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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.
Highlights
To enhance our understanding of the life histories of boring bryozoans, we examined colony development by (1) conducting settlement experiments and observing early growth patterns, (2) determining and comparing growth rates in Immergentia stephanieae Johnson & Schwaha 2024 and Immergentia cf. suecica Silén 1947 with or without supplemented food culture, (3) observing feeding behaviors, (4) evaluating the implications of species interactions on the substrate, and (5) updating the geographic distribution of immergentiids
Reproductive zooids Embryos develop from large macrolecithal oocytes subsequently moving from the zooidal coelom to the tentacle sheath of autozooids where the polypide has largely degenerated; these are referred to as reproductive zooids (Fig. 2)
The lophophore and digestive tract are degenerated but especially the apertural, parietal, tentacle sheath, and/or retractor musculature remain in reproductive zooids (Fig. 2 a–c)
Summary
Bryozoans are a phylum of sessile, colonial suspension feeders [1], with the exception of a few solitary forms [2–4]. They are classified into two clades, Phylactolaemata and Myolaemata, with the latter comprising Stenolaemata and Gymnolaemata [5]. Close to 400 species of ctenostomes have been described [7] compared to cheilostomes with over 6000 extant species [8]. The Immergentiidae constitute one of the four families of extant endolithic ctenostomes, the so-called boring bryozoans, owing to their ability to dissolve calcium carbonate by chemical means [9, 10]. The other endolithic ctenostome families are Penetrantiidae, Spathiporidae and Terebriporidae. Fewer than 50 extant boring bryozoan species have been described
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