Effects of sessile Protozoa on intracapsular oxygen tension and embryonic shell calcification in the muricid Chorus giganteus

Clusters of egg capsules deposited by some common marine Mollusca may suffer problems of a low diffusive oxygen supply to the embryos they contain, especially if the capsules are exposed to hypoxic seawater or attachment and growth of marine biofouling organisms. The present study was undertaken to determine the effects of severe biofouling by sessile Protozoa on intracapsular oxygen tension (IPO2) and the development of embryos contained in the egg capsules of the muricid snail Chorus giganteus. We also investigated the effects of ambient oxygen tension (EPO2) on IPO2. The presence of sessile Protozoa attached to the outer wall of the egg capsules significantly reduced the IPO2 compared to capsules not fouled by Protozoa. Clean capsules containing embryos showed an IPO2 of about 105 mm Hg, compared with about 92 mm Hg for protozoan-fouled capsules when both were immersed in air-saturated seawater at 12°C. The embryos in capsules without Protozoa grew normally, hatching in about 70 d as veliconch larvae, whereas the development of larvae in protozoan-fouled capsules showed impairment of shell formation and delay in hatching for up to 5 mo. Pre-hatch embryos at 60 d measured about 922 μm and had an ash content near 18 μm embryo–1; embryos in capsules covered by micro-organisms measured only about 783 μm, with an ash content of about 3 μm embryo–1 over the same time period. Our study suggested that the lack of larval calcification observed in the presence of sessile Protozoa on the outer wall of the egg capsules was probably related to reduced IPO2. Similarly, any factor reducing oxygen supply to encapsulated embryos (i.e. exposure to water masses with low oxygen content, biofouling, reduced water movement) could impair embryonic development, a significant phenomenon thus far not reported in C. giganteus.