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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