FISHERY BULLETIN: VOL. 75, NO. 2 



eastern part of the Gulf of Mexico — occur at 

 depths between 250 and 475 m. The species is 

 scarce in less than 256 m, and not abundant at 

 depths greater than 500 m. If the data are correct, 

 the male found in 137 m off Guyana at Oregon stn 

 1993 represents an extremely rare occurrence of 

 the shrimp in waters shallower than 180 m, as 

 does the presence of the species at 70 m (at Oregon 

 stn 2669, 18°31'N, 66°47.5'W, north of Puerto 

 Rico), reported by Bullis and Thompson ( 1965). Al- 

 though Roberts and Pequegnat (1970) stated that 

 this shrimp has been found at depths as great as 

 500 fm (915 m), there is no precise record of its 

 presence below 400 fm (732 m). Their statement 

 seems to have been based on a catch from the 

 Alaminos, in 289-472 fm (529-863 m); however, 

 their remaining records, as well as those of all 

 others, suggest that the specimens obtained in 

 that haul were caught in the shallower part of 

 the depth range cited. 



Throughout the Caribbean and northeastern 

 South America, the royal red shrimp seems to be 

 rather sparsely distributed; various explorations 

 by the Oregon and Oregon II in the region have 

 indicated a dense concentration only off Cabo de 

 la Vela, Colombia. 



Affinities -Pleoticus robustus can be separated 

 from P. muelleri, the only other western Atlantic 

 representative of the genus, by the following char- 

 acteristics: the densely pubescent body, the 

 relatively short prosartema, which does not over- 

 reach the distal margin of the first antennular 

 article, the presence of a branchiostegal spine, 

 the lack of an orbital spine, and the disposition 

 of the submarginal carina which is subparallel to 

 the free border of the carapace along its entire 

 length. The external genitalia of the two species 

 are also quite different: whereas in the petasma 

 of P. robustus the row of cincinnuli occupies the 

 entire median line, the ventromedian lobule is 

 flexible and entire distally, and the ventral costa 

 is plain, in P. muelleri the row of cincinnuli 

 extends only along the proximal 0.4 of the median 

 line, the ventromedian lobule is produced distally 

 in cornified oval and hooklike projections, and the 

 distal part of the ventral costa bears a flange along 

 the inner border. Also, the thelycum of P. 

 robustus exhibits a pair of anterior triangular 

 projections on the flexible anterior part of sternite 

 XIV, and a median ridge on sternite XIII, whereas 

 that of P. muelleri bears nothing more than 

 a pair of minute tubercles on the heavily 



sclerotized anterior part of sternite XIV, and a 

 strong median projection on sternite XIII. 



Spermatophore. -Compound spermatophore (as 

 attached to female) consisting of broad, dorso- 

 ventrally depressed geminate body, with con- 

 spicuous transverse fold at about midlength, and 

 bearing anterolateral wings; also provided with 

 sculptured lateral flaps, and produced postero- 

 lateral^ in short flanges (Figure 35). 



Ventral and lateral walls of each spermato- 

 phore (Figure 36A) thick, opaque, fusing im- 

 perceptibly, their anterior margins broad and 

 perpendicular to medial line. Spermatophore lack- 

 ing anterior lobe, deeply concave at base of wing, 

 there bearing conspicuous constriction; trans- 

 verse angular fold present at about midlength, 

 followed by depressed caudal half. Dorsomesial 

 wall (Figure 36C) largely translucent, with glob- 

 ular anterior evagination (Figure 36B) markedly 

 expanding lumen of sperm sac; posterior part of 

 latter attenuated caudally by close proximity of 

 opposing walls. Flap broad anteriorly and merg- 



FlGURE 35. — Pleoticus robustus, compound spermatophore 

 attached to female, 9 44 mm cl, west of Quita Sueho Bank, 

 western Caribbean (setae omitted). 



306 



