Skeletons in confusion: a review of astrophorid sponges with (dicho-)calthrops as structural megascleres (Porifera, Demospongiae, Astrophorida).

We present a review of astrophorid species possessing calthrops megascleres as structural megascleres (including species with dichotriaene modifications, but excluding mesotriaene and trichotriaene bearing species). Radiating oxeas characteristic of most astrophorids are lacking in such sponges, but auxiliary oxeas are apparently present in some species. These sponges are currently assigned to two families, Pachastrellidae with four nominal genera Dercitus, Stoeba, Dercitancorina, Halinastra (the latter two generally considered junior synonyms of Stoeba), and Calthropellidae with nominal genera Calthropella, Corticellopsis (usually considered a junior synonym), Pachataxa and Pachastrissa. Our review of many original specimens and extensive new material demonstrates the existence of considerable morphological similarity in megasclere shape and arrangement, and possible overlap of some microscleres of these sponges: pseudasters in Dercitus resembling euasters in Calthropella, ataxasters and other modified types in Calthropellidae resembling rhabds in a species of Dercitus. Pachastrellid representatives are proposed to be united in a single genus Dercitus, recognizable as (dicho-)calthrops bearing sponges with sanidaster-like microrhabds. Three subgenera, Dercitus s.s., Stoeba and the revived Halinastra are distinguished to accommodate species groups sharing additional characters. Dercitancorina is merged with Dercitus (Stoeba), because the type species, Dercitus lesinensis was found to be barely distinct from Dercitus (Stoeba) plicatus. Similarly, the calthropellid representatives are proposed to be united in a single genus Calthropella recognizable as calthrops bearing sponges with oxyasters. Three subgenera, Calthropella s.s., Pachataxa and Corticellopsis are distinguished to accommodate species groups sharing additional characters. The calthropellid genus Pachastrissa is considered a junior synonym of Calthropella s.s. because its type species, Pachastrella geodioides, is barely distinct from the type species of Calthropella, Calthropella simplex. Two species previously assigned to Dercitus or Stoeba (Dercitus loricatus and Stoeba natalensis) are excluded from the genus Dercitus as they do not fit with the emended and improved definition of the genus. One species assigned to Calthropella, Calthropella digitata, is excluded from that genus and assigned to the ancorinid genus Stelletta. Based on the similarity of the megascleres and their structure-less arrangement, we predict that the two reviewed genera will be found to be closely related and eventually will need to be accomodated in a single family, but independent molecular evidence is awaited before changes in the family classification, including a verdict on the validity of the family Calthropellidae, will be proposed. Our review included 38 valid species among which fourteen species and one subspecies appear to be new to science. Four of these were represented by insufficient material for a proper description, but the remaining ten are described as new species: Dercitus (Stoeba) senegalensissp. n., Dercitus (Stoeba) verdensissp. n., Dercitus (Stoeba) fijiensissp. n., Dercitus (Stoeba) bahamensissp. n., Dercitus (Halinastra) berausp. n., Dercitus (Halinastra) japonensissp. n., Dercitus (Halinastra) arubensissp. n., Dercitus (Halinastra) sibogaesp. n., Calthropella (Calthropella) xavieraesp. n., and Calthropella (Pachataxa) pyriferasp. n. The new subspecies, Dercitus (Dercitus) bucklandi lusitanicusssp. n. is described for southern East Atlantic populations of the nominal species. Several specimens assigned to existing species were found to be deviating to the extent that they are likely members of further undescribed species. These are briefly discussed to facilitate further studies of the diversity of the two genera. Species of both genera and the six subgenera, including deviating or insufficiently characterized specimens belonging to species not yet properly described, are keyed out. Distribution patterns are discussed.