Ione sarahae n. sp.

(Figs. 1B, 2–4)

Type material: Holotype, ovigerous female, 6.6 mm TL; allotype, male, 3.5 mm TL; from right branchial chamber of female Neotrypaea tabogensis (5.3 mm CL), Mata de Limón, Pacific coast of Costa Rica, in burrows, intertidal, coll. P. Hernáez, 16 March 2012 (MZUCR 3789-01). Paratypes: female, 2.8 mm TL, without male, from left branchial chamber of male of N. tabogensis (5.1 mm CL), Mata de Limón, Pacific coast of Costa Rica, in burrows, intertidal, coll. P. Hernáez, 21 January 2013 (MZUCR 3790-01); female, 4.3 mm TL, from right branchial chamber of male of N. tabogensis (4.4 mm CL), Mata de Limón, Pacific coast of Costa Rica, in burrows, intertidal, coll. P. Hernáez, 21 December 2012 (MZUCR 3791-01).

Type host: The burrowing shrimp Neotrypaea tabogensis (Sakai, 2005) (Fig. 1A).

Diagnosis: Ovigerous female with body shape obovate, asymmetrical, slight sinistral distortion up to 13° in adults, all segments of pereon distinct; head not fused with first pereomere, posterior margin nearly straight; antenna composed of five articles; barbula formed by four lamellae, posterior lamella with one tapered lobe on each side; oostegite 1 ornamented with small spines from base to middle region on outer surface; maxillipedal palp composed of three articles. Pereopods subequal in structure and size, dactyli progressively less pointed posteriorly, propodus not fused with carpus, carpus composed of two articles. Five pleomeres fused dorsally, laterally separated, with long strongly ramified featherlike lateral projections posteriorly; three pairs of biramous pleopods, reduced progressively in size posteriorly, pleopods 4 and 5 absent; uropods strongly segmented. Mature male with body elongated, clearly segmented dorsally and laterally, symmetrical; head separated from first pereomere; maxillipeds absent; antennula and antenna of 3 and 6 articles, respectively. Pereomeres separated dorsally, segmentation not visible ventrally, directed laterally. Pereopods subequal in size and length. Pleon shorter than pereon. Pleomeres progressively narrower posteriorly, distinctly separated; pleotelson truncated; uropods uniramous and digitiform.

Description: Holotype ovigerous female (MZUCR 3789-01): General body shape (Figs. 1A, 2) obovate, body segments separate, asymmetrical, slight sinistral distortion up to 13° in adults; total length 6.6 mm, maximal width (across pereomere 3) 3.4 mm. Head with hexagonal outline, approximately 1.5 times wider than long (length 1.1 mm, width 1.7 mm), straight anteriorly, nearly rounded posteriorly, not fused with first pereomere, dorsal surface smooth, lateral margins produced into obtuse angle (112°) on each side. Frontal lamina very long, extending beyond anterior margin of head, with one lateral leaflike projection on each side; eyes absent. Pereon broadest across pereomere 3, all pereomeres dorsally, ventrally and laterally distinct. Pereomere 1 curved forward around head reaching its lateral angles; pereomere 2 anterior margin curved forward, others progressively curved backward. All pereomeres without tergal projections. Coxal plates oval, not overlapping; dorsolateral bosses present.

Antennula (Fig. 3A) composed of three articles; basal article globose with sparse, short stiff setae distolaterally; second article subcylindrical with isolated stiff setae distally; distal article subcylindrical, shorter than second article, with tuft of short setae distally.

Antenna (Fig. 3B) composed of five articles; basal fifth article globose, not fused with fourth article, article 2 subquadrate, articles 3 and 4 cylindrical, with isolated setae distolaterally, distal article subcylindrical, shorter than former, with tuft of setae on tip.

Barbula (Fig. 3C) with two pair of smooth lamellae: posterior lamella with one tapered lobe on each side, middle region flat; each lateral lamella with one tapered lobe.

Maxilliped (Fig. 3D) articulated, prominent basal process anterolaterally, distal article subpyramidal; palp composed of three articles, distal article with 1 or 2 setae on tip.

Pereopods (Fig. 3E, F) subequal in size and structure; ischium widening distally in lateral view (Fig. 3E) or subquadrate (Fig. 3F), similar in length to merus; carpus two-segmented; propodus with anterior margin expended dorsally, not fused with carpus, inner margin with small crenulations used for attachment to host; dactyli progressively less pointed posteriorly.

Marsupium (Figs. 2B, 3G, H) with five pairs of oostegites, inner surface concave in both segments, ornamented with small spines from base to middle region of outer surface, similar in shape; oostegites overlapping, enclosing brood pouch (Fig. 2B); oostegite 1 smallest, each segment formed by two plates covering oral region completely; oostegites 2 and 3 largest; others similar in size. Oostegites 2–5 each with longitudinal carina, posterior margin of oostegite 5 covered with setae.

Length of pleon approximately 4.8 times smaller than head/pereon (Fig. 2A). Pleomeres 1-5 fused dorsally, laterally not separated, with long strongly ramified featherlike lateral plates posteriorly; ventral surface of pleomeres smooth; pleotelson subquadrate, approximately 1.4 times wider than long.

Pleopods 1-3 biramous, reduced progressively in size posteriorly; pleopods 4 and 5 absent, endopod much larger than exopod (Fig. 2B). Uropods (Fig. 2A, B) strongly segmented, lanceolate and biramous, outer ramus strongly ramified.

Allotype male (MZUCR 3789-01): Body (Fig. 4A, B) symmetrical, with lanceolate outline, unpigmented, elongated, distinctly segmented dorsally and laterally, body boundaries not visible ventrally; total length 3.5 mm, maximal width at fourth pereomere, 0.57 mm. Head (Fig. 4A, B) suboval, distinct from first pereomere, approximately 4 times as wide as long; anterolateral borders rounded and produced ventrally; eyes small, dark near to posterior margin of head.

Antennula and antenna (Fig. 4C, D) of 3 and 6 articles, respectively; antennula much shorter than antenna, antenna approximately 2.6 times longer than antennula; antennula with small tuft of setae at tip of distal article; antennae (Fig. 4A) extending beyond distal margin of head.

Maxillipeds absent.

Pereomeres (Fig. 4A) separated, directed laterally; midventral tubercles absent.

Pereopods (Fig. 4B, E) subequal in size and length, not visible in dorsal view; pereopods 1–7 used for attaching to female. Pereopod 1 prehensile; length of ischium more than twice that of merus; merus slightly widening distally; carpus two-segmented, armed with sclerotized setae distolaterally; dorsal margin of propodus expanded anteriorly, inner margin irregularly crenulate used for attachment to female; dactyli strongly arcuate with each tip slightly curved downward.

Pleon (Fig. 4A, B) shorter than pereon. Pleomeres 1-5 progressively narrower posteriorly, distinctly separated, lateral margins rounded; each pleomere with pair of digitiform and elongated lateral plates. Pleopods absent. Pleotelson truncated. Uropods uniramous and digitiform; anal cone present.

Distribution: Known only from the intertidal zone at the type locality of Mata de Limón, Pacific coast of Costa Rica, Costa Rica.

Etymology: The specific name sarahae is given to honor the young lady Sarah Villegas Abarca, daughter of the second author, who has filled her father's life with love.

Remarks: The new species described here is assigned to Ione, whose main diagnostic characters (cf. Latreille 1818; see also Boyko et al. 2013 and references therein) are readily recognized in the new species. They are as follows: female pleonal lateral plates with numerous filamentous rami, and biramous uropods (see Figs. 1B, 2A, B); male with five pairs of simple pleonal lateral plates, and uniramous uropods (see Figs. 1B, 4A, B). Ione now consists of seven species (Boyko et al. 2008; for details on synonymy, see also Boyko et al. 2017), viz: I. cornuta (parasite of Neotrypaea gigas (Dana, 1852)), I. ovata (parasite of N. uncinata (H. Milne Edwards, 1837)), I. sarahae n. sp. (parasite of N. tabogensis), I. taiwanensis (parasite of Callianidea typa H. Milne Edwards, 1837), I. thompsoni (parasite of Gilvossius setimanus (DeKay, 1844)), I. thoracica (parasite of Callianassa subterranea (Montagu, 1808)), and I. tubulata (host unknown). The new species described herein raises to two (with I. ovata) the number of eastern Pacific species of Ione, and is the only species known from the Pacific coast of Costa Rica and the whole of the tropical Eastern Pacific.

Morphologically, the new species differs from the six other congeners by the following combination of characters: (i) body of female slightly asymmetrical in Ione sarahae n. sp., but symmetrical in I. ovata and I. tubulata; (ii) the posterior margin of the head of female is nearly straight in the new species, whereas it is evenly rounded in I. thompsoni, and I. thoracica; (iii) the antenna of the female is composed of five articles in I. sarahae n. sp., whereas it is composed of four or six articles in I. thoracicus and I. taiwanensis, respectively; (iv) the frontal lamina of the female bears lateral leaflike projections in the new species, instead the posteriorly curved horn-shape projections present in I. cornuta; (v) the pleotelson of the male is truncated in the new species, whereas it is distally tapered in both I. thompsoni and I. thoracica (Bonnier 1900, pl. 1, fig. 2, 6; Richardson 1904, fig. 64, 68; Shiino 1964, fig. 2a; Bourdon 1976, fig. 17; Markham 1995, fig. 1c; Figs. 2A, 3B, 4A, B herein).