NO. 9 MARINE INVERTEBRATES, ALASKA — MacGINITIE 29 



Distribution depends more on the larvae's than the adults' becoming 

 acclimatized to new environments. If marine animals can do anything 

 well it is to reproduce — sometimes at the rate of half a billion per 

 year (Galtsoff, 1930; MacGinitie, 1934). Because of the advantages 

 of constant immersion in sea water and an enormous rate of reproduc- 

 tion the distribution of marine animals into or out of the Arctic should 

 not require a long evolutionary process. However, this would be 

 different for each species just as the rate of reproduction is different 

 for each species ; and the rate of evolutionary change is not neces- 

 sarily proportional to the number of offspring. Some groups of ani- 

 mals are much more adaptable than others in their ability to evolve 

 into new species. Some species live a few years ; others many. Some 

 groups (e.g., brachiopods) are in general inclined to stick to an 

 orthogenetic line while others (e.g., certain snails) become adapted 

 readily to a new environment. In other words, evolution is a slow 

 process in some groups and relatively rapid in others, with cor- 

 responding effects on distribution. Distribution, therefore, furnishes 

 as much information regarding evolutionary development as do almost 

 any other data. 



FOOD 



The writer classifies the food of marine animals under five head- 

 ings, which are listed in the order of their relative abundance: 

 I, Plankton; 2, detritus; 3, other animals; 4, debris; 5, seaweeds. 

 Actually a sixth could be given — self-cultured algal cells, used by the 

 flatworm Convoluta roscoffensis and the clam Tridacna. 



Plankton furnishes food for one great group of marine animals — the 

 pelagic or free-moving marine animals living in the ocean above the 

 bottom. This group, from the tiny microscopic larvae to great sharks, 

 is dependent directly or indirectly on the diatom pastures (which 

 form an important part of the plankton) of the oceans. In the waters 

 off Barrow base plankton is extremely plentiful in summer and great 

 swarms of euphausiids (krill), mysids, pteropods, and copepods are 

 present. This abundance of food accounts for the migration of the 

 baleen whales into the Arctic each summer. Since plankton is not so 

 abundant in winter, many invertebrates as well as vertebrates of the 

 Arctic store food in the form of oil or fat. 



Another great group feeds on detritus. It includes the burrowers 

 and semiburrowers of the ocean bottom, together with many sessile 

 animals that are anchored in the mud. In rocky regions where the 

 bulk of animal life is sessile, both plankton and detritus may be used 

 indiscriminately, for both consist of finely divided food particles. 



