A new symbiotic relationships between a carnivorous sponge and its annelid host

Sergi Taboada, Autonomous University of Madrid

Figure 1. MicroCT image of the worm Neopolynoe chondrocladiae n. comb. laying on a gallery of the sponge Chondrocladia virgata.

The grant awarded by the SRF was used to conduct a combined morphological and molecular study on the symbiotic relationships between carnivorous sponges of the genus Chondrocladia and annelid hosts of the family Polynoidae. As outlined in the proposal, the money awarded was mainly used to cover the molecular expenses and the costs for imaging (including SEM and microCT; see Figure 1). For the molecular analyses, we used almost 100 individuals of different species. These specimens were DNA-extracted and subsequently we amplified and sequenced four different genetic markers (18S, 28S, 16S, and COI)for a selection of the specimens to build a phylogenetic hypothesis and two genetic markers (16S, and COI)for all of them to build haplotype networks. Interestingly, the phylogenetic approach, with a special focus on the subfamily Polynoinae, recovered the genus Neopolynoe as polyphyletic. These results, combined with our morphological observations using light microscopy, SEM and micro-CT techniques, resulted in splitting the species Neopolynoe acanellae into two different species: on one hand the species Neopolynoe acannellae s.e., a species commonly found in association with the gorgonian Acanella arbuscula; on the other hand the species Neopolynoe chondrocladiae n. comb., always found in association with the carnivorous sponges Chondrocladia robertballardi and Chondrocladia virgata. The haplotype network analyses for the two species recovered similar patterns for both genetic markers for each of the species Neopolynoe acanellae showed relatively high nucleotide and haplotype diversity (i.e. genetic diversity) and a diffuse haplotype network. As the populations sampled were less than one Km away, nothing can be inferred about the dispersal capabilities of the species. However, the relatively high values of genetic diversity indicate very genetically diverse populations for this species, consistent with other shallow-water polychaete species studied at small scale. In contrast, N. chondrocladiae n. comb. showed significantly less genetic diversity than N. acanellae and star-like haplotype networks. Interestingly, the predominant ancestral haplotype for 16S in N. chondrocladiae n. comb. was shared between specimens collected in populations more than 900 km apart, which indicates long distance connectivity for this species. In addition to our molecular results, we were able to identify specialized chaetae in N. chondrocladiae n. comb. and the occurrence of open galleries in the host sponge, derived from a gradual overgrowth of the sponge to accommodate the worm, which suggests an obligate symbiotic relationship between worm and sponge.

The results derived from this study were presented by Ana Serra Silva as her Master Thesis (University College London). Also, these results have already been presented as oral presentations in two international conferences in 2018: the XXXVII Annual Meeting of the Hennig Society (Barcelona, Spain), and IV World Conference of Marine Biodiversity (Montreal, Canada). Importantly, a scientific publication with the systematic results of our work has recently been accepted for publication in the journal Deep-Sea Resarch Part I. Also, the results of our study have appeared in the media (https://www.nhm.ac.uk/discover/the-worm-that-lives-with-its-enemy.html) and were used in a Nature Live show entitled ‘Sleeping with my enemy’ in May 2018.