Schubert, Philipp (2003) Wechselwirkungen zwischen Opisthobranchiern und ihre Beuteorganismen. (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 103 pp.

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Abstract

This study represents the first investigation of the opisthobranchs and the interactions with their prey in the vicinity of the island of Helgoland (German Bight / North Sea). Particular emphasis was laid on the abundant nudibranch Polycera quadrilineata and its bryozoan prey species, including the abundant and ecologically important bryozoan Membranipora membranacea. In 2002 (April to December) 34 opisthobranch species were found during several scuba dives and identified in the laboratory. From these species 14 were not listed in the 'Check list of species - Helgoland' (Harms, 1993). One species (Geitodoris planata) is new to the opisthobranch fauna of the North Sea and Helgoland. The bryozoan Membranipora membranacea produces extended corner spines if exposed to chemical cues from or attacked by predators. In the present study I investigated the spine formation of colonies from Helgoland induced by chemical cues of predatorial nudibranchs and whether the defensive reaction deviates from known defenses of Pacific populations of M. membranacea. The induction water (hereafter referred to as 'inducer') was prepared by holding test species of nudibranchs in a vessel of prefiltered seawater. The inducer was added to the colonies for 5 days, spines were censused on the ath day and compared with a control treatment. This comparison revealed that the inducer from P. quadrilineata and the inducer from the herbivorous snail Gibbula cineraria failed to produce spines in the bryozoan. An abiotic treatment (desiccation) also failed to induce spine formation, but inducer from the predatorial nudibranch Onchidoris muricata induced significantly langer corner spines than the control treatment. As shown in previous investigations of Pacific colonies, these extended corner spines are an effective defense against 0. muricata. In these previous investigations corner spine length was always correlated with the formation of membranous spines. Membranous spines were not induced in the present study. This might be an indication of genetic differences between M. membranacea populations from the Atlantic and the Pacific. The nudibranch P. quadrilineata never induced spine formation, although it is the most important predator of M. membranipora near Helgoland. Probably the reasons are the high energetic costs and the lack of protection against this predator by spines. Food preferences of P. quadrilineata were tested using a 'multiple choice design' in which the nudibranch chose the preferred food by crawling into a chamber. Membranipora membranacea was pref erred over all other offered bryozoan species. The second and third preferred bryozoans were Electra pilosa and Bugula plumosa, respectively, between the remaining species no statistically significant preference was found. Neither the multi-chamber apparatus, nor the algae Laminaria hyperborea, basibiont of E. pilosa and M. membranacea, affected the choice of the nudibranchs, as revealed by additional experiments. A no-choice test confirmed the hierarchy found in the multiple-choice experiments. Another experiment tested the mucus 'trail-following' -behaviour of P. quadrilineata. The nudibranchs followed the trail of another individual significantly, which was prevented effectively by cleaning the apparatus with a brush. 'lngestive conditioning', the modification of prey preferences by recent feeding experience, was tested in another experiment and was shown to have no significant effect on the food preferences of P. quadrilineata. The last experiment investigated the food preferences of the herbivorous Gibbu/a cineraria. This snail clearly preferred kelp-fronds fouled with M. membranipora over unfouled fronds. This preference is probably due to higher biomass of microalgae on bryozoan colonies compared to unfouled fronds.

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