Phylogeny of the Apodan Holothurians (Echinodermata) inferred from morphology

The Apodida is an order of littoral to deep-sea, largely infaunal sea cucumbers with about 270 extant species in 32 genera and three families, Synaptidae, Chiridotidae and Myriotrochidae. In this study, I perform the first phylogenetic test of the taxonomic and palaeontological hypotheses about evolutionary relationships within Apodida by using cladistic analyses of 34 morphological characters. I introduce several previously unconsidered synapomorphic characters, examine the relationships between all recognized suprageneric taxonomic groups and assess the assumptions of monophyly for each family. Maximum-parsimony analyses of type species from 14 genera and use of three rooting methods recovered identical topologies at the subordinal level and subfamily level within Synaptidae. Overall, the current higher-level classification of Apodida was well corroborated. Within Synaptidae, the relationships (Synaptinae, (Leptosynaptinae, Rynkatorpinae)) are well supported. The monophyly of Chiridotidae was not supported and appears paraphyletic at the subfamily level. Calibrating the phylogenetic hypothesis of Apodida against the fossil record indicated that most higher-level divergences occurred within the Palaeozoic, unlike that of extant non-holothuroid echinoderms, which radiated in the early Mesozoic. Synaptidae appears to have radiated during the Lower Cretaceous. Alternatively, and if one discounts the considerable ghost lineage duration that this hypothesis requires, they may have radiated during the Eocene.     

Development and morphology of ciliary urns in the sea cucumber Synaptula hydriformis (Echinodermata: Holothuroidea)

Sea cucumbers (holothuroids) lack the only known echinoderm immune organ, the axial organ. Holothuroids of the families Synaptidae and Chiridotidae have coelomic organs, known as ciliary urns, that gather and excrete waste and, therefore, might function in immunity. Although ciliary urns are widely reported and illustrated in the literature, the process and histology of urn development remain unknown. Development and structure of ciliary urns were examined in Synaptula hydriformis using scanning electron, brightfield, and scanning laser confocal microscopy. Mature urns occurred on all three mesenteries in 10‐tentacled young and later growth stages, and developing urns were found in post‐pentactulae, 10‐tentacled young, and released juveniles. Developing urns were circular clusters of ciliated collar cells protruding from the mesentery. The cells increased in number to form the sessile cushion stage with a shallow lumen. The subsequent spoon‐shaped stage had a stalk and a deepened lumen with an extensive ciliary field where coelomocytes began to accumulate. Mature urns had a thin stalk and cornucopia‐shaped body with an abluminal epithelium of squamous cells and an adluminal epithelium of densely packed ciliated collar cells. Cell boundaries of the rim of mature urns and of the stalk and body of developing urns were outlined on one or both sides by microvilli and an elevated cell membrane. Ciliary urns resembling the cushion‐stage urns of S. hydriformis have been described in the sea star Archaster typicus. If urns in these groups are homologous, it is likely that cushion urns are plesiomorphic and that they are present and have been overlooked in other echinoderms.