Kleptoparasitism may be an additional or exclusive feeding mode in symbiotic associations of gastropods and echinoderms

Gastropods from the family Eulimidae have different life strategies in inhabiting their host echinoderms, but it is currently assumed that they derive most of their nutrition from the host’s tissues. This paper presents the results of stable carbon and nitrogen isotope analyses of different tissues of echinoderm hosts (crinoids Comaster nobilis and holothurians Holothuria atra ) and their ectosymbionts ( Annulobalcis wareni and Peasistilifer nitidula ). Our data show that the stable nitrogen isotope enrichment level in symbionts is practically indistinguishable from that of their hosts. The symbionts are depleted in 13 C compared to their hosts. We suggest that these molluscs may not feed on host tissues but instead obtain food particles from the environment, possibly stealing detritus particles, which, in turn, are food for these species of echinoderms. We revealed the proximity of the location and slight overlap of the crinoids’ visceral tissue and their symbiont tissues. This suggests a greater food variety for the symbionts. The detected differences in δ 13 C values and the almost complete absence of overlap of the holothurians’ body walls and symbionts allow us to conclude that in this symbiotic association, the hosts and symbionts rely on different food sources. In addition, the two studied echinoderm species differed significantly in δ 13 C signatures, which is confirmed by their different lifestyles. Thus, stable isotope analysis suggests that kleptoparasitism is at least an additional and possibly exclusive feeding mode for eulimids A. wareni and P. nitidula . However, this hypothesis will need to be followed up with morphological, biochemical, and experimental data.