Contributions to Zoology, 86 (3) – 2017Daniel Martin; Miguel A. Meca; João Gil; Pilar Drake; Arne Nygren: Another brick in the wall: population dynamics of a symbiotic species of Oxydromus (Annelida, Hesionidae), described as new based on morphometry
Discussion

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Life-cycle

Little is known about the life cycle of most commensal polychaetes and, when known usually does not differ much from that of their free-living relatives. Thus, it is expected to find planktonic larvae (responsible of dispersal and colonization) and benthic adults (with a somewhat reduced mobility) with the single main difference that the symbiotic mode of life replaces the free-living one during the benthic phase (Martin and Britayev, 1998). Larval settlement may occur on the bottom, being then followed by a juvenile migration towards the respective hosts (Davenport and Boolootian, 1966). However, it seems more likely that chemically mediated cues (either generated by the host or by the own symbiotic adults) driving larval settlement could be the most widespread behaviour among symbiotic polychaetes (parasites included) (Martin and Britayev, 1998).

As it occurs for the free-living species, this basic life-cycle scheme may vary in many ways as a result of the adaptation to a symbiotic mode of life. For instance, it can be simplified by reducing (or even eliminating) the free-living pelagic stage or become more complex by having one or more intermediate hosts, which are occupied when the preferred ones are not available or because they have more room to host several juveniles during growing period (Martin and Britayev, 1998). Herewith, intraspecific competition and aggression may play a major role in the associated relocation processes. In the case of O. okupa sp. nov., the only information known on the juveniles is that they have not been found inside S. plana, neither free-living, which has been confirmed during the studied period.

The highest number of small worms found inside S. plana occurred in late autumn, which could be considered as an indication of new symbiont’s recruitment into the host bivalves (after the population being more actively reproducing during summer). In turn, large adults tended to disappear around mid-winter. This may suggest that the life span of O. okupa sp. nov. may be of one year, with the adults dying after reproducing. However, sampling during successive years would be required to confirm this hypothesis, as well as to assess the regularity of the recruitment events.

The life cycle of O. okupa sp. nov. may be limited by the tidal regime characteristic of its intertidal habitat, while the highly abundant population living in M. pellucida seems to indicate that recruitment may occur mainly in the more favourable subtidal conditions. Thus, we propose a possible scenario in which the symbionts mainly inhabit certain areas of the Bay not submitted to periodical desiccation by tides, living in association with M. pellucida (but we cannot discard other possible unknown hosts), then reaching intertidal areas such as those at Río San Pedro either as larvae through tidal currents or by adult migration. Therefore, the intertidal area here studied could be at the limit of the ecological distribution of this bivalve endosymbiont in the Cádiz Bay region.

The absence of juveniles inside the studied hosts suggests that this phase may be free-living and that the colonization of S. plana occurs during the benthic phase of the life cycle of O. okupa sp. nov., whose adults are able to move up along Río San Pedro with tides. However, neither juveniles, nor adults have been found in the sediments surrounding the studied areas (P. Drake, per. observ.), likely because they may be quite rare.

The possible life cycle of O. okupa sp. nov. may thus consist of 1) a planktonic larval phase settling on soft bottoms, 2) free-living juveniles, and 3) adults able to select (whenever possible) and enter the hosts at a given size (i.e., >1.6 mm wide according the present results). If so, phase 3 may involve thigmotaxis and the highly specific host entering behaviour described by Martin et al. (2015). In fact, a comparable life cycle was described for the polychaete Neanthes fucata (Savigny in Lamarck, 1818) in hermit crabs, by Gilpin-Brown (1969) off Plymouth. The planktotrophic larvae of this nereidid settle directly on soft bottoms. The juveniles live in tubes for several months feeding on detritus and small benthic animals exactly as many of their free-living relatives. Then, 4-month old worms start to develop the ability to recognize the presence of potential hosts by the substratum vibrations produced by the hermit crab legs bouncing on the sediment surface, which triggers a characteristic host-entering behaviour that allows the worms to crawl on the hermit crab shell following the shell spirals by thigmotaxis (Gilpin-Brown, 1969). This complex life cycle uses different mechanisms that characterize the free-living nereidids (such as thigmotaxis or mucus production) as specific adaptations to the commensal mode of life, while the worm itself has no relevant morphological adaptations. Therefore, in addition to the similarity of the hypothesized life cycle of O. okupa sp. nov., both species also share the lack of evident morphological adaptations (maybe except for the reduction of the central antennae in the hesionid, whose significance in terms of adaptation to the symbiotic mode of life remains unclear) and an equivalent, highly specific host entering behaviour.

The mechanism of host-recognition behaviour in O. okupa sp. nov. is currently unknown, and the presence of a host-factor has not been demonstrated (Martin et al., 2012, 2015). However, it is well known that the bivalves hosting O. okupa sp. nov. may alternate between direct water filtration and deposit feeding by tapping on the sediment surface with their inhalant siphons. Thus, we may hypothesize that the symbiont, like N. fucata, may recognize the presence of a potential host by the movements of the inhalant siphon. Despite aforementioned similarities between the known hosts of O. okupa sp. nov., there is no direct evidence of the symbiont entering into M. pellucida (or any other potential host) in a similar way as into S. plana.