Plesionika exigua ( Rathbun, 1906 )

Chan, Tin-Yam, 2016, New records of the rare pandalid shrimp Plesionika exigua (Rathbun, 1906) (Crustacea: Decapoda: Caridea) in the western Pacific, Zootaxa 4205 (1), pp. 97-100 : 98-99

Plesionika exigua ( Rathbun, 1906)

( Figs. 1 , 2 )

Pandalus exiguous Rathbun, 1906: 916 , fig. 66, pl. 21-fig. 2 (type locality: Hawaii). Plesionika exigua — Chace, 1985: 47; De Grave & Fransen, 2011: 448; Li & Chan, 2013: 140, fig. 2H.

Material examined. Hawaii, “ Albatross ”: stn 3982, vicinity of Kauai, 233–40 fathoms, 10.06.1902, 1 male cl 4.7 mm, 1 ovig. female holotype cl 4.6 mm ( USNM 30549 ) ; stn 4062, northeast coast, 83–113 fathoms, 1 male cl 4.1 mm (USNM 30676); stn 4064, northeast coast, 63–107 fathoms, 1 male 3.9 mm ( USNM 30677 ).— New Caledonia, VOLSMAR- HUNTER, stn DW 41, 22o17.7’S, 168o41.2’ E , 250 m, 0 8.06.1989, 2 males cl 6.0 and 6.7 mm, 2 ovig. females cl both 4.9 mm (MNHN IU-2014-8775).— Vanuatu, SANTO, stn AT 77, 15o39.2’S, 167o03.2’ E , 142–171 m, 10.10.2006, 1 ovig. female cl 4.9 mm ( MNHN IU- 2014-8776 ).— Philippines, PANGAO 2005, stn DW 2374, 8o43.7’N, 123o14.0’ E , 101– 109 m, 28.05.2005, 1 damaged spec. cl about 4.9 mm ( NTOU M01615 ).— Ryukyu Islands , Kume Island, KUMEJIMA, stn T 73, 26o35.692’N, 126o68.87’ E , 96.4–186 m, 19.11.2009, 1 male cl 4.1 mm (NTOU M02026 ).

Description. Rostrum long and far exceeding scaphocerite, 1.5–2.0 times as long as carapace; curving downwards basally but recurved slightly upwards after passing eyes; armed dorsally only at basal crest and near tip; subapical teeth 2–3; basal crest distinct but low, bearing 6–7 teeth with bluntly barbed tips, teeth on basal crest progressively smaller posteriorly and with posterior most tooth often minute, posterior 4–5 teeth behind orbital margin (including tooth just above orbit) and movable, distal 1–3 teeth anterior to orbital margin and with distal tooth fixed while distal second and third tooth, if present, either fixed or movable; ventral border bearing 8–13 teeth along entire margin except near tip. Eyes kidney shaped and with large ocellus. Orbital margin regularly concave with ventral end truncate and steep. Antennal spine strong and elongated, pterygostomian spine smaller and not elongated. Stylocerite sharply pointed and reaching from near distal margin of eye to distal margin of basal antennular segment. Scaphocerite with distolateral tooth more or less reaching distal margin of lamella. Maxilliped III with epipod, distal segment 1.4–1.8 times as long as penultimate segment. Pereiopods with well-developed epipods on 2 anterior pairs, occasionally minute thread like epipods also present on pereiopod III. Pereiopod I microscopically chelate. Pereiopod II greatly unequal and with left side much longer, carpus of left side composed of 42–45 articles while that of right side composed of 11–14 articles. Posterior 3 pereiopods similar, pereiopod III overreaching scaphocerite by dactylus and about half propodus; dactylus 0.14–0.17 times as long as propodus, accessory spine slightly shorter than and abutting terminal spines, flexor margin armed with 3–4 distinct movable spines. Abdomen with somite III moderately arched dorsally and without posterior spine; pleura IV and V generally terminated posteroventrally into sharp tooth; somite VI 2.1–2.5 times as long as maximum height. Telson nearly as long as abdominal somite VI and armed with 4 pairs of dorsolateral spines and 2 pairs of posterior spines (outer pair larger). Eggs small and numerous, 0.3–0.5 mm in diameter and becoming about 0.5 X 0.7 mm in diameter when near hatching (i.e., eyed eggs).

Coloration. Body semi-translucent and scattered with fine red dots, dots on anterior carapace denser. Rostrum reddish with distal 1/4 whitish or having 2 broad white bands at tip and at area slightly anterior to level of tip of scaphocerite. Eyes dark brown. Posterior carapace, anterior abdominal somites and lateral surfaces of peduncles of pleopods with some white dots. Rows of iridescent yellowish green dots present on maxilliped III, abdominal tergites V to telson, tips of uropods and somewhat also at base of rostral crest and lateral margin of scaphocerite. Ovigerous females with broad reddish oblique strips on abdominal pleura I to IV. Eggs whitish.

Distribution. Western Pacific from Hawaii, Ryukyu Islands, the Philippines, New Caledonia and Vanuatu. At depths of 73– 426 m.

Remarks. The present material sampled at different localities is essentially similar. The holotype female from Hawaii (USNM 30549), nevertheless, has the left abdominal pleuron IV and right abdominal pleuron V not terminated into a distinct posteroventral tooth. As commented by Li & Chan (2013), the remaining parts of the badly damaged specimen from the Philippines fit well with the characteristics of P. exigua . Although Plesionika is a very large genus with 93 species known to date, only four species have the teeth on the rostral crest bluntly barbed terminally. These four species are very likely most closely related (i.e., the “ P. exigua ” species group). Plesionika exigua is generally considered to be rather different from the other three species in lacking epipod at the pereiopod III as well as having more postrostral teeth (4–5 versus 1–3; Fig. 1 A–C) and the dactyli of posterior pereiopods bearing posterior spines (3–4 versus 0; Fig.

1F). Instead of it, a small epipod is present on both pereiopods III in the Vanuatu specimen examined. Thus, the presence or absence of epipods on the pereiopods is not so constant as previous thought for the species of this group ( Chace 1985; Li & Komai 2003). As P. longidactylus is extremely similar to P. pumila (and occur at similar localities) except for lacking epipod on the pereiopod IV, more detailed studies will be needed to determine if these two species are really valid.

Figures

FIGURE 1. Plesionika exigua (Rathbun, 1906), New Caledonia, VOLSMAR-HUNTER, stn DW 41: A, D – F, male cl 6.0 (MNHN IU- 2014 - 8775); B, male cl 6.7 mm (MNHN IU- 2014 - 8775); C, ovig. female cl 4.9 mm (MNHN IU- 2014 - 8775). A, B, carapace and anterior appendages, lateral; C, anterior carapace, lateral; D, posterior abdomen and tail fan, lateral; E, propodus and dactylus of left pereiopod III, lateral and setae omitted; F, dactylus of left pereiopod III, lateral and setae omitted. Scales: A – D = 1 mm, E = 0.5 mm, F = 0.25 mm.

FIGURE 2. Plesionika exigua (Rathbun, 1906), Kume Island, Japan, KUMEJIMA, stn T 73, male cl 4.1 mm (NTOU M 02026).