Spirorbis spirorbis

Abstract: Seaweeds are key species of the Baltic Sea benthic ecosystems. They are the substratum of numerous fouling epibionts like bryozoans and tubeworms. Several of these epibionts bear calcified structures and could be impacted by the high pCO2 events of the late summer upwellings in the Baltic nearshores. Those events are expected to increase in strength and duration with global change and ocean acidification. If calcifying epibionts are impacted by transient acidification as driven by upwelling events, their increasing prevalence could cause a shift of the fouling communities toward fleshy species. The aim of the present study was to test the sensitivity of selected seaweed macrofoulers to transient elevation of pCO2 in their natural microenvironment, i.e. the boundary layer covering the thallus surface of brown seaweeds. Fragments of the macroalga Fucus serratus bearing an epibiotic community composed of the calcifiers Spirorbis spirorbis (Annelida) and Electra pilosa (Bryozoa) and the non-calcifier Alcyonidium hirsutum (Bryozoa) were maintained for 30 days under three pCO2 conditions: natural 460±59 µatm, present-day upwelling1193±166 µatm and future upwelling 3150±446 µatm. Only the highest pCO2 caused a significant reduction of growth rates and settlement of S. spirorbis individuals. Additionally, S. spirorbis settled juveniles exhibited enhanced calcification of 40% during daylight hours compared to dark hours, possibly reflecting a day-night alternation of an acidification-modulating effect by algal photosynthesis as opposed to an acidification-enhancing effect of algal respiration. E. pilosa colonies showed significantly increased growth rates at intermediate pCO2 (1193 µatm) but no response to higher pCO2. No effect of acidification on A. hirsutum colonies growth rates was observed. The results suggest a remarkable resistance of the algal macro-epibionts to levels of acidification occurring at present day upwellings in the Baltic. Only extreme future upwelling conditions impacted the tubeworm S. spirorbis, but not the bryozoans.

Abstract: Brood protection is characteristic of the Spirorbinae although the mode of protection differs widely between species (Bailey, 1969; Knight-Jones, Knight-Jones & Vine, 1972). The way in which these protected eggs are successfully fertilized has been a subject of speculation (Gee & Williams, 1965; Franzen, 1956; Potswald, 1968). Individuals of the Spirorbinae are simultaneous hermaphrodites and the possible role of self-fertilization in their reproduction has been considered (Gee & Williams, 1965; Potswald, 1968). In the past, these problems have been discussed with the tacit assumption that no spermatheca is present in the Spirorbinae.

Abstract: Among the marine invertebrates whose settlement behaviour is well studied, the polychaete tubeworm, Spirorbis spirorbis L. ( = borealis Daudin), the barnacle, Balanus balanoides, and the oyster, Ostrea edulis, are different in form whether in the free-swimming or adult stage, yet the settlement behaviour patterns are basically similar (Crisp, 1973). The larvae have a sequence of movements during exploration of the substratum, with each activity initiated by a hierarchy of stimuli. Given a positive set of responses to a suitable habitat, the distances moved on the substratum are limited progressively so that finally the larva investigates a very restrictred area. When the exact site for settlement is determined the larva ceases to move, attaches permanently and immediately commences metamorphosis into the adult form. However, if certain stimuli are lacking or obnoxious to the searching larva, it recommences the initial free-swimming activity, in order to reach alternative substrata.