual organisms like Calanus and larval fish despite the fact that it possesses only light-sensitive “eye spots” instead of true eyes (fig. 228a).

Among smaller important plankton forms, mention should also be made of the tintinnids, radiolarians, foraminifera, Noctiluca, and other planktonic Protozoa that engulf such small organisms as chance carries within their reah. These are plankton feeders but not filter feeders. That some may exercise a degree of selective feeding is indicated by the tintinnids, some of which are found regularly to contain only the shells of silicoflagellates, while others select certain coccolithophores, the coccoliths or armor of which they use in building their shells.

Among the more or less obvious preying plankton feeders may be placed many fishes, notably herring, mackerel, sardines, and others of this type (p. 896) which either select out individual animals of the plankton or filter quite indiscriminately by the aid of the gill rakers, which form a net through which water entering the mouth must pass in its course over the gills and out under the gill coverings (fig. 240). The fineness of the net or branchial sieves formed by the gill rakers varies with different types of fishes, and in unclogged condition determines the minimum size of the planktonic organisms that can be sieved out for food. Even the menhaden, Brevoortia tyrannus, with a notably fine branchial sieve, is unable to retain organisms as minute as coccolithophores and small diatoms and infusoria. In the herring the sieve is much coarser, and though these fish are known to select out organisms individually the gill rakers must assist materially in retaining many of the smaller Crustacea. The stomach of a single herring has been found to contain more than 60,000 copepods. It should be noted that though much of plankton feeding may appear indiscriminate, yet a good deal of selection does occur, since swarms of specific prey may be selected and followed, as is evidenced in the herring and in the filter-feeding whalebone whales. The plankton-feeding fishes are swift swimmers but the usefulness of this ability must be in large measure to escape their enemies, the typical large predators (see below), although most plankton feeders do in part also prey upon other smaller fishes.

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It is a strange fact that the largest animals, that is, certain of the whales, are plankton feeders, living upon great masses of very small animals. These are the Mystacoceti or whalebone whales, of which the blue whale or sulphur bottom, the largest of all living animals, is an example (fig. 76 a, p. 314). In the mouth of this type of whale are suspended the closely set plates of whalebone (fig. 241) through the frayed ends of which water is passed while the planktonic euphausiids, copepods, pteropods, and so forth, which make up the principal diet, are filtered out. Whales are most abundant in waters rich in planktonic life and, as indicated in fig. 244, p. 904, their numbers may be correlated with the abundance of planktonic food of their preference.

Numerous other examples could be given from diverse animal groups to illustrate the manner in which nature exploits the supply of microscopic but vastly numerous and scattered particulate food floating freely in the water. Any considerable fluctuations in the abundance or distribution of the planktonic food must quickly affect the plankton feeders and, in due time of course, other types of feeders as well.

More will be said later (p. 901) about general problems involving filter feeding, production, and population density.