Amongst the most significant metazoan taxa associated with gastropod molluscs is the endoparasitic copepod family Splanchnotrophidae. Currently it contains five genera with highly modified morphology and exclusively infesting nudibranch and sacoglossan sea slug hosts. The present study is a first approach towards reconstructing their phylogeny and evolution. Cladistic analysis of 109 morphological characters including 24 known splanchnotrophid species resulted in a fully resolved strict consensus tree that is discussed in morphological, functional, and geographical frameworks. Alternative topologies are also explored. Originating from paraphyletic Philoblennidae, the Splanchnotrophidae emerge as sister group to the genus Briarella. Unique synapomorphies, such as the bizarre body shapes and successive reduction of mouthparts, are discussed as adaptive traits to endoparasitism that evolved only once within copepods infesting shell‐less heterobranch gastropods. The ancestrally Indo‐Pacific Splanchnotrophidae split up into a clade of the still Indo‐Pacific genera Ceratosomicola and Arthurius, sister to a clade composed of the monophyletic amphi‐American genus Ismaila and European Splanchnotrophus emerging from paraphyletic Lomanoticola. Although initial radiation of Briarella and Splanchnotrophidae is likely to have involved chromodoridid nudibranch hosts, later phylogenies of parasites and their hosts are incongruent; intriguingly, host shifts from nudibranch to only distantly related sacoglossan species occurred at least two times independently. Such remarkable ecological plasticity is assumed to have driven splanchnotrophid diversification. Topological hypotheses and historical biogeographical and evolutionary scenarios inferred herein can be tested by future molecular research. © 2013 The Linnean Society of London

The Splanchnotrophidae is a family of highly modified endoparasitic copepods known to infest nudibranch or sacoglossan sea slug hosts. Most splanchnotrophid species appear to be specific to a single host, but some were reported from up to nine different host species. However, splanchnotrophid taxonomy thus far is based on external morphology, and taxonomic descriptions are, mostly, old and lack detail. They are usually based on few specimens, with intraspecific variability rarely reported. The present study used molecular data for the first time to test (1) the current taxonomic hypotheses, (2) the apparently strict host specificity of the genus Ismaila and (3) the low host specificity of the genus Splanchnotrophus with regard to the potential presence of cryptic species. Phylogenetic analyses herein used sequences of the barcoding region of the cytochrome oxidase I (COI) gene from 40 specimens representing 13 species of five genera. Species delimitation approaches include distance and barcoding gap analyses, haplotype networks and diagnostic nucleotides. Molecular results are largely compatible with the commonly accepted, morphology-based taxonomy of the Splanchnotrophidae. Strict host specificity could be confirmed for two Ismaila species. COI analyses also supported the idea that Splanchnotrophus angulatus is host-promiscuous. In Ismaila, morphology seems more suitable than barcoding to display speciation events via host switches in a recent Chilean radiation. In Splanchnotrophus, some genetic structure suggests ongoing diversification, which should be investigated further given the inadequate morphology-based taxonomy. The present study thus supports the presence of two different life history strategies in splanchnotrophids, which should be explored integratively.

The Splanchnotrophidae are a worldwidedistributed family of endoparasitic copepods, utilising shellless opisthobranch gastropod hosts. Using scanning electron microscopy, we describe three new Ismaila Bergh, 1867 species infesting nudibranch hosts from southern Chile. Ismaila volatilis spec. nov. infests the proctonotid Janolus sp. and differs from all congeners by the size and number of dorsal bulges, the number of processes on the maxilla, and the thickness of thoracic appendages. Ismaila chaihuiensis spec. nov. was found in the doridoidean Diaulula punctuolata (D'Orbigny, 1837) and is diagnosed by a pore situated on a prominent bulge above the labrum. Ismaila genalis spec. nov. from a polycerid Holoplocamus papposus Odhner, 1926 host differs from its congeners in the size and form of the ventral bulges present on the head. These discoveries further broaden the range of splanchnotrophid host taxa; they are in line with earlier hypotheses of strict host specificity of Ismaila species and support Chile as a hotspot for Ismaila radiation. Herein, we present a key to the identification of all 14 Ismaila species.In Sulawesi (tropical Indo-West Pacific), Arthurius gibbosa spec. nov. infests the sacoglossan Elysia macnaei Marcus, 1982. The new species differs from both congeners by the short and stubby dorsal bulges. Uniquely among splanchnotrophids, long thoracic appendages are absent. Arthurius Huys, 2001 thus is the morphologically most divergent genus of the Splanchnotrophidae, but it is well characterized by the loss of several mouthparts and the preference for sacoglossan hosts. Currently, the distribution of the genus is limited to the Indo-West Pacific.