Pycnogonid affinities: a review

Early authors regarded Pycnogonida (sea spiders) either as aquatic arachnids, 'degraded' crustaceans or as some sort of intermediate form between the two. Subsequently, pycnogonids were either placed among the Chelicerata or considered as an isolated group, unrelated to other arthropods. The latter model is untenable under phylogenetic systematics and recent cladistic studies have supported one of two alternative hypotheses. The first is the traditional Chelicerata s.lat. concept, i.e. (Pycnogonida + Euchelicerata). This, however, has only one really convincing synapomorphy: chelate chelicerae. The second hypothesis recognizes (Pycnogonida + all other Euarthropoda) and has been recovered in various 'total evidence' studies. Morphologically some characters - the presence of gonopores on the trunk and absence of a labrum, nephridia and intersegmental tendons - support Cormogonida (Euarthropoda excluding pycnogonids). Advances in developmental biology have proposed clear interpretations of segmentation homologies. However, so far there is also a confrontation of the two hypotheses depending on whether the last walking leg segment is considered part of the prosoma. In this case pycnogonids have too many prosomal segments compared with Euchelicerata; perhaps implying they are not sister groups. Alternatively, if part of the postprosomal region, the last leg pair could correspond to the chilarial segment in euchelicerates and its uniramous state could be apomorphic with respect to other euarthropods. Molecular phylogenies need to be more rigorously analysed, better supported by data from different sources and technique-sensitive aspects need to be explored. Chelicerata s.lat. may emerge as the more convincing model, yet even the putative autapomorphy of chelicerae needs to be treated with caution as there are fossil 'great appendage' arthropods in the early Palaeozoic which also have a robust, food-gathering, pair of head limbs and which may lie on the chelicerate, or even the euarthropod, stem lineage.