In situ environmental conditions and molecular identification of the photosymbiotic marine worm Symsagittifera roscoffensis

Known as the Roscoff worm or mint-sauce worm, Symsagittifera roscoffensis is an Acoel distinguishable due to the presence of symbiotic alga Tetraselmis convolutae, held beneath the epidermis. Isolated populations of S. roscoffensis span a broad geographical range along the north-eastern Atlantic coast, from Wales to Portugal. The only known population of the worm in the United Kingdom was discovered in Wales decades ago, but very little research has been conducted since. For 13 months, we measured how environmental conditions such as temperature, salinity and light intensity coincided with population size at the Welsh field site. To establish phylogenetic relationships among the different populations and their algal symbionts, we designed new polymerase chain reaction (PCR) oligonucleotides to assess the nucleotide diversity of the mitochondrial cytochrome c oxidase I subunit (COI) gene in gDNA extracted from representative worms across their known range (Wales, France, Portugal, Spain, and Guernsey). We also targeted the 18S rRNA gene of their algal symbiont, Tetraselmis convolutae. We observed temporal shifts in environmental factors coinciding with fluctuating worm colony size, notably temperature. Based on the molecular data, the worm exhibited different ecotypes across locations, while the algal symbiont showed little genetic variation.