Understanding of the conditions encouraging the evolution of kleptoparasitism has been limited by the preponderance of attention focused on a limited number of taxa (predominantly birds). However, most animal taxa contain kleptoparasitic members. In many ways, adult birds are aberrant because most are acrobatic, capable of three-dimensional movement, and steal from hosts that are capable of similar feats. The mobility of kleptoparasites and their hosts falls along a continuum, with the acrobatic birds at one end and sessile hosts and kleptoparasites at the other. Considering sedentary systems requires amendments to the criteria previously outlined as necessary for kleptoparasitism. Kleptoparasitic interactions between agile participants are ephemeral, involve numerous hosts over the lifetime of the kleptoparasite, and usually rely on vision. Large concentrations of hosts and large, high-quality food items are important in mobile systems. By contrast, the long-term, intimate associations of sedentary interactions demand that the host not consume the kleptoparasite's offspring and that the host must either have a longer life cycle than the kleptoparasite or be located in aggregations that have staggered life cycles (so the kleptoparasite can move to a new host when the original host dies). Expanding studies of kleptoparasitism to other taxa will differentiate between the selective pressures and evolutionary responses that are universal among kleptoparasitic symbioses and those that are restricted to certain groups.

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Individuals of the marine snail Trichotropis cancellata can either suspension feed or steal food from tube-dwelling polychaete worm hosts (kleptoparasitism). Because kleptoparasitism is facultative in this case, the performance of both members can be evaluated separately and together.

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Although theory pinpoints the reliability of cues as an important condition for the evolution of inducible defenses, the specificity of cues is poorly known in most systems. The bryozoan Membranipora membranacea produces long, energetically-costly spines in response to a trophically specialized nudibranch Doridella steinbergae, but the range of nudibranchs that trigger this response is unknown. We tested chemical cues from 14 nudibranch species from 4 suborders, the snail species Lacuna vincta, and an abiotic treatment (desiccation) for their ability to induce spines. Both corner spines (arising from the corners of the zoecium) and membranous spines (arising from the frontal membrane of the zooid) were observed. Of the nudibranch species tested, 57% induced corner spines in at least 1 trial: 4 predators of M. membranacea (Onchidoris muricata, Polycera zosterae, Triopha catalinae, and D. steinbergae) and 4 non-predators (Coryphella sp., Archidoris odhneri, Cadlina luteomarginata, and Dirona albolineata). Although many nudibranchs occasionally triggered spines, the spine response was most reliably and fully developed in response to the bryozoan's primary predator, D. steinbergae. All other species that induced corner spines, except Coryphella sp., failed to induce spines in all trials. Membranous spines were sometimes produced in response to D. steinbergae, O. muricata, P. zosterae, A. odhneri, and Coryphella sp. The last 2 species are not known to prey on M. membranacea. Neither corner nor membranous spines were ever induced by Discodoris sandiegensis, Dendronotus frondosus, Dendronotus diversicolor, Tritonia festiva, Flabellina trilineata, Phidiana crassicornis, or desiccation. Of these non-inducers, only P. crassicornis feeds on M. membranacea, and this species typically causes little damage. There was no phylogenetic pattern among nudibranchs inducing spines. The production of corner and membranous spines were correlated. Corner spines appeared to have a lower induction threshold than membranous spines; the latter are therefore a more conservative indication of induction. Counter to the hypothesis of Adler & Grün-baum, the spine-inducing chemical cue(s) from D. steinbergae is probably not a mating pheromone, as D. steinbergae egg masses and pre-reproductive slugs induced spines. We conclude that M. membranacea often produces spines in response to predators that are deterred by spines, but seems surprisingly responsive to cues from some benign nudibranch species. KEY WORDS: Chemical ecology · Defense · Nudibranch predators · BryozoansResale or republication not permitted without written consent of the publisher