Stellatoniscus shieldsi, Oanh & Boyko, 2020

Oanh, Le Thi Kieu & Boyko, Christopher B., 2020, Cancrion khanhensis sp. nov. (Crustacea: Isopoda: Entoniscidae) infesting Monomia haanii (Stimpson, 1858) (Crustacea: Brachyura: Portunidae) from Nha Trang Bay, Khanh Hoa, Vietnam, with remarks on larval stages of entoniscids and description of a new family, genus and two new species of hyperparasites, Zootaxa 4894 (3), pp. 366-386 : 383-384

Stellatoniscus shieldsi sp. nov.

LSID urn:lsid:zoobank.org:act:C3DF7EB6-2762-4A8A-810F-9B3BDE28D380

Tiarinion texopallium Shields & Ward, 1998: 591–594 , fig. 4.— Poore et al., 2002: 132 (in part, cryptoniscus larva only).

Type material. Holotype cryptoniscus larva, 800 µm, ex mature holotype female Tiarinion texopallium Shields & Ward, 1998 (QM W222015), 7.59 mm, ex female Tiarinia cornigera ( Latreille 1825) , 10.8 mm CL, Mermaid Cove, Lizard Island, Queensland, Australia, 3–5 m, 14.40’S, 145.28’E, 1 Mar 1992, coll. J. D. Shields ( QM W22272 [not W22016 as cited in Shields & Ward , 1998].

Etymology. The new species name is given after Jeffrey D. Shields in honor of his many years of parasitological research.

Description. Cryptoniscus larva body fusiform, strongly convex dorsally, length 800 µm, maximum width 125 µm. Cephalon anterior margin round; eyes positioned dorsolaterally. Body pigmentation lacking. Antennula expanded distolaterally, mesially approximated, covering almost entire ventral cephalon; first segment flattened, broad, 6 distal, rounded to subacute projections; proximomesial area demarcated, smooth; proximal margin with single spine and 2 short setae slightly distally; second segment cylindrical, projecting distolaterally, smooth; exopodite short, cylindrical, endopodite longer, distally tapered, epipodite short, subtriangular, distally extended, all three segments with numerous long setae. Antenna with 4 peduncular and 5 flagellar articles, smooth.

Pereon with at least some coxal plates covered with scales (see Remarks). Pereopods 1, 2 and 3–7 dissimilar in size and shape. Pereopod 1 stout, dactylus approximately propodus length, with scale-like spinules on outer surface, propodus rounded, carpus with area of ridges ventrally, merus mostly smooth with few ridges distoventrally, ischium smooth, basis smooth. Pereopod 2 larger than 1, dactylus reaching to distal margin of merus, surface smooth; propodus smooth with 1 spine-like seta on ventral margin; carpus smooth with 1 distoventral spine-like seta. Pereopods 3–7 subequal in size and shape, much slenderer than 1 and 2; dactylus smooth, recurved, approximately half length of propodus, those of pereopods 6 and 7 with small setae on dorsal and ventral surface; propodus slender, slightly medially inflated, 2 stout spine-like setae on distoventral surface; carpus subtriangular with single distoventral spine-like seta on at least pereopod 6; merus approximately twice as long as carpus, distally expanded with 1 distoventral seta and 1 distodorsal seta; ischium and basis subequal in size and shape, dorsomedial surface expanded into broad lobe.

Pleon ventrally smooth; five pairs of biramous subequal pleopods; sympod subquadrate, distomesial corner with 2 long setae; exopods and endopods short, subquadrate, each with 5 long terminal setae. Pleotelson rounded. Uropods biramous, composed of subquadrate sympod with single long seta at distolateral corner, endopod nearly twice as long as exopod, exopod with 2 long distal setae and pair of lateral short setae.

Distribution. Lizard Island, Queensland, Australia, 3– 5 m.

Remarks. The cryptoniscus larva now designated as the holotype of Stellatoniscus shieldsi sp. nov. was originally described as one of two paratype cryptoniscus larvae of Tiarinion texopallium Shields & Ward, 1998 . Despite the description being incomplete and the illustrations containing numerous errors (see below), it is clear from the biramous condition of the pleopods that this is a cryptoniscoid cryptoniscus, not a bopyroid cryptoniscus and was therefore incorrectly considered a larval stage of T. texopallium . However, until the discovery of Stellatoniscus khanhensis sp. nov., the affinities of this larva could not be determined. It is clear that the cryptoniscus of Stellatoniscus tentaculus sp. nov. and S. shieldsi sp. nov. share many characters in common and belong to the same genus. These characters include the shape of the antennular plate (expanded with demarcated mesial region), pereopod 1 with ridges on dactylus, carpus and merus, pereopod 2 larger than pereopod 1 and morphologically different (no ridges, much larger dactylus, tapered propodus), pereopods 3–7 different from 1 and 2 (proportionally shorter dactylus than pereopod 2, scarcely recurved, ischium and basis with dorsomedial expansion), scales on coxal plates. None of these characters are known from any other cryptoniscoid larvae. However, it needs to be pointed out that the description of the cryptoniscus larva from Tiarinion texapallium is incomplete in several characters and the illustrations are wrong or misleading (based on examination of the badly damaged specimen). Errors include: there are only 2 distal antennular segments not 3 as drawn ( Shields & Ward 1998: fig. 4C), the peduncular segments of the antennae are very much like those seen in S. tentaculus gen. nov., sp. nov. and not as drawn ( Shields & Ward 1998: fig. 4B), pereopod 1 is much larger compared to pereopod 2, nearly the same size as the first 4 antennal peduncular segments, than drawn ( Shields & Ward 1998: fig. 1A). Unfortunately, the examined specimen is so damaged that the coxal plates of the pereomeres cannot be discerned and pereomeres 2 and 3 are completely missing. Despite this, the holotype of S. shieldsi sp. nov. can be distinguished from the cryptoniscus of S. tentaculus gen. nov., sp. nov. by the following characters: antennular plate mesial demarcated region smooth, general shape rounded, and with 7+ distal teeth vs. with horizontal ridges, general shape angular, and with 7 distal teeth in S. tentaculus gen. nov., sp. nov.; coxal plates with small region of scales vs. covered with scales in S. tentaculus gen. nov., sp. nov., posterior pereopods with weakly recurved dactylus vs. strongly recurved dactylus in S. tentaculus gen. nov., sp. nov. The sizes of the larvae are also very different: 1.8–2.2 mm for S. tentaculus gen. nov., sp. nov. and 800 µm for S. shieldsi sp. nov. It should be noted that we were unable to examine the other cryptoniscus larva cited by Shields & Ward (1998) due to the onset of the COVID-19 pandemic; this specimen (USNM 284171) should be examined to confirm its conspecificity with the larva designated as holotype of S. shieldsi sp. nov.

The description and illustrations of the female, male, and epicaridium larva of T. texopallium are reasonably accurate except that the epicaridium larva was described and illustrated as having biramous pleopods when it actually has uniramous pleopods as do all other epicaridium larvae of entoniscids described to date. There are two segments present on each pleopod but they are the sympod and exopod (the endopod is lacking) and this is considered to be the uniramous condition.