Bioacoustics of Robertsella meridionalis

Remarks. Robertsella was until now known from one species, R. mystica , from the northwestern Atlantic (South Carolina, Gulf of Mexico , Florida, and Dry Tortugas ). Although Robertsella meridionalis n. sp. is represented by only one adult male, it can be distinguished from its northern counterpart by a suite of carapace and appendage characters. In Robertsella meridionalis n. sp. (i) the well-developed lateral expansion of the third abdominal somite is in contact with thoracic sternite 7 and thus covers the gonopore and almost the entire penis ( Figs. 1D , 2A ) (much shorter abdominal lateral expansion that is not in contact with sternite 7 thus leaving the gonopore and most of the proximal part of penis uncovered in R. mystica ; Figs. 1 H, 2E); (ii) abdominal suture 3/4 is evident along its entire length, less so near the axis only ( Figs. 1D , 2A ) (abdominal suture 3/4 evident at lateral notch only in R. mystica ; Figs. 1 H, 2E); (iii) when the abdomen is closed, the abdominal suture 5/6 is located well above the thoracic sternal suture 6/7 ( Figs. 1D , 2A ) (abdominal suture 5/6 and thoracic sternal suture 6/7 aligned to each other in R. mystica ; Figs. 1 H, 2E); (iv) the lateral margins of the thoracic sternite and episternite 7 are straight, aligned to each other and subparallel in relation to the body axis ( Fig. 1D ) (sternal and episternal margins distinctly convex in R. mystica ; Fig. 1 H); (v) the portion of the thoracic sternite 8 not covered by the lateral expansion of the third abdominal somite is proportionally larger than in R. mystica ( Figs. 2A , E); (vi) the meri of pereiopods 2–5 are densely and coarsely tuberculate ventrally( Figs. 2D , 4B) (ventral surface of P2–P5 meri almost smooth in R. mystica ; Figs. 2 H, 4D); (vii) the cheliped ischial ridge (pars stridens of the stridulatory mechanism, see below) is densely and coarsely tuberculate ( Fig. 1C ) (ischial ridge less tuberculate in R. mystica ); (viii) pterygostomial and branchiostegal regions are densely and coarsely tuberculate (Fig. 4A) (pterygostomial and branchiostegal regions much less tuberculate in R. mystica ; Fig. 4C); (ix) the tubercles in both the branchiostegal margin and the transversal row of tubercles above the branchiostegal margin are much denser and stronger than in R. mystica (Figs. 4A, C); and (x) frontal region of carapace strongly deflexed ( Fig. 1A ) (frontal region distinctly less deflexed in R. mystica ; Fig. 1 E). Robertsella meridionalis n. sp. and R. mystica probably produce sound by stridulation. Both species have an ischial ridge on the mesial side of the right and left chelipeds (par stridens, see Guinot-Dumortier & Dumortier 1960) that rubs against a scrapper (plectrum) on the carapace, a curved tuberculate edge located above the inhalant water opening. The chelipeds basis-ischium is fused together into a single article providing additional leverage for the rasping movement.