Heteromysis cocoensis n . sp . ( Crustacea : Mysida : Mysidae ) from coastal waters of Isla del Coco , Costa Rica

A survey of the invertebrate fauna of coral reef hard bottom communities in the shallow waters of Isla del Coco yielded a new species of mysid belonging to the genus Heteromysis S. I. Smith, 1873. Heteromysis (Olivemysis) cocoensis, n. sp. was collected from coral rubble at depths of 8 to 34 m. It differs from its congeners by having male pleopods 1, 3, and 4 with modified setae. Within the subgenus Olivemysis Băcescu, 1968, the new species is morphologically most similar to Heteromysis ekamako Wittmann and Chevaldonne, 2017 from the Pacific, Heteromysis gomezi Băcescu, 1970, Heteromysis mayana Brattegard, 1970, and Heteromysis rubrocinta, Băcescu, 1968 from the Western Atlantic, and Heteromysis dardani Wittmann, 2008, Heteromysis wirtzi Wittmann, 2008, and Heteromysis sabelliphila Wittmann and Wirtz, 2017 from the Eastern Atlantic. However, H. cocoensis n. sp. is distinguished from these six apparently closely related species by the following combination of characters: flagellate, modified setae on articles 1 and 3 of the antennular peduncle, and setation of thoracic endopod 3, male pleopods 1, 3 and 4, uropodal endopods, and the apical and lateral margins of the telson. A diagnostic table separating these eight species is given.


New species of Heteromysis from Costa Rica
Nauplius, 26: e2018012 inTroduCTion Price (2004) and Price et al. (2009) published annotated compilations of the published literature for the members of the family Mysidae Haworth, 1825 sensu lato reported from the Pacific waters of the Americas and specifically Costa Rica, respectively.Based on these reports and our review of the more recent literature, there are no published descriptions or records of mysids from Isla del Coco (5 o 30'-5 o 34'N 87 o 02'-87 o 06'W).
The marine biodiversity of Isla del Coco, which lies 550 km West Southwest of the Costa Rican mainland (see Fig. 1), is relatively well known, with more than 1,100 species reported to date (Cortés and Wehrtmann, 2009;Sibaja-Cordero et al., 2016).Notwithstanding, new taxa continue to be discovered.Between 2007 and 2011, during surveys of coral reef hard bottom habitats, an undescribed species of mysid belonging to the genus Heteromysis S. I. Smith, 1873 was discovered.The description of this new species is presented herein.

MaTerial and MeThods
Mysids were sampled from coral reefs along the northeastern end of Isla del Coco and the north side of its satellite Isla Manuelita in Oct. 2007, May 2008, and Jul. 2011.Using SCUBA, coral rubble was collected manually, brought to the surface, and gently washed in seawater.Specimens collected in 2008 were captured alive and preserved in 95% ethanol.Specimens taken in 2007 and 2011 were preserved in a 3% formalinseawater solution and later transferred to 70% ethanol for examination.Body length was determined as being the distance from the anterior margin of the carapace to the posterior margin of the telson, excluding setae.Type material and additional specimens are deposited in the University of Costa Rica, School of Biology, Zoology Museum (MZUCR) and the National Museum of Natural History (USNM).Diagnosis.Antennular peduncle, article 3 with distomedial flagellate seta possessing small tubercles distally, article 1 with distodorsal flagellate seta; thoracic endopod 3 with 7-9 robust flagellate setae with microdentations on medial margin of carpopropodus; male pleopod 1 with 6 robust spiniform simple setae along distomedial and distal margins; male pleopod 3 with 6 stout flagellate setae on distal margin; male pleopod 4 with 28-36 small simple setae (no distal flagella) on distal margin; uropodal endopod armed with 2-3 spiniform setae along medial margin in region of statocyst; telson, lateral margins armed along posterior 0.3-0.4length with 6-7 spiniform setae per margin (including apical setae); outer apical seta 3.3-5 times longer than inner; cleft depth 0.25-0.30times telson length, cleft completely armed with 16-21 spinules.
Eye (Fig. 2A): moderately large, oval, distal end of eyestalk slightly wider than cornea with ocular tooth on anteromedial margin; cornea large, oval, occupying distal third of eye.
Mandible (Fig. 2D): cutting edges typical of genus, molar processes with strong grinding surfaces on
Labrum (Fig. 2E): typical of genus.oval, posterolateral margins rounded with dense clusters of fine setae; anterior margin not produced into a spiniform process.
Marsupium: females with pairs of developed oostegites on bases of the medial margins of thoracopods 7 and 8, respectively.
Penes: penes cylindrical, stiff with smooth cuticle and no discernable setae; about 6 times longer than wide; extending forward to base of thoracopod 6.
Etymology.The name refers to the type locality, Isla del Coco.
Habitat.Heteromysis (Olivemysis) cocoensis n. sp.collected from coral rubble washings with sponges, algae, and hydroids in depths of 8-34 m.Collecting techniques were too general to determine if this species was associated with a specific host; however, many species of Heteromysis are associated symbiotically with benthic invertebrates such as sponges, anthozoans, ophiuroids, and shells inhabited by hermit crabs (see Fukuoka, 2005 for details).
Within the Pacific members of the subgenus Olivemysis, H. cocoensis n. sp.appears to be most morphologically similar to H. ekamako, described from marine caves in the Marquesas Islands in the central Pacific, 6,000 km west of Isla del Coco.Both species have similar telsons, uropods, and third thoracic endopods.However, H. cocoensis n. sp.differs from the central Pacific species by having (1) a modified seta on articles 1 and 3 of the antennular peduncle, as opposed to a modified seta on all three articles; and (2) male pleopods 1, 3, and 4 modified as opposed to male pleopods 2, 3, and 4. In addition, the new species has male pleopod 3 with 28-36 apparently nonflagellate setae as compared to 13-26 flagellate setae for H. ekamako (see Tab. 1).
Of the Atlantic members of the subgenus, H. cocoensis n. sp.most closely resembles H. mayana, which is distributed throughout the Caribbean (Price and Heard, 2004), and H. gomez and Heteromysis rubrocincta Băcescu, 1968 from Cuba from the W. Atlantic as well as three species from the E. Atlantic: H. dardani and H. wirtzi from Madeira Island, and Heteromysis sabelliphila Wittmann and Wirtz, 2017 from the Cape Verde Islands.Although the telsons of these seven species are relatively similar, the outer: inner length ratio of the apical telsonal setae of H. cocoensis n. sp. is greater (3.3-5.0)than the other six species (1.8-3.0).The new species is further distinguished from its W. Atlantic congeners by having (1) the telson with lateral setae restricted to the posterior 40% of the telson as opposed to 50-70%; and (2) male pleopods 1, 3, and 4 modified as opposed to male pleopods 1-4 or 3-4 modified.Male pleopods 1 and 2 of H. gomezi and H. mayana have a single, large spiniform seta at the distal end rather than 6 robust, spiniform setae along medial and distal margins of pleopod 1 and a normal pleopod 2 for H. cocoensis n. sp.In addition, the new species has 28-36 small, apparently non-flagellate  1) 2-3 spiniform setae on the uropodal endopod rather than 4-5 and (2) 7-9 flagellate setae on the carpopropodus of thoracic endopod 3 rather than 0 (see Tab. 1).