Study of the population of the symbiotic polychaete Oxydromus humesi hosted by the bivalve Scrobicularia plana in Cádiz Bay

Meca Jimenez, Miguel Angel
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Oxydromus humesi is a strict bivalve endobiont, probably a parasite, living in association with Tellina nymphalis in north of Pointe-Noire (Congo mangrove swamps, West Africa) and Scrobicularia plana (intertidal) and Psammotreta cumana (subtidal) in Cádiz Bay salt marshes (southern Atlantic coast of Iberian Peninsula). The symbiont shows a regular distribution (one, exceptionally two or three symbionts per host, being one male, one female and one small worm) with intraspecific competition and complex host-entering behaviour observed in experimental conditions. This work aims to study the population dynamics of the above-mentioned species hosted by S. plana in Cádiz Bay to increase knowledge on the biology of O. humesi and to contribute to the growing knowledge on symbiotic polychaetes. The population of the symbiont in the intertidal area of Rio San Pedro (Cádiz Bay) seems to reproduce actively through all the year, with a higher number of ripe females in spring and, particularly, in summer. Although S. plana is very abundant in the studied area, with a large amount of specimens having the size most preferred by the symbiont (i.e., from >26 mm to about 36 mm long), the prevalence tends to be very low (usually <5% and reaching up to ~20%). This fact, together with the presence of a highly infested population (>85% in the specimens longer than 20 mm) of P. cumana in Río San Pedro mouth, lead us to discuss on the environmental requirements and host-specificity of O. humesi. We suggest that the studied intertidal could be at the limit of the ecological distribution of this symbiont in the Cádiz Bay region. The symbiotic hesionid seems to show a seasonal pattern of prevalence, which may be related with its reproduction (the periods with more ripe females tended to coincide with those of lower prevalence). Taking into account these data, we propose that males undertake reproductive migrations probably to improve the possibilities of fertilization, whilst the females stay throughout the whole life inside the host. The life-cycle of O. humesi remains unknown since the species’ discovery in the coasts of the Republic of Congo. However, our data allow us to suggest that its life span may be of one year, with the adults dying after reproducing ​
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