The Significance of Micro-animals
One may ask what influence this type of development of the plants has had on the animal population of the sea. Along what lines has the animal economy evolved to exploit this vast supply of microscopic plant food in the sea? A plant population consisting mainly of scattered individuals that are microscopic in size must impose certain restrictions and requirements upon the animal grazers that depend upon it for nourishment. It is significant that in the sea the chief grazers, representing the main bulk of the zooplankton, are also microscopic or semimicroscopic in size and vast in numbers. Foremost among the grazers of the pelagic region are placed the copepods, the diet of which has been shown by direct analysis to consist mainly of diatoms, dinoflagellates, and other micro-plants, but many other small herbivores, especially protozoans, euphausiids, and larval stages of larger invertebrates also graze directly upon the phytoplankton.
Through all the tiny grazers of the sea nature accomplishes two important ends: first, complete utilization of even the minutest particles of primary food; and, second, transformation of the organic material of these plants into animal substance of size sufficiently large to be caught and utilized by carnivorous forms. The large number of carnivorous animals that occur in the sea is abundant evidence that the numbers of microscopic grazers must indeed be great (p. 896). Much economy would result if the larger animals could feed directly upon the plants in the manner of the large terrestrial animals, but the nature of the environment prohibits the growth of large plants except in a narrow fringe along shore and, except for a small amount in the eel grasses, no seeds with concentrated nutriments are produced as on land for immediate use or as a store to be drawn upon during periods of low vegetative growth. It is true that some fishes, particularly the herrings, feed to some extent directly upon minute plants, especially during their larval stages (Lebour, 1924a), and an exceptional few, such as the menhaden with their notably fine filtering apparatus, feed partly on diatoms and dinoflagellates throughout life (Bigelow, 1926).
In our concept of plant-animal relations, the plants are to be looked upon chiefly as autotrophic organisms capable of converting inorganic material to organic material, thus making it available to animals as particulate food. Lohmann found in the Bay of Kiel that for every multicellular animal there were one thousand protozoan and seven thousand protophytan forms. It is extremely difficult to obtain reliable measurements of the relative volume of plant and animal substance in the sea, but, since only through the endothermic process of photosynthesis can the solar energy be bound and become available for the building of animal substance, it is obvious that the mass of plant material produced must be greater over a long period of time than the mass of animal material. It appears that even at periods of only moderate production the very rapid rate of reproduction in unicellular plants is sufficient to maintain the zooplankton population, even though the bulk of plants actually present at any moment may be less than that of the animals. It should be emphasized that the rapidity of reproduction of the nanno- and microplankton is as important as the bulk present in the water as food at any given time. Lohmann calculated that even though the mass of plants may fall below that of animals in the Bay of Kiel, yet plant production usually exceeds animal consumption during the summer months. The excess may be 29 mm3/100 l of sea water. During winter both production and consumption fall off, but during midwinter, January and February, there is a production deficiency of −0.8 mm3/100 l of sea water.
It must be pointed out that some of the organic material produced by plants is lost to the animals through solution in the sea water; the content of dissolved organic matter in the sea runs as high as six or more milligrams per liter of sea water (up to three milligrams of carbon per liter; p. 250). This represents much more organic material in the sea in solution than exists there as particulate food at any one time, and any organisms that are capable of reclaiming this dissolved organic material have an important role in the economy of the sea. Bacteria doubtless serve this purpose (p. 911), but other saprophytic forms may also play a part. Some dinoflagellates are believed to be saprophytic, but their utilization of dissolved organic matter from the water in such dilute concentrations has not been demonstrated. Attention has already been called to certain “olive-green cells” regularly collected in deep water from the South Atlantic by the Meteor. These have a maximum distribution below the euphotic zone and Hentschel (1936) believes them to live heterotrophically. If this is true, they are important in reclaiming dissolved organic matter and building it into bodies of suitable size for use by filter-feeding animals at mid-depths.
In the following analysis of food relationships our purpose is not so much to learn the habits of individual animals, though to do so is extremely important and indispensable in understanding the biology of
In studying the food relationships of the marine organisms, it is perhaps most convenient for our purpose to group the animals according to the kind or source of food upon which they subsist and according to general methods of feeding, remembering at the same time that the feeding habits of many are unknown and that many have habits not clearly confined to any one of these categories but overlapping more or less into others. The larval and adult stages may also differ both in food required and in the method of procuring it. The first two groups given below are based on the source and kind of food used and, incidentally, on the method of feeding. The last two groups are based mainly on methods of feeding, but this feature itself results largely from the nature of the food.