A new species of Mesochra (Copepoda: Harpacticoida: Canthocamptidae) from a coastal system of northern Colombia with a key to the American species

Plankton samples obtained from the Laguna Navio Quebrado, La Guajira, northern Colombia, yielded the first record of the genus Mesochra Boeck, 1865 for Colombia. It is represented by an undescribed species, M. huysi sp. nov., which appears to be closely related to M. parva Thomson, 1946 and M. pseudoparva Gómez & Fiers, 1997. However, the new species can be distinguished from these two species by the insertion point of the inner seta of P1 ENP1, the length/width ratio of P1ENP, the inner seta of P1ENP2, the relative length of P3 and P4 ENP, by details of elements of P5 EXP female, the structure of the anal operculum, the remarkable ornamentation of the male P3 EXP1-2, the apophysis of the male P3 ENP2, and the claw-like or flame-shaped element of male P3 ENP3. This is the third record of a species of the genus from the Caribbean. A key to the 11 species of Mesochra known from the Americas is provided. http://zoobank.org/urn:lsid:zoobank.org:pub:02DD40CD-ECBA-4A5D-AE80-14D185376272


Introduction
The Family Canthocamptidae was established by Sars (1906) by upgrading the subfamily Canthocamptinae Brady, 1880. It is the largest family in the Harpacticoida, currently comprising about 627 species, of which only 17% are known from the Neotropical region (Boxshall and Defaye 2008). Most canthocamptids inhabit fresh water (Wilson 1971) but some genera can dwell in coastal marine habitats or in brackish water, e.g. Cletocamptus and Mesochra Boeck, 1865 (Boxshall and Defaye 2008); the latter genus has been reported also from coral reef environments (Sarmento and Parrera Santos 2012). Among the 52 valid canthocamptid genera, Mesochra is one of the largest, currently comprising more than 45 valid species (Boxshall and Halsey 2004;Gaviria-Melo et al. 2013).
A key for the identification of the American species of the genus is also presented.

Material and methods
Biological samples of benthos and plankton were taken from the Laguna Navío Quebrado, Colombia (11°25ʹ N, 73°05ʹ W) from April to December 2012, mainly in the littoral areas with vegetation (macrophytes and mangrove) but also from open water in areas adjacent to shallow oyster banks. Water salinity was measured with a WTW 3111 conductivity meter. Water samples were collected using a 25 l bucket at both vegetation areas and open water. Samples were filtered with a standard zooplankton net (45 μm mesh) and fixed and preserved in 70% ethanol. Copepods were sorted from the original samples. Dissected specimens and appendages were mounted in glycerine and sealed with Canada balsam or Entellan®. Drawings of the mounted appendages were prepared at 1000× magnification with the aid of a camera lucida adapted to a compound microscope. The specimens were measured in lateral position, from the anterior end of the cephalic area to the posterior margin of the caudal rami. Morphological nomenclature follows Huys and Boxshall (1991) and Huys et al. (1996). The following abbreviations are used in the text and tables: P1-P6, first to sixth swimming legs; EXP, exopod; ENP, endopod. The type specimens examined were deposited in the collection held at the Museo de Colecciones Biológicas de la Universidad del Atlántico (UARC), Barranquilla, Colombia and in the Collection of Zooplankton at El Colegio de la Frontera Sur in Chetumal, Mexico (ECO-CHZ).

Description of female
Body fusiform, slightly tapering posteriorly ( Figure 1A). Habitus in dorsal view as in Figure 1A, in lateral view as in Figure 1B. Total body length, measured from tip of rostrum  to posterior margin of caudal rami ranging from 336 to 403 μm (average 374 μm, n = 10; holotype: 336 μm). Rostrum distinct, flat in dorsal view, subtriangular, with rounded tip in lateral view. Dorsal surface of cephalosome and free prosomites with incised hyaline frill. Transverse row of microspinules on dorsal surface of first three pedigerous free somites, fourth free somite without dorsal spinulation. Genital double-somite with vestigial suture visible in ventral view, first genital somite with row of small spinules on mid-dorsal surface and posterior frill; second genital somite with transverse row of spinules and weak frill along posterior margin on dorsal surface ( Figure 1A); lateral and ventral margins with larger set of spinules ( Figures 1B, 4D). First and second free urosomites with mid-dorsal row of minute spinules and row of spinules along posterior margin, spinules increasing in size laterally and ventrally ( Figure 4D). Anal somite with separate transverse rows of minute spinules on mid-dorsal surface, distal margin with short row of spinules along insertion of caudal rami. Anal operculum rounded, ornamented with slender spinules ( Figure 4F). Caudal rami ( Figure 4F) subquadrate, with six setae, with spinules along ventral surface. Seta II with accompanying spinules, and about as long as caudal rami, seta III 1.2 times as long as seta II, seta V about 4.2 times longer than seta IV and seta VII as long as seta II.
Antenna. Allobasis with two short setae, one proximal and one on subdistal position. Exopod short, cylindrical, 1-segmented, with three unequally long terminal setae, middle setae being longest. Endopod 1-segmented, inner margin with row of spinules on proximal 1/3 of segment, lateral armature represented by two subequal spines plus two outer spinules distally. Distal margin with five setal elements, two spines, two geniculate setae and one stout seta furnished with spinules ( Figure 2A).
Mandible. Gnathobase with strong, wide teeth and one plumose dorsal seta, basis elongate, armed with one basipodal seta and row of long spinules. Endopodal lobe armed with four setae ( Figure 2B).
Maxillule. Praecoxal arthrite with two setae. Distal margin with five strong curved claws and four setal elements. Coxa and basis partially fused; coxal endite with two slender setae. Basis with one seta. Endopodal and exopodal rami fused to basis, each represented by four and four setae, respectively ( Figure 2C).
Maxilla. Syncoxa ornamented with row of minute spinules, with two endites, each with three setal elements, two of them wide-based, spinulated, the other slender. Basis forming strong claw with one slender seta. Endopod represented by three slender setae ( Figure 2D).
Maxilliped. Syncoxa furnished with subdistal row of spinules, with inner distal plumose seta reaching half-length of basis. Basal segment robust, ornamented with longitudinal row of long spinules from proximal margin to distal 1/3 of segment; distal outer margin with row of small spinules. Endopod represented by weakly curved claw with an accompanying proximal seta ( Figure 2E). P1 ( Figure 3A). Protopod ornamented with row of spinules along posterior margin, coxa with proximal rows of minute spinules, additional rows also along outer and posterior margins. Basis with curved row of spinules on middle surface; segment with strong spinules along distal margin, particularly near insertion of endopod; with outer and inner spine, latter stout, straight, reaching about 1/3 of ENP1. ENP 2-segmented, longer than EXP, first segment reaching well beyond length of EXP, about 7 times as long as wide, inner seta on first segment inserted almost at the half of segment; second segment ornamented with outer row of spinules, armed with three elements, inner subdistal one shorter than segment. EXP 3-segmented, reaching beyond insertion of inner seta on ENP1, EXP1 and EXP2 without inner setae, third segment with four elements, two of them geniculate. P2 ( Figure 3B). Coxa and basis with weaker ornamentation than in P1. Basis with slender outer basipodal seta. ENP 2-segmented, shorter than EXP, not reaching midlength of EXP3. ENP1 with outer seta, without inner seta, ENP2 with five setal elements. EXP 3segmented, EXP1 without inner seta, EXP2 with short inner seta, EXP3 with five elements. P3 ( Figure 3C). Coxa and basis with weaker ornamentation than in P1, as in P2. Basis with slender outer basipodal seta. ENP 2-segmented, barely reaching distal end of EXP2; ENP1 with outer seta, without inner seta; ENP2 with five elements. EXP 3-segmented, EXP1 and EXP2 without inner setae, third segment with six elements. P4 ( Figure 3D). Coxa and basis with weaker ornamentation than in P1, as in P2 and P3. Basis with slender outer basipodal seta. ENP 2-segmented, short, reaching only proximal 1/3 of EXP1; ENP1 with inner seta, without outer seta; ENP2 with five setal elements. EXP 3-segmented, EXP1 with inner seta, third segment with six elements, one of them a thickened pectinate seta (arrowed in Figure 3D). P5 ( Figure 4C). Baseoendopod with single seta. EXP rounded, shorter than endopodal lobe, with five elements, inner middle distal seta longer than inner distal spine. Endopodal lobe armed with five elements ( Figure 4E). Setal formula (Arabic numerals = setae, Roman numerals = spines) of P1-P4 as in Table 1. P6 ( Figure 1D) vestigial, represented by small crescent-shaped lobe ( Figure 1D), with one short seta.

Description of male
Habitus resembling that of female except for narrower prosome and urosome ( Figure 4D). Smaller than female, total body length, measured from tip of rostrum to posterior margin of caudal rami ranging from 263 to 315 μm (average 289 μm, n = 5; holotype: 263 μm). Somitic ornamentation as in female. Urosomites 1-3 with ventral row of large spinule along posterior margin ( Figure 4D) which are coarser than those along the dorsal surface. Row of spinules along ventral margin of preanal somite with discontinuous row of spinules, as opposed to female, with continuous such row. Anal somite as in female, with smaller spinules on ventral surface and along insertion of caudal rami. Armature of caudal rami as in female.

Etymology
The new species is named after Dr Rony Huys (Natural History Museum, London, UK), for his outstanding contributions to the taxonomic knowledge of the Harpacticoida.
The male of M. huysi sp. nov. differs from the male of its congeners M. pseudoparva, M. pacifica and M. parva by the features of the outer spinules on the first and second segments of P3 EXP; such spinules are remarkably strong in M. huysi while in the other three species, M. parva (Hamond 1971, fig. 20), M. pacifica (Gómez and Fiers 1997, fig. 7b) and M. pseudoparva (Gómez and Fiers 1997, fig. 13a) these elements are slender, regular spinules. The apophysis of P3 ENP2 differs among these species. This structure does not reach the distal end of P3ENP3 in M. pseudoparva (Gómez and Fiers 1997, fig. 13a), M. pacifica (Gómez and Fiers 1997, fig. 7b) and M. huysi ( Figure 4B) but in M. parva it is clearly shorter, not reaching the midlength of P3ENP3 (Hamond 1971, fig. 20). The claw-like or flame-shaped elements in P3 ENP3 are unreported or absent in both M. parva (Hamond 1971, fig. 20) and M. pacifica (Gómez and Fiers 1997, fig. 7b), but one is present in M. huysi (arrowed in Figure 4B) and two in M. pseudoparva (Gómez and Fiers 1997, fig. 13a, b). As mentioned by Gómez and Fiers (1997), these structures are likely to be overlooked by its size and position in some cases or even confused with other setal elements; their homologies or function cannot be established yet. The male P5 EXP has six setal elements in M. huysi, M. pseudoparva (Gómez and Fiers 1997, fig. 13c) and M. pacifica (Gómez and Fiers 1997, fig. 7c), while only five are present in M. parva (Hamond 1971, fig. 21). Following Wells' (2007) key, our specimens are grouped among several species of Mesochra (key KG 33) (M. sewelli Lang, 1948, M. meridionalis Sars, 1905, M. pseudoparva, M. parva, M. wolskii Jakubisiak, 1933, M. lindbergi Petkovski, 1964, M. rostrata Gurney, 1927and M. aestuarii Gurney, 1921 by its possession of a combination of characters including the segmentation of P1 rami, the number of segments on P2ENP and P4ENP, and the number of setae on the EXP3 of P2-P4. The new species is then compared (KG 33/2) with M. pacifica and M. suifunensis Borutsky, 1952, both sharing the same ornamentation of the anal operculum, armature of ENP2 of P2-P4 and armature of male and female fifth legs. Mesochra suifunensis clearly differs from the new species, M. huysi, in the lack of inner seta on ENP1 of P2 and P3; these setae are present in the new species. Also, the male of M. suifunensis lacks an apophysis in the P3, which is present in M. huysi. Therefore, the unique combination of characters of these specimens from Colombia appears to be enough evidence to consider them as belonging to a new species of Mesochra.

Distribution and ecology
Hamond (1971) recognized 31 valid species of Mesochra and presented a key based on Lang's (1948) work. Fiers and Rutledge (1990) recognized, apart from Mielke's (1974) Mesochra sp., 34 valid species. Gómez and Fiers (1997) added M. pacifica and M. pseudoparva. The number of species increased with the addition of three species from Iceland (Gómez and Steinarsdóttir 2007), and one from South Korea (Lee and Chang 2008). Gaviria-Melo et al. (2013) recognized up to 45 species of Mesochra; hence, with the addition of this new species from Colombia, the number of species in the genus rises to 46. Overall, from the valid species of Mesochra, only about 25% of them have been recorded in the Americas, Brazil harbouring the highest number of species of this genus (Reid 1998).
According to Fiers and Rutledge's (1990) and to our comparative analysis, only 11 species (M. lindbergi Petkovski, 1964, M. dulcicula Jakobi, 1956, M. suifunensis Borutsky, 1952, M. meridionalis Sars, 1905, M. aestuari Gurney, 1921, M. wolskii Jakubisiak, 1933, M. rostrata Gurney, 1927, M. sewelli Lang, 1948, M. parva Thomson, 1946, M. pacifica Gómez & Fiers, 1997, and M. pseudoparva Gómez & Fiers, 1997 have a 2-segmented P1 ENP combined with a 222 spine formula (number of spines on the ultimate exopodal segment of P2-P4). The new species M. huysi can be assigned to this group. Only four species of this group (M. dulcicula, M. pacifica, M. pseudoparva and the new species, M. huysi) are known to be distributed in the Americas. There are only 10 species of the genus known from the continent (Gómez and Fiers 1997;Suárez-Morales et al. 2006;Sarmento and Parrera Santos 2012;Gómez and Morales-Serna 2014), a figure that reaches 11 with the addition of the new species. Only two of these species of Mesochra have been known from the Caribbean, the nominal M. pygmaea (Claus, 1863) from Barbados (Coull 1970), which is probably a species complex (Wells 2007), and a Mesochra sp. from the Mexican Caribbean coast (Suárez-Morales et al. 2006). This is the third record of a species of this genus from the Caribbean.
Mesochra huysi is currently known from a single locality, Laguna Navío Quebrado (Colombia). It was found in the limnetic region associated to a salinity of 28 psu. This large (surface area = 10.7 km 2 ) and shallow (depth range = 30-110 cm) lagoonal system has a temperature ranging between 28 and 31°C, and pH values were 7.8-8.3. This is the same kind of lagoonal environment from which its closest congeners, C. pacifica, C. parva and C. pseudoparva (Hamond 1971;Gómez and Fiers 1997) Wells (2007) this nominal species is probably a widely distributed species complex; American records should be treated and compared in detail.