EFFECTS OF FLOW SPEED ON GROWTH OF BENTHIC SUSPENSION FEEDERS

In separate experiments in 1991 and 1992, the sensitivities of growth rates of six species of benthic suspension feeder to flow speed were t ested in a series of turbulent pipe flows. Species examined were the c heilostome bryozoan Membranipora membranacea (1991); the serpulid poly chaete Pseudochitinopoma occidentalis (1991); and the barnacles Balanu s glandula (1991), B. crenatus (1992), Semibalanus cariosus (1992), an d Pollicipes polymerus (1992). In both experiments, animals were expos ed to one of five constant, narrow ranges of speed that varied from ab out 2-15 cm s-1. Growth rates of Membranipora and Pseudochitinopoma in 1991 declined significantly and monotonically with increasing flow sp eed, despite evidence that at faster flows there were greater concentr ations of suspended food available and higher particulate fluxes. In c ontrast, there was no detectable relationship between speed and growth of B. glandula over the same range of flow speeds in 1991. Results of the 1992 experiment indicated variability in growth responses among t hree species of barnacle. Growth rates of S. cariosus and P. polymerus were insensitive to flow speed, whereas growth rates of B. crenatus i ncreased from low speeds to a maximum at an intermediate speed of abou t 8 cm s-1, and then tended to decrease at higher speeds. Combined res ults of the two experiments indicate that the growth response of anima ls to flow was most obviously related to the relative flow energy of t he animal's natural habitat. Growth rates of animals that typically ex perience relatively weak flows (P. occidentalis, M. membranacea, and B . crenatus) were affected significantly by flow speed, whereas growth rates of animals from comparatively high-energy environments (B. gland ula, S. cariosus, and P. polymerus) were relatively insensitive to flo w speed. In contrast, animal morphology and behavior were not obviousl y related to growth responses to flow. A key to understanding the impa ct of flow on a suspension-feeder's growth may be the animal's ability to handle and process particles that have impacted the feeding appara tus and to deliver them to the point of ingestion.