On the enigmatic symbiotic polychaete ‘Parasyllidea’humesi Pettibone, 1961 (Hesionidae): taxonomy, phylogeny and behaviour

Abstract

The hesionid genus Parasyllidea differs from Oxydromus in lacking median antennae. It was originally described to include a single species, P. humesi, known only from its original description. This was based on specimens from mangrove swamps at Pointe-Noire (Republic of Congo, West Africa), living endosymbiotically with the bivalve Tellina nymphalis. Lately, the genus included P. blacki and P. australiensis. A new population of P. humesi was recently found at the upper intertidal level of Rio San Pedro salt marsh in Cádiz Bay (eastern Atlantic, Iberian Peninsula). It was also living endosymbiotically, but with another bivalve, Scrobicularia plana. Some Iberian and Congolese specimens revealed the presence of a small papilla-like central antenna associated with the prostomial median ridge, which raised some doubts on the validity of the genus Parasyllidea. A phylogenetic analysis based on the mitochondrial COI and 16S and the nuclear 18S and 28S genes confirms Parasyllidea as a junior synonym of Oxydromus. Therefore, in this paper, P. humesi is fully re-described as Oxydromus humesi comb. nov. The worm has never been reported as free-living. Previously, the association appeared to be an obligate symbiosis, closer to parasitism, as infested hosts had lower relative biomasses than non-infested ones and the worm did not occur locally inside any other bivalve co-habiting the intertidal salt marsh. The finding of a highly infested population (> 85% in the specimens longer than 20 mm) of a new host at the lower subtidal part of Rio San Pedro mouth, the bivalve Psammotreta cumana, led us to discuss the host-specificity of O. humesi. In addition, the observation of living specimens during sampling and laboratory handling enabled detailed observations of the host-entering behaviour of the specimens living with S. plana, which are also described and illustrated. Living, uninfested specimens of P. cumana have not been obtained, preventing us from checking the host-entering behaviour in the new host. The significance of the intraspecific attacks observed in experimental conditions is also discussed.

Does polyxenous symbiosis promote sympatric divergence? A morphometric and phylogeographic approach based on Oxydromus okupa (Annelida, Polychaeta, Hesionidae)

“…The width of the tenth segment (parapodia included) was used as a proxy for worm size (WW, μm). Due to cirrostyles alternating short and long through the segments (Martin et al, 2015(Martin et al, , 2017, all parapodial characters were measured for two parapodia (from chaetiger 10 to 30, depending on the specimen) bearing long and short cirrostyles and indicated by adding L and S to the end of the acronym (e.g., "PNCLL" and "PNCLS", meaning posterior neurochaetal lobe from parapodia having long and short cirrostyles, respectively). WW, WL, NS and WWP were measured under a Nikon SMZ645 stereomicroscope equipped with a micrometric ocular, while the rest of the characters using a Motic BA210 binocular microscope equipped with a TOUPCAMTM U3CMOS digital camera, managed by the ToupView 3.7 software.…”

“…The reliability of these classifications was assessed by cross-validating the original matrices. The variables included in the discriminant functions were selected according to the statistics F index and Wilk's Lambda Martin et al (2015Martin et al ( , 2017. by a threshold significance level of 0.05.…”

“…The aims are to assess whether host and environmental preferences affect morphology, gene flow and demography, ultimately attempting to clarify diversification mechanisms in marine invertebrate symbionts. The symbiotic hesionid polychaete Oxydromus humesi (Pettibone, 1961) was described as belonging to Parasyllidea by Pettibone (1961), based on a single population living in association with the tellinid bivalve Austromacoma nymphalis (Lamarck, 1818) in mangrove swamps in the Republic of Congo (Martin et al, 2015). Two populations were later reported as symbionts of bivalves, i.e., the semelid Scrobicularia plana (da Costa, 1778) in the intertidal of Río San Pedro and the tellinid Macomopsis pellucida (Spengler, 1798) in the subtidal of the Bay of Cádiz, both at the southern Atlantic coast of the Iberian Peninsula (Martin et al, 2012(Martin et al, , 2015.…”

“…The symbiotic hesionid polychaete Oxydromus humesi (Pettibone, 1961) was described as belonging to Parasyllidea by Pettibone (1961), based on a single population living in association with the tellinid bivalve Austromacoma nymphalis (Lamarck, 1818) in mangrove swamps in the Republic of Congo (Martin et al, 2015). Two populations were later reported as symbionts of bivalves, i.e., the semelid Scrobicularia plana (da Costa, 1778) in the intertidal of Río San Pedro and the tellinid Macomopsis pellucida (Spengler, 1798) in the subtidal of the Bay of Cádiz, both at the southern Atlantic coast of the Iberian Peninsula (Martin et al, 2012(Martin et al, , 2015. The Iberian populations were finally described as belonging to a new species based on morphometric analyses, i.e., Oxydromus okupa Martin, Meca & Gil in Martin et al (2017).…”

“…The population associated with S. plana was studied in detail in terms of behaviour, infestation characteristics, and life cycle (Martin et al, 2015(Martin et al, , 2017. It showed a regular distribution (i.e., a single symbiont individual per host), intra-specific aggression, a complex host-entering behaviour, and low (usually <5%) and seasonally fluctuating infestations that were closely related with the reproductive cycle (e.g., males leaving their hosts during spring/summer, likely for fertilization purposes).…”