Pseudostegias trisagitta, Williams & Boyko, 2016

Williams, Jason D. & Boyko, Christopher B., 2015, Abdominal bopyrid parasites (Crustacea: Isopoda: Bopyridae: Athelginae) of diogenid hermit crabs from the western Pacific, with descriptions of a new genus and four new species, Raffles Bulletin of Zoology 64, pp. 33-69 : 61-65

Pseudostegias trisagitta n. sp.

(Figs. 16, 17)

Material examined. Mature holotype female (8.9 mm), mature allotype male (2.3 mm) ( USNM 1283428), infesting intersex Calcinus minutus (2.9 mm SL), inhabiting shell of Drupella rugosa , Oriental Mindoro, Puerto Galera, Coco Beach, coll. JDW, 14 January 1999.

Type locality. Philippines, Oriental Mindoro, Puerto Galera: Coco Beach (13°30’N, 120°56’E) .

Description. Holotype female (Fig. 16A, B): Body length 8.9 mm, maximal width 3.8 mm, head length 1.5 mm, head width 0.9 mm, pleon length 2.7 mm. Body longer than broad; pereon straight. All body regions and pereomeres distinctly segmented but pereomeres II–V with indistinct segmentation medially (Fig. 16A).

Head 1.5 times longer than wide with weakly convex lateral margins, anterior margin convex, posterior margin weakly convex (Fig. 16A). Small eyespots anterolaterally. Antennule of three articles, distal ends of terminal article with setae; antenna of five articles, distal end of terminal article and subdistal region of third article with setae (Fig. 16C). Maxilliped (Fig. 16D) with elongate, triangular anterior lobe lacking palp; smaller, rounded posterior lobe without spur. Barbula (Figs 16E) with one short, smooth tapering lobe anterolaterally.

Pereon of seven distinct pereomeres, broadest across pereomeres IV, V, tapering anteriorly and posteriorly. Pereomere I posterior margin overlapped by head, pereomere II posteriorly convex, pereomeres III–VII posteriorly straight, pereomeres II–VII with small posterolateral flaps near base of pereopods, most pronounced on pereomeres II, III, V, VI. First oostegites not extended over head (Fig. 16A, B). Oostegites completely enclosing brood pouch; posteriormost oostegite with fringe of setae on posterior margin. First oostegite anterior lobe irregularly rounded, posterior lobe with ovate proximal region and elongate tapering posterior region, internal ridge with three short thin smooth projections mesially (Fig. 16F, G). Pereopods I–V subequal in size with ventrally projecting meral segments (Fig. 16H–J), pereopods VI, VII with distinctly more slender bases than pereopods I–V (Fig. 16A). Pereopod I (Figs. 16A, H) anterior to head, pereopods II, III parallel to head, distinctly longer gap between pereopods V, VI (Fig. 16A).

Pleon with six pleomeres, segmentation distinct. Pleomeres I–IV with elongate lateral plates approximately three times as long as wide (Fig. 16A) and weakly biramous anteroposteriorly directed pleopods (Fig. 16B). Pleomere V with pair of dorsally indented and triangular medially positioned lateral plates on either side of a small medial knob (Fig. 16J) and pair of biramous laterally directed pleopods similar in shape and proportion to lateral plates of segments I–IV (Fig. 16A). Pleomere VI with uniramous uropods, similar in shape and proportion to pleomeres I–IV lateral plates; single dorsally indented and triangular medially positioned lobe, identical to lateral plates of pleomere V (Fig. 16J).

Allotype male (Fig. 17): Length 2.3 mm, maximum width 0.8 mm, head length 0.2 mm, head width 0.4 mm, pleon length 0.6 mm. Head subovate, widest proximally, tapering anteriorly, medially fused with pereomere I (Fig. 17A). Eye pigmentation present at distal lateral junction of head and first pereomere (Fig. 17A). Antennule of three articles (Fig. 17B), all articles with distal setae. Antenna (Fig. 17C) of seven articles, extending posterolaterally from head, distalmost article with tuft of long setae, proximal four articles with distolateral setae.

Pereomeres IV, V broadest, tapering anteriorly and posteriorly. All pereomeres directed laterally with rounded lateral margins (Fig. 17A). Pereopods isomorphic (Figs. 17B, C, D), all articles distinctly separated, dactyli recurved and straight, touching base of propodus in raised medial region on ventral surface.

Pleon (Fig. 17A, B) equilaterally triangular with slightly expanded anterior portion, tapering posteriorly, all pleomeres fused with lateral indications of segmentation. No midventral tubercles, pleopods, or uropods (Fig. 17B).

Etymology. Named for the distinctive shapes of the structures on the fifth pleomere ventral surface resembling three diverging arrowheads.

Distribution. Known only from the Philippines in Oriental Mindoro province.

Host. Calcinus minutus .

Size range (Length). Female: 8.9 mm, male: 2.3 mm.

Remarks. In nearly all aspects, excepting the pleopod length/width proportions and the shape and proportions of the dorsal lobes on the fifth pleomere, the female of the new species resembles most closely that of P. dulcilacuum . The female of the new species has a much lower pleopod length to width ratio than P. dulcilacuum . We considered that the distinctive shape of the lobes on the fifth pleomere might represent an artifact of preservation but we can find no other altered structures on the female that would indicate this; additionally, the proportions of the lobes are quite different from those seen in the most similar species, P. dulcilacuum . The new species has two dorsally triangular subequal lobes on the fifth pleomere and one on the sixth pleomere while P. dulcilacuum has three dorsally rounded lobes with the anteriormost two being much larger than the posterior one. A fourth smaller rounded lobe is seen on the new species at the junction of the three triangular lobes; this lobe could be homologous with the fourth lobe seen in some specimens of P. dulcilacuum (e.g., Fig. 13C) but it has not been described from most specimens of P. dulcilacuum . Another character of interest is the number of lobes of the barbula which presents as a single lobe in the new species but has been variously reported without comment as one or two lobes in P. dulcilacuum . It is not clear whether this represents intraspecific variation. The males of the new species and P. dulcilacuum are very similar, albeit that most males reported for P. dulcilacuum have proportionally longer pleons.

Calcinus minutus has not been previously reported as a host of any Pseudostegias species , and we found no specimens referable to P. dulcilacuum on this host during the present study.

D

Fig. 17. Pseudostegias trisagitta n. sp., male allotype, USNM 1283428 (A–D). A, dorsal view; B, ventral view; C, left antennae and pereopod 1; D, right pereopod 7. Scale bars = 500 µm [A, B], 50 µm [C, D] .