PRATT, M.C.: Growth and survival of an erect and encrusting bryozoan in different flow habitats

Bryozoans are active filter feeding invertebrates that can exhibit a wide variety of different colonial growth forms. These growth forms can roughly be divided into species that are erect or encrusting. I compared the effects of hydrodynamics on the growth and survival of an encrusting and an erect bryozoan. Membranipora membranacea has an encrusting growth form and is found almost exclusively on large flat macroalgae (such as kelps) in relatively high and low flow habitats. Bugula pacifica has an erect growth form and is generally found on hard substrata (such as docks or rocks) in relatively low flow habitats. These two species do occasionally overlap in habitat: both can be found on large macroalgae in low flow environments. Previous data suggest that Membranipora can capture more particles than Bugula at both slow (2.5cm/s) and fast (10cm/s) flow velocities. However, it remains to be seen whether a higher particle capture success translates into faster growth and/or higher survival rates. I measured the growth (change in area) and survival rate of Membranipora and Bugula colonies in the field in a high and low flow habitat over a month and a half time period. Growth rate did not vary greatly between high and low flow habitats for either species, but survival rate did vary between species and between habitats. Membranipora had a higher survival rate than Bugula in both flow habitats, and both species had a higher survival rate in the slower flow habitat than in the higher flow habitat. These results suggest that hydrodynamics can have a large effect on survival of bryozoans based on their growth form.