Spathione asprosdovrima, Boyko & Van Der Meij, 2018

Spathione asprosdovrima sp. nov.

Remarks

The finding of a bopyrid parasite in the marsupium of a gall crab was unexpected, as only species of Rhopalione Pérez, 1920 were known to be adapted to this environment, and gall crabs and pinnotherids, although both thoracotreme brachyurans, are not closely related ( Wetzer, Martin & Boyce, 2009; van der Meij & Schubart, 2014; Tsang et al., 2014). However, Spathione gen. nov. is indeed very closely related to Rhopalione , which contains four species (and one nomen nudum), all of which are parasitic in the marsupia of pinnotherid crabs. This is not, therefore, a case of convergent evolution of the bopyrids towards marsupial pouch parasitism, but rather a diversification of a prior preference towards marsupium parasitism from the original host to other crabs with similar structures. The only such crabs where females bear enlarged marsupial abdomens are species in the Pinnotheroidea ( Aphanodactylidae and Pinnotheridae ) and Cryptochiroidea ( Cryptochiridae ). Which taxon was the original host for the common ancestor of Rhopalione and Spathione gen. nov. cannot be determined or even suggested, because there is no lengthy fossil record for gall crabs (but see below), and the oldest pinnotherid (a species of Viapinnixa Schweitzer & Feldmann, 2001 ) is only Danian (Paleocene, 61.6–66 Mya; Brösing, 2008) and appears to belong to Pinnothereliinae, species whose females do not have the requisite marsupial pouch needed for Rhopalione to be able to occupy it. Schweitzer et al. (2010) did later move Viapinnixa to Pinnotherinae but without giving any rationale, and it is not clear that this is the proper placement (S. T. Ahyong, personal communication). Recently, De Angeli & Ceccon (2015) described a putative fossil gall crab, Montemagrechirus tethysianus , from Italian Eocene deposits, but this is not a cryptochirid (see Klompmaker & Boxshall, 2015; Klompmaker, Portell & van der Meij, 2016). Currently, gall crabs in the fossil record date back only to the Pleistocene and Pliocene and are known only from their crescentic pits as an ichnotaxon ( Klompmaker, Portell & van der Meij, 2016).

Superficially, Rhopalione and Spathione gen. nov. species appear very similar, but the two genera can be distinguished easily by numerous characters, as follows: female head subquadrate in Spathione gen. nov. vs. head rounded in Rhopalione ; female and male antennae of four articles each, antennules of two articles in Spathione gen. nov. vs. five articles and three articles, respectively, in Rhopalione ; first oostegite with rounded posterior margin in Spathione gen. nov. vs. with sharp point in Rhopalione ; coxal plates small in Spathione gen. nov. vs. well developed in Rhopalione ; female first perepods approximately half as large as seventh pereopods in Spathione gen. nov. vs. pereopods isomorphic in Rhopalione ; tergal projections of pereomeres IV–VII similar in shape to lateral plates of pleomeres vs. tergal projections of pereomeres IV– VII not extended and not resembling lateral plates of pleomeres in Rhopalione ; female pleopods biramous as two separate tubular stalk-like rami extending from ventral surface of lateral plate in Spathione gen. nov. vs. pleopods as biramous as pair of flattened oval structures arising from common junction at base of ventral lateral plate in Rhopalione ; males with four pleomeres, lacking pleopods, plus pleotelson in Spathione gen. nov. vs. males with five pleomeres, rounded pleopods on each segment, plus pleotelson in Rhopalione ; and female uropods as short, rounded lateral lobes of pleotelson in Spathione gen. nov. vs. posteriorly extended and foliaceous in Rhopalione .

Rhopalione View in CoL has been placed in Ioninae sensu lato (= Keponinae Boyko, Moss, Williams & Shields, 2013 View in CoL ; e.g. Markham, 1992; Boyko et al., 2013; An et al., 2014), but several characters, particularly the biramous pleopods of the female and lack of midventral tubercles and presence of a non-bifurcated pleotelson of the male, support its transfer herein to Pseudioninae View in CoL . Spathione View in CoL gen. nov. is likewise placed in Pseudioninae View in CoL and shows the additional character of the male having fewer than five pleomeres; this character is variable in Pseudioninae View in CoL , whereas males of species in Keponinae View in CoL have five pleomeres.

Etymology

The generic name is a derived from a combination of the Greek σπάθη (spáthē), meaning saber, because of the extended, non-lamellar, lateral plates on the pleon, and Ione , a genus of bopyrid itself and also a common suffix for other bopyrid genera.