Pteropagurus McLaughlin & Rahayu, 2006

McLAUGHLIN, Patsy A. & Rahayu, Dwi Listyo, 2008, Pteropagurus and Catapagurus (Decapoda, Anomura, Paguridae): resource sharing or “ any port in a storm ”?, Zoosystema 30 (4), pp. 899-916 : 901-902

Genus Pteropagurus McLaughlin & Rahayu, 2006 emended

Pteropagurus McLaughlin & Rahayu, 2006: 56 . — McLaughlin 2007: 504.

TYPE SPECIES. — Pteropagurus inermis McLaughlin & Rahayu, 2006 by original designation.

EMENDED DIAGNOSIS. — Eleven pairs of biserial phyllobranchiate gills. Rostrum triangular, broadly rounded or obsolete; lateral projections well developed or obsolete. Ocular peduncles moderate to stout, corneas prominently dilated or not; ocular acicles triangular, each often with marginal or submarginal terminal spine. Antennal peduncles with supernumerary segmentation. Mandible with entirely calcified cutting edge. Maxillule with external lobe of endopod obsolete, internal lobe with 1 stiff seta. Maxilla with endopod reaching to or considerably overreaching distal margin of scaphognathite. First maxilliped with slender exopod. Second maxilliped without distinguishing characters. Third maxilliped with crista dentata reduced, no accessory tooth. Sternite of third maxillipeds (thoracic somite IX of Pilgrim 1973) unarmed. Sternite of chelipeds (thoracic somite X) quite narrow, incompletely fused to much broader sternite of second pereopods. Sternite of third pereopods (thoracic somite XII) sexually dimorphic or not: varying from only slightly broadened and not produced posteriorly to broad and prominently produced posteriorly.

Chelipeds long, slender, right appreciably stouter, but not always longer. Second pereopods somewhat to markedly shorter than third; dactyls with or without ventral corneous spines. Fourth pereopods semichelate or weakly subchelate, propodal rasps with few scales; no preungual process. Fifth pereopods chelate, rasps moderately well developed.

Males with elongate, stout right or left sexual tube directed posteriorly and externally and curving up over anterior portion of pleon; alternate coxa with or without papilla or short tube; left pleopods 3 and 4 uniramous or unequally biramous; pleopod 5 uniramous. Females with paired gonopores or unpaired left; no paired and modified first pleopods, unpaired biramous left pleopods 2-4, pleopod 5 uniramous as in males. Pleon somewhat reduced, straight; pleomere 1 partially fused with last thoracic somite, tergites 2-5 weakly delineated or not; tergite 6 chitinous. Uropods symmetrical; protopods each with or without posteriorly directed spine; exopods elongate, endopods reduced, both with 1 or 2 marginal rows of scales. Telson without transverse incisions, with slight to moderately well developed median cleft or concavity; terminal margins rounded or oblique, unarmed, with few minute bristles, very small tubercles, or tiny spines.

REMARKS

In proposing their new genus Pteropagurus, McLaughlin & Rahayu (2006) noted certain morphological attributes of the two assigned species, P. inermis and P. spina that they considered adaptations to the cylindrical pteropod shells that served as carcinoecia. These included the reduced and straight, albeit somewhat swollen pleons, the distinctly elongate third pereopods, the broadened and posteriorly lengthened sternite of the third pereopods, and the carriage of the uropods and telson that were routinely folded under the pleon. However, their interpretations were based on data gleaned primarily from P. inermis , for which both males and females were available for examination. The broadening of sternite XII (sternite of the third pereopods) occurred in males of P. inermis , but not in females of either species and therefore was considered a sexually dimorphic character of the genus. When P. spinulocarpus was recognized, McLaughlin (2007) found that the broadening and elongation of this sternite occurred in both sexes. Now, with the discovery of a male of P. spina , elongation of the sternite of the third pereopods can no longer be viewed as a direct adaptation to the pteropod carcinoecium. Neither male nor female of this species exhibit any unusual lengthening of the sternite. McLaughlin & Rahayu (2006) suggested that the dimorphic sternal elongation might be correlated to an increase in the breadth of the cephalothorax in males of P. inermis . However, with the information currently available for all three species, another hypothesis appears more feasible. The third pereopods are longer than the second in both males and females of all three species, but markedly so only in males of P. inermis and both sexes of P. spinulocarpus where the sternite is also lengthened. In contrast, females of P. inermis and both male and female of P.spina lack the pronounced elongation of the sternite of the third pereopods and of the third pereopods themselves. Thus it may be that the posterior development of the sternite of the third pereopods is in response to the exceptional development of this third pair of appendages.

Development of the sexual tube in P.spina presents a more serious dilemma for emendation of the generic diagnosis. Males of both P. inermis and P. spinulocarpus are characterized by an elongate sexual tube developed from the coxa of the right fifth pereopod that is directed toward the exterior and curves up over the dorsal surface of the pleon on the right side of the body. The sexual tube of the just discovered male of P. spina develops from the coxa of the left fifth pereopod, is directed toward the exterior and curves up and over the dorsal surface of the pleon from left to right, and extends almost to the gonopore of the right coxa on the ventral body surface. This disparity in sexual tube development could easily be considered justification for the establishment of a second genus of pteropod-dwelling hermit crabs. Additionally, the moderately slender ocular peduncles and corneas lacking appreciable dilation, together with the development of a prominent rostrum only in P. spina tends to support such a distinction. However, as McLaughlin & Rahayu (2006) emphasized, P.inermis and P. spina share several synapomorphies and these are similarly shared with P. spinulocarpus . Additionally, the magnitude of differences in lengths of the second and third pereopods and the elongation of the sternite of the third pereopods appear to be clinal, with P. spina being the least, P. spinulocarpus the most advanced, with P. inermis intermediate and dimorphic. With only one male of P. spina yet known, it is not possible to determine with certainty that the left sexual tube development in this species is typical or represents an abnormality. Clearly, there is no abnormal development in the tube itself other than its unusual length. However, one of the authors (PMcL) has observed the atypical development of the right sexual tube in two specimens of Catapagurus franklinae McLaughlin, 2004 , where the tube is directed from right to left across the ventral body surface as it is in species of Cestopagurus Bouvier, 1897 . Consequently, we believe that it is more appropriate to consider sexual tube development in Pteropagurus species variable, until such time as additional data prove the contrary.