Among the benthic organisms the animate factors of the environment are manifest not only in predacity and competition for food but also in competition for favorable space for attachment or burrowing. The competition for space is most obvious in sessile forms such as barnacles and mussels of the intertidal zone, where overcrowding must lead to elimination as growth proceeds, and many pelagic larvae of sessile forms perish by failure to find attachment at all. A foremost problem in oyster culture is the curtailment of oyster production that results in beds in which the slipper shell Crepidula is a serious competitor with the oyster for food and space. In English waters as much as twenty tons of Crepidula have been removed from certain beds daily, and some beds there and elsewhere have been completely abandoned for culture because of this pest.
Many good examples of the influence of biological factors on the benthic populations are also offered by studies of other pests on oyster beds (Orton, 1937). Among these we may mention the common drill Urosalpinx cinerea, which may destroy 50 per cent of the young oysters settling on a bed. In Australia whole beds have been destroyed in a few days by yet another predator, the mangrove crab Scylla serrata. The extent to which sea stars are a factor in reducing the yield of oysters in Long Island Sound is indicated by reports that as much as 500,000 bushels are annually killed by these pests in that small area alone (Galtsoff and Loosanoff, 1939).
In the above, attention has been directed to the effect of predacity on the prey, but we may shift the emphasis and consider an instance wherein the predator is the object of economic interest.
According to quantitative investigations of benthic invertebrates with reference to their utilization as food for the bottom-feeding plaice, it has been calculated by Blegvad (1930) that over an area of 3445 square miles in the Kattegat there is produced, as a minimal figure, about 230,000 tons of “first-class” plaice food. (The food is considered first-class when it is of a type available to young as well as adult animals, and second-class when it can be eaten only by the grown animals.) This amount of food is believed sufficient to produce 12,800 tons of plaice. Animals known to compete for the food of the plaice take about one tenth of the total.
These few examples selected from a field of study intensively pursued for economic reasons are doubtless illustrative also of similar biological aspects that enter into the control of any population in the sea (see also fig. 249, p. 937).