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 tested in a series of turbulent pipe flows. Species examined were the cheilostome bryozoan Membranipora membranacea (1991); the serpulid polychaete Pseudochitinopoma occidentalis (1991); and the barnacles Balanus glandula (1991), B. crenatus (1992), Semibalanus cariosus (1992), and Pollicipes polymerus (1992). In both experiments, animals were exposed to one of five constant, narrow ranges of speed that varied from about 2-15 cm s-1. Growth rates of Membranipora and Pseudochitinopoma in 1991 declined significantly and monotonically with increasing flow speed, despite evidence that at faster flows there were greater concentrations of suspended food available and higher particulate fluxes. In contrast, 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 three species of barnacle. Growth rates of S. cariosus and P. polymerus were insensitive to flow speed, whereas growth rates of B. crenatus increased from low speeds to a maximum at an intermediate speed of about 8 cm s-1, and then tended to decrease at higher speeds. Combined results of the two experiments indicate that the growth response of animals to flow was most obviously related to the relative flow energy of the animal's natural habitat. Growth rates of animals that typically experience 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. glandula, S. cariosus, and P. polymerus) were relatively insensitive to flow speed. In contrast, animal morphology and behavior were not obviously related to growth responses to flow. A key to understanding the impact of flow on a suspension-feeder's growth may be the animal's ability to handle and process particles that have impacted the feeding apparatus and to deliver them to the point of ingestion.