Contributions to Zoology, 86 (3) 181-211 (2017) Another brick in the wall: population dynamics of a symbiotic species of Oxydromus (Annelida, Hesionidae), described as new based on morphometry Daniel Martin1,*, Miguel A. Meca1, João Gil1, Pilar Drake2 & Arne Nygren3 1 Centre d’Estudis Avançats de Blanes (CEAB-CSIC) – Carrer d’Accés a la Cala Sant Francesc 14. 17300 Blanes, Girona, Catalunya, Spain 2 Instituto de Ciencias Marinas de Andalucía (ICMAN-CSIC), Avenida República Saharaui 2, Puerto Real 11519, Cádiz, Spain 3 Sjöfartsmuseet Akvariet, Karl Johansgatan 1-3, 41459, Göteborg, Sweden 1 E-mail: [email protected] Key words: Bivalvia, Cádiz Bay, Hesionidae, Iberian Peninsula, NE Atlantic Oxydromus, symbiosis, Tellinidae urn:lsid:zoobank.org:pub: D97B28C0-4BE9-4C1E-93F8-BD78F994A8D1 Abstract Results ............................................................................................. 186 Oxydromus humesi is an annelid polychaete living as a strict bi- Morphometry ........................................................................... 186 valve endosymbiont (likely parasitic) of Tellina nymphalis in Population size-structure ..................................................... 190 Congolese mangrove swamps and of Scrobicularia plana and Infestation characteristics .................................................... 190 Macomopsis pellucida in Iberian saltmarshes. The Congolese Discussion ....................................................................................... 193 and Iberian polychaete populations were previously considered Morphometry and taxonomy ............................................... 193 as belonging to the same species, the latter showing regular dis- Infestation characteristics .................................................... 194 tribution, intra-specific aggressive behaviour, and complex host- Life cycle ................................................................................... 197 entering behaviour. The fresh Iberian samples enabled us to un- Relationships between life–cycle and infestation dertake consistent morphometric analyses, as well as to further characteristics ......................................................................... 198 analyse the characteristics of the association and the population Current knowledge on hesionid symbionts ....................... 199 dynamics of the Iberian population hosted by S. plana. Among Acknowledgements ....................................................................... 202 the morphological differences between the Congolese and Iberi- References ...................................................................................... 202 an specimens, leading to the description of the latter as Oxydro- Appendix ........................................................................................ 208 mus okupa sp. nov., the most important are: 1) longer cephalic appendages, 2) greater distance between the eyes, 3) larger dorsal cirrostyle in relation to the corresponding dorsal lobe and cirro- phore. Moreover, dorsal and ventral lobes are similar in length, Introduction with the tip of the former reaching the tip of the latter in O. okupa sp. nov., while the dorsal lobe is much shorter than the ventral The Hesionidae is a well-known family of polychaetes one in O. humesi. Mature adults of O. okupa sp. nov. occurred whose representatives are most often free-living in during the whole study period, with a higher percentage of ripe shallow waters, although some occur in the deep-sea females in spring and, particularly, in summer. Numerous host (Summers et al., 2015) and some lives as symbionts of specimens showed the symbiont’s most preferred shell length (>26 - 36 mm). However, the prevalence was very low (usually other invertebrates (Martin and Britayev, 1998; Miller <5%) and showed a clear seasonal pattern, being lower during and Wolf, 2008; De Assis et al., 2012; Martin et al., spring/summer. This suggests that males are able to leave their 2012, 2015; Britayev et al., 2013; Chim et al., 2013). hosts during this period, most likely to improve fertilization by Oxydromus humesi (Pettibone, 1961), which is one directly entering or approaching a host occupied by a ripe fe- of these symbiotic hesionids, has been reported as a male, while females usually remain inside. Based on the new re- Tellina nym- sults, the current knowledge of symbiotic Hesionidae and their strict bivalve-associate that is hosted by relationships with invertebrate hosts is updated and discussed. phalis Lamarck, 1818 in Loango (Congo mangrove swamps) and by Scrobicularia plana (Da Costa, 1778) and Macomopsis pellucida (Spengler, 1798) (reported Contents as Macoma cumana (O.G. Costa, 1830)) in Cádiz Bay, Iberian Peninsula (Pettibone, 1961; Martin et al., 2012, Introduction .................................................................................... 181 2015). The Iberian hosts show similar morphologies Material and methods .................................................................. 182 and modes of life. However, in Cádiz Bay, M. pellucida Downloaded from Brill.com10/10/2021 11:51:28AM via free access 182 Martin et al. – Life cycle of a new species of symbiotic Oxydromus (Annelida) LA A PS PP HL DAE DAPE DPE HW Fig. 1. Oxydromus okupa sp. nov. Schemes of anterior end (a), two mid- body segments (b) and one mid-body pa- B rapodia (c), showing the measurements WW S used in the morphometric analyses. WW: worm width with parapodia (µm); WWP: WWP worm width without parapodia (µm); L HW: head width (µm); HL: head length (µm); LA: length of lateral antenna (µm); PP: length of palpophore (µm); PS: length of palpostyle (µm); DAE: distance DCS between anterior eyes (µm); DPE: dis- tance between posterior eyes (µm); DCP DAPE: distance between anterior and DL posterior eyes (µm); DL: length of dorsal C lobe (µm): DCP: length of dorsal cirro- phore (µm); DCS: length of dorsal cirro- VL PNCL style (µm); PNCL: length of posterior neurochaetal lobe (µm); VL: ventral lobe; VC: length of ventral cirri (µm); L: long VC cirrostyle; S: short cirrostyle. occurs mainly in the subtidal (E. Pascual, pers. ob- peared to be indistinguishable and a preliminary com- serv.), whilst it is very rare in the intertidal (Subida et parison of chaetal morphology and appendage meas- al., 2013). In turn, S. plana occurs in intertidal salt- urements based on the available specimens was there- marshes (like Río San Pedro), where it shows very high fore inconclusive (Martin et al., 2015), which lead us to abundances (Subida et al., 2011; Drake et al., 2014). undertake a more robust study based on morphometry. Based on occasional sampling of mainly S. plana Therefore, the following questions were addressed: associates, it has been inferred that the association ap- Were hosts < 20 mm infested during the seasonal cy- parently affects negatively the metabolism of the host cle? Did the symbionts prefer any host size among (and so it is considered to be closer to a parasitism), that those > 20 mm? Were the prevalence and intensity of infestations did not occur in hosts measuring less than infestation constant along time? Had the symbiont a 20 mm in shell length, that all symbionts seem to be seasonal trend in reproductive traits or size-class struc- adults (which are not sexually dimorphic), and that ture? These observations were compared with the few there are no relationships between worm’s size and available data on symbionts infesting M. pellucida. shell length (Martin et al., 2012, 2015). However, noth- Based on the new results, the current knowledge on the ing is known of the seasonal characteristics of the as- symbiotic relationships involving hesionid polychaetes sociation, as well as of the life cycle of the symbiont. worldwide is summarized and discussed. 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 biogeo- Material and Methods graphic isolation of the respective populations. The fact that no fresh specimens were available from the Con- Collection details and sampling were explained by golese population (i.e. the original specimens were Martin et al. (2015). Specimens of the host bivalves formalin-fixed) prevented a molecular approach. Mor- were collected: monthly, from April 2011 to May 2012 phologically, animals of the two known populations ap- and in January 2013, at Río San Pedro, 36°31’56.28”, Downloaded from Brill.com10/10/2021 11:51:28AM via free access Contributions to Zoology, 86 (3) – 2017 183 6°12’53.28” (S. plana); in January 2013 at Río San Pedro population, 25 specimens were selected on the basis of mouth, 36°31’47.28”, 6°14’52.80” (M. pellucida). The the quality of the available specimens (as all of them specimens of S. plana from May 2011 were damaged were similar in size). The following characters were se- during sampling. All applicable international, national, lected and measured (Fig. 1): WW, worm width without and/or institutional guidelines for the care and use of parapodia (WWP, µm), worm length (WL, µm), num- animals were followed. Possible non-biotic influences ber of segments (NS), head width (HW, µm), head in the observed patterns could not be determined be- length (HL, µm), length of lateral antenna (LA, µm), cause no environmental data were recorded during the length of palpophore (PP, µm), length of palpostyle (PS, study period. µm), distance between anterior eyes (DAE, µm), dis- In the laboratory, the collected specimens