Prionocrangon Wood-Mason and Alcock, 1891

Kim, Jung Nyun & Chan, Tin-Yam, 2005, A revision of the genus Prionocrangon (Crustacea: Decapoda: Caridea: Crangonidae), Journal of Natural History 39 (19), pp. 1597-1625 : 1598-1601

Genus Prionocrangon Wood-Mason and Alcock, 1891

Prionocrangon Wood-Mason and Alcock 1891, p 361 ; Alcock 1901, p 123; Balss 1914, p 71; de Man 1920, p 308; Yaldwyn 1960, p 45; Chace 1984, p 55; Christoffersen 1988, p 46; Holthuis 1993, p 300.

Description

Body small, laterally compressed; integument not particularly firm, smooth; generally whitish in colour.

Rostrum short, spiniform, slightly ascending. Carapace having only median carina, armed with 4–12 spines; antennal spine small; branchiostegal spine moderately strong, directed forward; hepatic and pterygostomian spines absent; orbital cleft and longitudinal suture absent.

Abdomen lacking median carina or with low median carina on fourth and fifth somites; first to fifth pleura rounded ventrally. Telson tapering to rounded or subtruncated apex, narrowed posterior to mid-length, armed with three pairs of dorsolateral spines; apex bearing several to many long spines. Interlocking mechanism present between telson and uropods.

Thoracic sternum narrow, with small median tubercle posteriorly. Abdominal sternites without distinct median or preanal spine.

Eye lacking cornea; eyestalk triangular or drawn out to bluntly cylindrical or villiform extremity, usually with ventral denticle or tubercle.

Antennular peduncle ( Figure 5B ) with proximal segment elongate, usually five times as long as second segment, without longitudinal carina on ventral surface; stylocerite leaf-like, tapering to acute spine, but sometimes spine considerably reduced; flagella sexually dimorphic as in other crangonids; dorsal flagellum thicker, longer and subdivided in much more articles in males than in females.

Antenna ( Figure 5C ) with scaphocerite slender, more than five times as long as wide and with lateral margin concave, distolateral spine terminal; carpocerite elongate, usually reaching beyond distal end of scaphocerite; flagellum much shorter than body.

Mandible ( Figure 5D ) moderately slender, divided distally into four blunt teeth. Maxillule ( Figure 5E ) with proximal endite rounded distally, bearing many marginal setae; distal endite strongly curved mesially, mesial margin narrowly truncate with six spines arranged in two rows; palp with somewhat expanded distal part, curved mesially, with several setae distally. Maxilla ( Figure 5F ) with palp rounded distally, mesial margin bearing short setae; endite poorly developed; scaphognathite with posterior lobe slightly truncate and only bearing short setae. First maxilliped ( Figure 5G ) with palp elongate and flattened, bearing long distomesial setae; exopod with peduncle shorter than palp, mesial and lateral margins bearing short setae, caridean lobe narrow, lash moderately short; epipod large, triangular, not bilobed. Second maxilliped ( Figure 5H ) with endopod pediform, dactylus with five long mesial spines and dense setae distally; exopod with peduncle reaching distal margin of carpus of endopod, lash moderately short; rudimentary epipod present. Third maxilliped ( Figure 5I ) heavily setose especially on dorsal margin, overreaching tip of scaphocerite by distal segment; distal two segments slightly dorsoventrally compressed, ultimate segment slightly longer than penultimate segment, with tip rounded and curved dorsally; antepenultimate segment laterally compressed, subequal to length of distal two segments combined, produced distodorsally into lobe-like projection and bearing small proximoventral tubercle; exopod with short lash; epipod vestigial with fine setae distally.

First pereopod ( Figure 5J ) without exopod; palm ( Figure 5K ) slender, more than 3.5 times as long as wide, with fixed distomesial spine (‘‘thumb’’), cutting edge strongly oblique; merus with small subdistal spine on dorsal margin but always lacking distolateral and mesial spines. Second pereopod ( Figure 5L, M ) not chelate, reaching proximal part of chela of first pereopod, setose dorsally and ventrally; merus and ischium partially fused. Third pereopod ( Figure 5N ) very slender, usually overreaching tip of scaphocerite by dactylus and propodus; carpus longer than distal two segments combined; merus shorter than carpus or ischium. Fourth and fifth pereopods ( Figure 5O, Q ) robust; dactyli ( Figure 5P ) short, flattened laterally; propodi to ischia heavily setose dorsally and ventrally; propodi shorter than carpi; meri and ischia partly fused.

Pleurobranchs of fourth to eighth thoracic somites inclined forwards.

Second ( Figures 5S , 6C ) to fifth pleopods with endopods slender, 0.2–0.6 times as long as exopod, lacking appendix interna in both sexes; male second pleopod with appendix masculina ( Figure 6D ) well developed, but shorter than endopod. Eggs large, 1.12– 2.45× 0.88–1.76 mm in diameter.

Branchial formula as follows (r, rudimentary):

Type species

Prionocrangon ommatosteres Wood-Mason and Alcock, 1891 by monotypy. Gender: feminine.

Species included

Prionocrangon ommatosteres Wood-Mason and Alcock, 1891 , P. pectinata Faxon, 1896 , P. dofleini Balss, 1913 , P. curvicaulis Yaldwyn, 1960 , P. formosa sp. nov., P. demani sp. nov., and P. paucispina sp. nov.

Distribution

Bay of Bengal, Andaman Sea, Indonesia, the Philippines, Taiwan, Pacific coast of central Japan, New Caledonia, New Zealand, Caribbean Sea; at depths of 200–2556 m and probably burrowing deep in mud.

Remarks

The genus Prionocrangon is unique in the Crangonidae in the following characters: rostrum spiniform, carapace bearing a row of dorsal spines, lateral surfaces of carapace without any spine or carina, interlocking mechanism present between telson and uropods, eyestalk reduced and lacking cornea, proximal segment of antennular peduncle elongate, scaphocerite with very narrow blade, epipod of second maxilliped vestigial, antepenultimate segment of third maxilliped with lobe-like distal projection, meri and ischia partly fused in second, fourth and fifth pereopods, and propodi shorter than carpi in fourth and fifth pereopods.

The interlocking mechanism between the telson and uropods is developed by the strong median hook on the anterodorsal part of the uropodal exopod together with the anteromedian concavity of the uropodal endopod forming a socket which clips the ventrolateral tubercle of the telson. This mechanism operates when the abdomen is in subcircular form ( Figure 4A ). Such a mechanism is not known in the other crangonids, as well as other caridean shrimps. Yaldwyn (1960) pointed out that the blind and white coloration, coupled with their stout fourth and fifth pereopods and their spatulate dactyli strongly suggest that Prionocrangon has a burrowing habit (see also ‘‘Remarks’’ under P. dofleini ). The interlocking mechanism may also be related to such a lifestyle.

Chace (1984) mainly used the shape of the degenerated eyes and the telson, as well as the spination along the posterior margin of telson to separate the species within the genus. In the present study the above characters, together with the length of the rostrum and the last two pereopod dactyli, and the presence or absence of a dorsal carina on the abdomen were found to be useful in delineating the species, at least to groups. Several small characters, such as the shape of the posterolateral process on the sixth abdominal somite as well as the presence or absence of lobes or projections on the endopods and protopods of the pleopods, are also helpful in separating species.

Members of this genus shows a typical Tethys distribution, and its geographical range is now slightly expanded in the West Pacific. The bathymetrical distribution is also shown to be much wider. It is interesting to note that species with triangular eyestalks, e.g. P. pectinata , P. demani , and P. paucispina , inhabit deeper waters (1033– 2556 m deep) than others with the eyestalks drawn out to bluntly cylindrical or villiform extremities, e.g. P. ommatosteres , P. dofleini , P. curvicaulis , and P. formosa (200– 891 m deep).

Figures

Figure 4. Prionocrangon dofleini Balss, 1913, female (CL 9.2 mm) from Tosa Bay, Japan. (A) Entire animal, lateral; (B) carapace and anterior cephalic appendages, dorsal; (C) first to fifth abdominal somites, dorsal; (D) telson, dorsal; (E) distal part of telson, dorsal.

Figure 5. Prionocrangon dofleini Balss, 1913, female (CL 9.2 mm) from Tosa Bay, Japan, appendages dissected from left side. (A) Eye, lateral; (B) antennule, lateral; (C) antenna, dorsal; (D) mandible, external and internal; (E) maxillule, external; (F) maxilla, external; (G) first maxilliped, external; (H) second maxilliped, external; (I) third maxilliped, lateral; (J) first pereopod, lateral; (K) chela of first pereopod, ventral; (L) second pereopod, lateral; (M) dactylus, same, lateral; (N) third pereopod, lateral; (O) fourth pereopod, lateral; (P) dactylus, same, lateral; (Q) fifth pereopod, lateral; (R) first pleopod, ventral, exopodal setae omitted; (S) second pleopod, ventral, exopodal setae omitted.

Figure 6. Prionocrangon dofleini Balss, 1913. (A) Male (CL 7.4 mm) from Taiwan, TAIWAN 2001 stn CP 104; (B–D)male (CL 7.4 mm) from Sagami Bay, Japan; (E) ovigerous female (CL 9.8 mm) from unknown locality in Japan; (F) female (CL 9.9 mm, same lot). (A) Carapace and anterior cephalic appendages, lateral; (B) left first pleopod, ventral; (C) left second pleopod, ventral; (D) appendix masculina, same, dorsal; (E, F) distal part of telson, dorsal.