Current knowledge on hesionid symbionts
Thirty out of 170 currently known hesionid species live as commensals of other invertebrates (Martin and Britayev, 1998; Miller and Wolf, 2008; De Assis et al., 2012; Martin et al., 2012, 2015; Britayev et al., 2013; Chim et al., 2013), representing around 18% of all the known hesionid species and about 6% of the known symbiotic polychaetes (Table 10). Symbiosis seems to be restricted to the clade Ophiodrominae, which includes the genus Oxydromus. It has multiple origins within the family Hesionidae because Oxydromus and Gyptis, two of its most representative genera with symbiotic species, are not closely related phyllogenetically (Ruta, et al., 2007). Moreover, both are species-rich genera and there is no evidence indicating whether commensalism arose once or multiple times within each of them.
Commensal hesionids are involved in about 65 different associations (Table 10). Except for the polyxenous species of Oxydromus, such as for O. flexuosus (Delle Chiaje, 1827) and O. pugettensis (Johnson, 1901) with up to 10 and 11 hosts, respectively, most symbiotic hesionids are monoxoneus, occurring in only one (15 species) and two (6 species) hosts (Table 10). Hesionid hosts include species from very different taxonomic groups, a variety among symbiotic polychaetes only comparable to that of polynoids (Martin and Britayev, 1998). The most common hosts are, however, echinoderms (particularly starfishes and sea urchins) and polychaetes (Table 10).
Only one genus, Oxydromus, includes symbiotic species living in association with bivalves. These are O. pugettensis, O. humesi and O. okupa sp. nov. Although nothing is known on the relationships between O. pugettensis and its bivalve host, the polychaete seems to be able to detect at a certain distance the presence of at least two of its host starfishes, P. miniata and Luidia foliolata (Grube, 1866) (Davenport et al., 1960). Also there are some indications of mutualistic behaviour in their relationships with one of its echinoid hosts, the sand dollar Clypeaster humilis (Leske, 1778) (Storch and Niggemann, 1967). The existing analyses of the behaviour of O. okupa sp. nov. in experimental conditions do not prove the existence of a host-factor in its relationships with S. plana, like in the case of O. pugettensis and its host starfishes. In turn, the species shows an elaborated and complex host-entering behaviour, which leads the worm to enter inside the host bivalve mainly through the inhalant siphon (Martin et al., 2015). Moreover, the presence of O. okupa sp. nov. caused a significant reduction in the soft-body biomass of the infested hosts, compared to the non-infested ones, which may imply affectation of the host’s metabolism according to Bierbaum and Ferson (1986) thus leading to a relationship closer to parasitism (Martin et al., 2012). Such a negative influence has been previously reported for other symbiotic polychaetes living in the mantle cavity of bivalves, such as the deep-sea hydrothermal vent polynoids B. seepensis (Britayev et al., 2007). However, contrary to B. seepensis, in the case of O. okupa sp. nov. no damages in the tissues of S. plana were observed (Martin et al., 2012). In B. seepensis, tissue damages were considered as a secondary effect of the worm’s feeding inside the host caused by powerful jaws of the polynoid, and not a voluntary ingestion. Oxydromus okupa sp. nov. lacks jaws so that, even in the case of having a feeding mode similar to that B. seepensis, the hesionid seems to be able to avoid causing involuntary damage to the host tissues.
Despite the overall growing knowledge on symbiotic hesionids, O. okupa sp. nov. is probably the best known representative of the group to date. Nevertheless, further studies are required to complete the knowledge on this species, which becomes apparent if we compare it with O. humesi, with which it shares a similar morphology and a closely related host that is several thousands of kilometres away from Cadiz Bay. In other words, the present study is just another brick in the wall, which hopefully will encourage further research on the complex relationships between the symbiotic species of Oxydromus and their hosts.