Ecology of endolithic bryozoans: colony development, growth rates and interactions of species in the genus Immergentia.

Abstract

Boring bryozoans dissolve calcium carbonate substrates, leaving unique borehole traces. Depending on the shell type, borehole apertures and colony morphology can be diagnostic for distinguishing taxa, but to discriminate among species their combination with zooidal morphology is essential. All boring (endolithic) bryozoans are ctenostomes that, along with other boring taxa, are common in benthic communities. The growth rates of such bryozoans, including Immergentiidae, are largely unknown. For the first time laboratory experiments were conducted to determine growth rates and early colony development of the intertidal species Immergentia stephanieae and the subtidal species I. cf. suecica from Roscoff, France. In growth experiment 1, ancestrular growth rates varied, with the highest rates in I. stephanieae at 96.5 µm day^-1 and the lowest at 1.1 µm day^-1, during the period of August to October, in which the number of reproductive zooids was comparably higher than in other months of the year. Immergentia cf. suecica had a higher proportion of reproductive zooids from December to March compared to other months. In growth experiment 2, the bryozoans were fed a culture mixture of Chaetoceros calcitrans and Tisochrysis lutea which was compared with a control. The growth rate of small colonies of comparable size was greater in the food-enriched samples compared to the control (non-enriched). In larger colonies, the trend differed with greater growth (cystid appendage expansion) rate reported for some samples in the control. In food-enriched samples ancestrulae of I. stephanieae grew at 23 µm day^-1 and I. cf. suecica 9.3 µm day^-1 while no growth was observed in the control of I. cf. suecica, but 0.4 µm day^-1 was reported for I. stephanieae. Growth patterns in the early developmental stages showed that the budding patterns from the ancestrulae were the same for both species, with different enantiomorphic tendencies. Inter- and intraspecific interactions are also discussed. The distribution of immergentiids is presented, as are records from new locations and the greatest subtidal depth of collection reported to date.