The Effect of Grain Size on the Distribution of Small Invertebrates Inhabiting the Beaches of Puget Sound

Abstract

The position with respect to the general configuration of the shore‐line and the direction and force of the currents of five localities in Puget Sound is such that they represent a series of decreasing exposure, Richmond Beach being the most exposed, Bainbridge Island the most sheltered locality. This is reflected in the composition of the substrate, the former locality having the coarsest, the latter the finest substrate. Perpendicular to this horizontal gradient there runs a vertical gradient due to the tide, the coarse grades being deposited in the upper, the fine grades in the lower intertidal.The intertidal distribution of at least some of the more common species of invertebrates on the beaches is not so much determined by levels of tidal water as by the pattern of distribution of certain grades of substrate. For example, species of this sort will penetrate into the upper intertidal if the preferred substrate occurs there, but they will remain in the lower intertidal if the substrate is confined to this zone.On the beaches there exist mixtures of sand which constitute barriers separating major faunal components from each other. It is assumed that substrates with a median diameter of approximately 200µ constitute such a barrier separating the bulk of interstitial sliders from a great number of burrowing animals. The former can move only in sand coarse enough to maintain an interstitial system; the latter, for mechanical reasons, will find fine sand more favorable than coarse. This distinction, however, does not apply to nematodes which are able to move even in the interstices of fine sand. There are various ways in which grain size and shape can influence the distribution of the fauna. These types of relationship are discussed.