On the whole, it is animals of this type that are best known to the layman, for they are usually relatively large and their feeding habits tend towards the spectacular. We have already had occasion to mention a few of the preying animals such as the jellyfish, arrow worm, certain copepods, plankton-feeding fishes, and whalebone whales because of their role in utilizing the food offered by the micro- and macroplankton. The fishes and whales of this type were considered, in part, also as filter feeders.
Fast-swimming predators, among which are the surface fishes tuna, barracuda, and salmon, are provided with well-developed eyes and efficient teeth to aid in capture of their prey, which consists largely of plankton-feeding fishes. It should be noted here that such plankton-feeding fishes as the herring, sardine, menhaden, mackerel, anchovy, and alewife are fishes of exceptionally great abundance and are eagerly sought as food by the larger preying animals of the pelagic region. Some idea of their numbers may be gleaned from the fact that over 500,000 tons of sardines have been taken in in one year from California waters alone (Scofield, 1937), and the menhaden yield of the east American coast may reach 400,000 tons in a year (Tressler, 1923). Such fishes must therefore stand as an important link between the animal plankton and the larger piscine predators unprovided with direct means of gaining sustenance from the small planktonic life. High in this complex food pyramid are also the toothed whales and other marine mammals.
The pelagic fishes of great depths are also predators but they are of relatively small size, usually ranging upward to only a few centimeters. This small size may well be correlated with the scarcity of food at these depths. The diminutive sizes not only represent a conservation of organic material in growth, but also, owing to the increased ratio of
Among the nonplankton-feeding predator mammals of the sea are the toothed whales, for example the sperm whale (fig. 76b, p. 314), which is provided with teeth only in the lower jaw and which dives to great depths for its favorite food the squids, including the giant squid, the largest of all invertebrates. Other cetacean predators are the killer whales, porpoises, and dolphins, animals adapted to swimming with incredible speed and provided with teeth in both upper and lower jaws. To these must be added the seals, the sea lions, and the walruses. The first two catch their prey (fish and crustaceans) with very well-developed but ordinary teeth, while the last are specialized with long tusks with which to dig shellfish from the bottom.
Many benthic animals are predaceous, living upon each other and upon other bottom animals already discussed as users of detrital and finely divided food occurring on the bottom. The list includes many bottom-living fish called “bottom feeders” or “ground fish.” The best known among these are the plaice, flounders, halibut, croakers, cods, and rays which live on the crustaceans, shellfish, worms, and coelenterates of the bottom community.
Sea stars are notably voracious feeders on bivalve molluscs, a single specimen having been known to devour five or six clams in a day. Predaceous snails, distinguished by their long siphons, are also enemies of bivalves and other molluscs, drilling a neat hole through the shell and eating the soft contents.
The interrelations of the organisms of the sea are diagrammatically summarized in fig. 242. The volumes indicated are not based on computations and should be considered as being only very roughly proportional
The marine bacteria are a vital link in the nutritional relationships of all marine organisms, but it is more convenient to discuss these later under a special heading (p. 908).
The main features of the interrelations of marine organisms.