THE BODY PLAN OF THE CYPHONAUTES LARVA OF BRYOZOANS PREVENTS HIGH CLEARANCE RATES: COMPARISON WITH THE PLUTEUS AND A GROWTH MODEL

¹ Department of Zoology and Friday Harbor Laboratories, 620 University Road, Friday Harbor, Washington 98250

The cyphonautes larva of bryozoans is anomalous among ciliary suspension feeders with upstream particle capture. This is because the length of the ciliated band that produces the feeding current does not increase disproportionately relative to body length during larval development and growth. Comparisons with previous studies of other upstream collectors (mostly the pluteus of the echinoid Dendraster excentricus or other echinoplutei) demonstrated a striking deficiency in feeding capabilities of the cyphonautes. (1) In comparison to the pluteus the ciliated band generating the feeding current of the cyphonautes was short both absolutely and relative to larval body size as protein. (2) During larval growth, the length of the ciliated band of the cyphonautes decreased relative to protein in the whole body whereas the length of ciliated band of the pluteus was nearly isometric with body protein. (3) The ratio of metabolic capacity (by electron transport system assay) to protein content was similar for the advanced stage pluteus and cyphonautes. If respiratory rate is proportional to metabolic capacity in these larvae, then the cyphonautes does not compensate for low feeding capacity by reduced respiratory rate. (4) The velocity of the current across the ciliated band and the length of lateral cilia was similar for the cyphonautes and pluteus. Therefore, maximum clearance rates per unit length of ciliated band were not unusually high for the cyphonautes. (5) The cyphonautes was inferior in rate of capture of small (2 µm) spheres relative to 10 µm spheres in comparison to plutei feeding on the same suspension. (6) The pluteus and cyphonautes were similar in the maximum sizes of spheres ingested. (7) In two experiments the cyphonautes was more selective of spheres flavored by incubation with the alga Dunaliella tertiolecta than was the pluteus. (8) Different patterns of allometric growth of ciliated bands were incorporated in a growth model based on the difference between allometry of gain in organic carbon through feeding and loss of organic carbon through respiration. The model predicted that a cyphonautes form required a much greater concentration of food than a pluteus form to gain the same organic material in the same time. Thus the cyphonautes proved inferior to the pluteus in quantitative measures of capacity for suspension feeding. The highly conservative differences in larval body plans result in differing capabilities for feeding that are likely to influence larval growth rates and the evolution of life histories.

This article has been cited by other articles:

				M. G. Hadfield, E. J. Carpizo-Ituarte, K. del Carmen, and B. T. Nedved Metamorphic Competence, a Major Adaptive Convergence in Marine Invertebrate Larvae Integr. Comp. Biol., October 1, 2001; 41(5): 1123 - 1131. [Abstract] [Full Text] [PDF]