Pinnotheridae De Haan, 1833

Family Pinnotheridae De Haan, 1833 View in CoL View at ENA

Pinnotheridea De Haan, 1833: 34.

Diagnosis. Carapace circular (mainly Pinnotherinae ) to ellipsoidal (mainly Pinnixinae and Pinnixulalinae), dorsally vaulted to flat, regions generally ill-defined, carapace cuticle usually poorly calcified in female pinnotherines. Buccal cavern semicircular, large. Mxp3 obliquely placed in buccal cavern; ischium and merus indistiguishably fused (mainly pinnotherines) or with or without superficial suture (mainly pinnixines and pinnixualines), ischiomerus produced mesiosubdistally, somewhat hatchet-shaped; palp 2- or 3-segmented, small, 3-segmented species with slender to narrowly spatulated dactylus inserted subdistally on lateral margin (in situ) of propodus; ambulatory legs mostly symmetrical but bilaterally asymmetrical in some pinnotherines. Male pleonal somites and telson free, telson not usually markedly wider than pleonal somite 6. G1 simple, slender to rather stout, straight to broadly curved. G2 small. Female pleonal somites and telson free, covering most of thoracic sternum in large female.

Remarks. This study formally transfers Tetrias and Parapinnixa + Sakina from Pinnotheridae to their own families, Tetriasidae fam. nov. and Parapinnixidae , respectively. The monophyletic Pinnotheridae sensu stricto is the largest family in Pinnotheroidea and is comprised of nearly 300 species in 3 subfamilies and 59 genera ( Ng et al. 2008; Ng & Ho 2016; Palacios Theil et al. 2016; Ahyong 2018; Ng 2018; Ng et al. 2019; Palacios Theil & Felder 2020; Ng & Ahyong 2022). Pinnotheridae exhibits high morphological diversity, especially between subfamilies. For instance, crabs from Pinnixinae and Pinnixulalinae mostly live in the tubes or burrows of other marine benthos (e.g. mud shrimps and annelids). They have a much wider carapace (i.e. high aspect ratio carapace) compared to more circular carapace observed in the species from Pinnotherinae that predominantly live endosymbiotically with bivalve hosts, or occasionally with gastropod, ascidian and echinoderm hosts ( Hultgren et al. 2022).