The Hesionidae is a well-known family of polychaetes whose representatives are most often free-living in shallow waters, although some occur in the deep-sea (Summers et al., 2015) and some lives as symbionts 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).
Oxydromus humesi (Pettibone, 1961), which is one of these symbiotic hesionids, has been reported as a strict bivalve-associate that is hosted by Tellina nymphalis Lamarck, 1818 in Loango (Congo mangrove swamps) and by Scrobicularia plana (Da Costa, 1778) and Macomopsis pellucida (Spengler, 1798) (reported as Macoma cumana (O.G. Costa, 1830)) in Cádiz Bay, Iberian Peninsula (Pettibone, 1961; Martin et al., 2012, 2015). The Iberian hosts show similar morphologies and modes of life. However, in Cádiz Bay, M. pellucida occurs mainly in the subtidal (E. Pascual, pers. observ.), whilst it is very rare in the intertidal (Subida et al., 2013). In turn, S. plana occurs in intertidal saltmarshes (like Río San Pedro), where it shows very high abundances (Subida et al., 2011; Drake et al., 2014).
Based on occasional sampling of mainly S. plana associates, it has been inferred that the association apparently affects negatively the metabolism of the host (and so it is considered to be closer to a parasitism), that infestations did not occur in hosts measuring less than 20 mm in shell length, that all symbionts seem to be adults (which are not sexually dimorphic), and that there are no relationships between worm’s size and shell length (Martin et al., 2012, 2015). However, nothing is known of the seasonal characteristics of the association, as well as of the life cycle of the symbiont. Also, when re-describing O. humesi, Martin et al. (2015) discussed the possibility of having different species at the two known locations due to an hypothetical biogeographic isolation of the respective populations. The fact that no fresh specimens were available from the Congolese population (i.e. the original specimens were formalin-fixed) prevented a molecular approach. Morphologically, animals of the two known populations appeared to be indistinguishable and a preliminary comparison of chaetal morphology and appendage measurements based on the available specimens was therefore inconclusive (Martin et al., 2015), which lead us to undertake a more robust study based on morphometry.
Therefore, the following questions were addressed: Were hosts < 20 mm infested during the seasonal cycle? Did the symbionts prefer any host size among those > 20 mm? Were the prevalence and intensity of infestation constant along time? Had the symbiont a seasonal trend in reproductive traits or size-class structure? These observations were compared with the few available data on symbionts infesting M. pellucida. Based on the new results, the current knowledge on the symbiotic relationships involving hesionid polychaetes worldwide is summarized and discussed.