Mysidopsis gemina, Heard Rita Vargas, 2019

Mysidopsis gemina n. sp.

( Figs. 1–3 View Figure 1 View Figure 2 View Figure 3 )

Type material. Holotype: adult male (length [L] 6.4 mm), Costa Rica, Puntarenas, Playa Caldera , 09°92’75”N 84°71’64”W, grey sand beach, kicknet, intertidal zone, depth 1.0 m, R. Heard, O. Breedy and R. Vargas coll., 15 Sep 2010, MZUCR 2843-02 . — Paratypes: 1 adult male (L 5.1 mm), 4 ovigerous females (L 5.0, 5.1, 5.5, 5.4 mm), same collection data as holotype, MZUCR 2843-03 ; 2 adult males (L 6.2, 5.9 mm), Costa Rica, Puntarenas, Playa Caldera , 09°92’75”N 84°71’64”W, grey sand beach, kicknet, intertidal zone, depth 0.5 m, R . Heard and R. Vargas coll., 12 Sep 2010, MZUCR 2817-02 ; 2 adult males (L 5.4, 5.1 mm), 2 ovigerous females (L 5.4, 5.5 mm), same collection data as holotype, GCRL 6601 View Materials .

Additional material examined. 8 males, 15 ovigerous females, 7 non-ovigerous females, Costa Rica, Puntarenas, Playa Caldera , 09°92’75”N 84°71’64”W, grey sand beach, kicknet, intertidal zone, depth 0.5 m, R . Heard and R. Vargas coll. , 12 Sep 2010, MZUCR 2817-01 ; 3 males, 1 non-ovigerous female, Costa Rica, Guanacaste, Bahía Culebra, Playa Panama, 10°35’27.6”N 85°39’32.4”W, grey sand beach, kicknet, intertidal zone, depth 1.0 m, R GoogleMaps . Heard, O. Breedy and R. Vargas coll. , 14 Sep 2010, MZUCR 2822-1 ; 5 males, 12 ovigerous females, 11 non-ovigerous females, Puntarenas, Costa Rica, Playa Caldera , 09°92’75”N 84°71’6”W, grey sand beach, kicknet, intertidal zone, depth 1.0 m, R . Heard, O. Breedy and R. Vargas coll. , 15 Sep 2010, MZUCR 2843-01 ; 2 males, 3 ovigerous females, Costa Rica, Guanacaste, Playa Ocotal , 10°32’52.8”N 85°43’19.2”W, rocky area, intertidal, algal washings, Mar 2014, R GoogleMaps . Heard, O. Breedy and R. Vargas coll., MZUCR 3122-02 ; 1 ovigerous female, Costa Rica, Puntarenas, Cabo Blanco, Poza San Miguel , 09°34’40.8”N, 85°8’16.8”W, tide pools at night with light, kicknet, R GoogleMaps . Vargas and A. Carrillo, 21 Apr 2012, MZUCR 3240-01 .

Diagnosis. Antennal scale lanceolate, apex rounded with indistinct suture; carapace with anterior margin produced into an acutely pointed triangular rostrum reaching mid-length to distal end of article 1 of antennular peduncle; carpopropodus of thoracic endopods 3–8 3-articulated; male pleopod 1 with 2-articulated linguiform endopod; male pleopods 2–5 with enlarged membraneous, transparent pseudobranchial lobes (exites) on basal article of endopods; telson entire, linguiform, with rounded to truncate apex; marked sexual dimorphism, female, lateral margins with 17–20 short spiniform setae, increasing in length posteriorly, apex with two pairs of subequal, long, stout spiniform setae, outer pair curved inward, inner pair straight to slightly curved, about 0.3 length of telson; outer-most apical setae 1.7–2.2 times length of most posterior lateral setae; male, lateral margins with 20–23 short spiniform setae, increasing in length posteriorly; apex with three pairs of fairly stout spiniform setae, inner-most pairs subequal in length; about 0.2 length of telson; outer-most apical pair 1.4–1.7 times length of most posterior lateral setae; uropodal endopod with 16–20 spiniform setae along inner median margin from region of statocyst to apex.

Description. General body form ( Fig. 1A View Figure 1 ): moderately slender, adult males to 6.4 mm and females to 5.5 mm; carapace with anterior margin produced into an acutely pointed triangular rostrum extending mid-length to distal end of article 1 of the antennular peduncle; posterior dorsal margin broadly emarginate, partially exposing thoracic somite 8; antero-lateral corners rounded.

Eyes ( Fig. 1A View Figure 1 ): cornea large, globular, broader than eyestalk, occupying distal 45% of eye, extending to article 2 of antennular peduncle.

Antennular peduncle ( Fig. 1B View Figure 1 ): more robust in males than females, shorter than antennal scale; article 1 subequal in length with article 3, disto-lateral epiprocess with 3–5 plumose setae, apophysis near disto-medial margin with 5–9 plumose setae; article 2 short, with 8–9 slender simple (smooth) and plumose setae and 1 blade-like plumose seta on disto-medial margin; article 3 with one long simple seta and one short plumose seta mid-lateral; group of 4–5 long simple setae on disto-medial margin; distal border with dorso-medial lobe bearing two tooth-like processes and 3–5 plumose setae; male lobe (appendix masculina) large and densely setose with no indentation on medial margin; outer antennular flagellum slightly thicker than inner.

Antenna ( Fig. 1C View Figure 1 ): scale lanceolate, extending well beyond peduncle, 5.5–7 times as long as maximum width; outer margin slightly concave to straight; inner margin slightly convex; apex with indistinct suture, tip about 0.15 scale length; all margins setose; peduncle 3-articulated; article 1 (not shown in Fig. 1C View Figure 1 ) slightly shorter than article 3 and about half as long as article 2; article 2 with 1 long and 2 short plumose setae on inner distal margin and 1 simple seta on outer distal margin; article 3 with 2 long simple setae and 3–4 shorter plumose setae on inner distal margin and 2 simple setae along outer distal margin; sympod with small tooth on distolateral corner.

Labrum ( Fig. 1D View Figure 1 ): well rounded, wider than long, middle half of bilobed posterior margin with cluster of short fine setae on one side of emargination and short coarse setae on the other.

Mandibles ( Fig. 1E View Figure 1 ): palp 3-articulated; article 1 small with no setae, article 2 with mostly simple setae on outer, inner, and distal margins; article 3 about 0.6 times as long as article 2, mostly with simple setae, but distal half with a long, curved spiniform seta armed bilaterally in mid-part with barbs, and 7–9 shorter spiniform setae with similar barbs along their entire lengths.

Gnathobasic surfaces of both mandibles without molar process; left mandible, incisor process with 6–7 teeth, lacinia mobilis with 5–6 teeth, spine row with three teeth and series of smaller spine-like teeth and bristles; right mandible, incisor process with 5–6 teeth, lacinia mobilis spherical process with 4 teeth, spine row with 2 large teeth and series of smaller teeth and bristles.

Maxillule ( Fig. 1F View Figure 1 ): outer lobe apex with 9 strong serrate setae; inner lobe with two apical microserrated setae and one medial microserrated seta along outer margin.

Maxilla ( Fig. 1G View Figure 1 ): exopod relatively narrow, with 7–9 plumose setae; endopod 2-articulated, distal article oval, about 2.4 times as long as wide, with 3 submarginal simple setae and series of simple and plumose setae along apex and inner margin; coxal endite with 2–3 plumose and 2 barbed setae; basal endite bilobed, with simple and plumose setae and 1 serrate seta on inner margins.

Thoracic endopods: endopod length increases in order of endopods 1, 2, (6, 7, 8 subequal), 5, (3, 4 subequal) ( Figs. 2A–G View Figure 2 , 3A View Figure 3 ); thoracic endopod 1 ( Fig. 2A View Figure 2 ): short and robust, typical of genus; ischium and merus fused (merischium), two times carpus length, slightly shorter than combined propodus and dactylus lengths, smooth setae along medial margin; carpus with 2–5 simple setae on medial margin, three serrate setae along lateral margin; propodus bent inward, several simple setae and one serrate seta on distolateral margin, 2–4 simple setae on disto-medial corner; dactylus densely setose with simple setae, strong curved terminal seta; thoracic endopod 2 ( Fig. 2B View Figure 2 ): ischium 0.7–0.8 length of merus; merus slightly shorter than carpopropodus, with one serrate and one simple seta on distolateral corner; carpopropodus with one strong pectinate and one plumose seta on disto-lateral corner; dactylus with three strong pectinate setae and two plumose setae on disto-lateral margin and inner face, strong curved terminal seta; thoracic endopods 3–8 ( Figs. 2C–G View Figure 2 , 3A View Figure 3 ): ischium length as compared to merus length decreases steadily from 1.4 (endopod 3) to 0.9 (endopod 8); merus 1.1–1.4 times as long as carpopropodus, one serrate seta on disto-lateral corner ( Fig. 2C View Figure 2 ); carpopropodus 3-articulated; middle article 0.4–0.8 length of other two articles, one serrate seta on disto-lateral corner of proximal two articles except endopod 8; dactylus with undulating medial margin, armed with strong, curved terminal seta.

Penes: (not figured) length 0.8–1.1 basal plate length of exopod of thoracopod 8; well developed, cylindrical, each penis with two apical lobes, one with two large simple setae, outer face with series of 10 plumose setae.

Thoracic exopods: basal plates with small tooth on outer distal corner; exopod 1 with 8 articles, exopods 2–8 with 9 articles ( Fig. 2B View Figure 2 )

Pleopods: female, uniarticulate setose plates ( Fig. 3B, C View Figure 3 ); pleopod 5 longest, reaching about 0.6 length of abdominal somite 5; male pleopods well developed; pleopod 1 ( Fig. 3D, E View Figure 3 ), 2-articulated endopod linguiform with 1 distal simple seta; article 1 with pseudobranchial lobe (exite) normally developed with 4–5 setulose setae; anterior face with group of 7–10 simple setae; exopod 8-articulated, about three times length of endopod; pleopods 2–5 biramous, endopods and exopods 8-articulated; pseudobranchial lobes enlarged, membraneous and transparent with 5 setulose setae; pleopod 4 ( Fig. 3F View Figure 3 ), exopod with terminal article having 1 simple subapical seta and one long barbed apical spiniform seta as long as the 4–5 distal articles combined.

Telson ( Fig. 3G, H View Figure 3 ): entire, linguiform, 1.4–1.6 times as long as wide at base, about 0.8 length of last abdominal somite; lateral margins moderately concave, apex rounded to truncate; marked sexual dimorphism, female, lateral margins with 17–20 short spiniform setae, increasing in length posteriorly, apex with two pairs of subequal, long, stout spiniform setae, outer pair curved inward, inner pair straight to slightly curved, about 0.3 length of telson; outer-most apical setae 1.7– 2.2 length of most posterior lateral setae; male, lateral margins with 20–23 short spiniform setae, increasing in length posteriorly; apex with three pairs of fairly stout spiniform setae, inner-most pairs subequal in length; about 0.2 length of telson; outer-most apical pair 1.4–1.7 length of most posterior lateral setae.

Uropod ( Fig. 3I View Figure 3 ): exopod 1.2–1.3 times as long as endopod, 1.4–1.5 times as long as telson, outer margin straight to slightly concave, inner margin convex; endopod 1.1–1.3 times as long as telson, outer and inner margins slightly concave and setose, inner margin with 16–20 mostly subequal spiniform setae from region of statocyst to apex.

Colour (in preserved specimens): body uniformly brown to beige with diffuse brown pigmentation irregularly distributed on eyestalks, carapace, thorax, abdomen, telson, and appendages; heavy pigmentation near posterior ventral margins of abdominal somites, dorsal base of telson, and posterior-most pair of oostegites.

Etymology. The species name is a Latin adjective with female gender referring to the similarity of the new species to its Atlantic cognate, M. furca Bowman, 1957 .

Habitat. Collected from sandy beaches in depths of 0.5 to 1.0 m.

Type locality. Costa Rica, Puntarenas, Playa Caldera (09°92’75”N 84°71’64”W), grey sand beach, depth 1 m.

Distribution. At present known only from the provinces of Guanacaste and Puntarenas along the Pacific coast of Costa Rica.

Remarks. Mysidopsis gemina n. sp. has its closest morphological affinities with M. furca . These two species, respectively, appear to represent Eastern Pacific and Western Atlantic cognate or sister species. Both differ from other known members of the genus Mysidopsis by the distinctive sexual dimorphism expressed by their telsons. The new Costa Rican species is distinguished from M. furca by a variety of characters, including the (1) acutely pointed triangular rostrum (blunt in M. furca ), (2) male pleopods 2–5 with enlarged pseudopodal lobes on the endopods (lobes normally developed in M. furca ), (3) male first pleopod having a biarticulated endopod (uniarticulate in M. furca ), and (4) fewer number of spiniform setae (16–20) on the uropodal endopod (20–45 in M. furca ). For additional morphological details for these and other characters for distinguishing the two sister species, see Tab.1.