Phylogeography of the Atlanto-Mediterranean brittle star <i>Ophioderma longicaudum</i> (Ophiuroidea, Echinodermata) with new gonad histology and genetic data from Algeria

Abstract In the Ophioderma longicaudum species complex, previous studies identified five biological species: three broadcast spawners, including O. longicaudum s.s., and two brooders. This study explores the phylogeography of the complex, incorporating novel samples from its Western range: Algeria and the French Atlantic coast. We analysed 66 new COI and 65 new EF1 sequences alongside 869 published COI and 116 EF1 sequences. Based on these markers, all new specimens belonged to O. longicaudum s.s. This species displayed limited genetic differentiation among localities and a nearly significant isolation-by-distance pattern, although the most abundant haplotypes spanned from the Eastern Mediterranean to the Atlantic. This weak genetic structure, unusual for a species with lecithotrophic larvae, likely reflects large effective population sizes. A survey of gonad developmental stages showed that most Algerian individuals spawned between July and August but some showed reproductive activity in March—a period unreported for broadcast spawners—suggesting possible brooding in Algeria. However, additional unlinked genetic markers are needed to include the Algerian and Atlantic genotypes in a global multilocus biogeographical dataset in order to delimit species. As new populations of this abundant species are discovered, this study, with its updated genetic database, will be invaluable for integrative taxonomy.