A new species of Monstrilla (Copepoda, Monstrilloida) from the plankton of a large coastal system of the northwestern Caribbean with a key to species Author Suarez-Morales, Eduardo Author Castellanos-Osorio, Ivan A. text ZooKeys 2019 876 111 123 http://dx.doi.org/10.3897/zookeys.876.38400 journal article http://dx.doi.org/10.3897/zookeys.876.38400 1313-2970-876-111 DD7E5DEB446344A4A485B5071CA3578B 8E16ED537DA45725AA532353003F99ED Monstrilla chetumalensis sp. nov. Figures 2 , 3 , 4 , Table 1 Material examined. Holotype adult male (ECO-CH-Z-10330), Chetumal Bay, near Mexico-Belize international border ( 18°26'54"N ; 88°04'00"W ) on 27 July 1997 by I. Castellanos-Osorio. Specimen partially dissected, cephalothorax and urosome in a vial, ethanol-preserved. Appendages including antennules and legs 1-4 mounted on semi-permanent slide with glycerine, sealed with acrylic varnish. Etymology. The epithet of the new species is a toponym that refers to Chetumal Bay, the type locality of this species. Diagnosis. Small-sized male Monstrilla (0.73 mm), with body divided in relatively short, robust prosome, pedigerous somites 2-4 tapering posteriorly, and slender urosome. Cephalothorax with low, rounded medial rostral projection, with both dorsal and ventral cuticular ornamentation. Antennule 5-segmented geniculate antennules. Geniculation between segments 4 and 5. Fifth pedigerous somite separated from preceding somite. Posterolateral margins produced and partially overlapping succeeding somite, visible in lateral and dorsal views. Somite with two small rounded ventral processes visible in lateral view. Legs 1-4 with outer sea on basis; exopods and endopods 3-segmented. Leg 5 absent. Genital somite with dorsal field of transverse striations; ventral genital complex represented by short shaft with distal laterally diverging lappets with rugose anterior surface, branches with dorsally directed apical spiniform processes, probably representing opercular flaps; lappets connected medially by dentate margin. Caudal rami with four subequally long caudal setae. Description of adult male holotype. Body shape and tagmosis as usual in male Monstrilla ( Huys and Boxshall 1991 ; Suarez-Morales 1993 , 1996 , 2003 ) ( Fig. 2A, B ). Total body length of holotype individual 0.69 mm, measured from anterior end of cephalothorax to posterior margin of anal somite. Cephalothorax representing 47.5% of total body length. Succeeding pedigerous somites 2-4 each with pair of biramous swimming legs; pedigerous somites 2-4 combined accounting for 31% of total body length in dorsal view. Cephalic region wide, bilaterally protuberant in dorsal view, narrower than cephalothorax; outer margin of cephalic protuberances corrugate. Pair of small dorsal pit setae present between antennulary bases; ventral anterior surface also with two pit setae (1, 2 in Figs 2A , 3A , respectively). Forehead moderately produced, weakly rounded, with coarsely rugose anterior margin and field of transverse striations on dorsal anterior surface; no other cephalic ornamentation discernible on dorsal anterior surface ( Fig. 2A ). Cephalothorax robust, 0.36 mm long, representing 47.5% of total body length; dorsal surface with scattered dorsal pores ( Fig. 2A ). Midventral oral papilla moderately protuberant ( Fig. 2B ), located at about proximal 1/3 (0.31) along ventral surface of cephalothorax. Pair of relatively small lateral pigment cups moderately developed, separated by length of less than one eye diameter, weakly pigmented; ventral cup slightly larger than lateral cups. Preoral ventral surface with low, wide-based rounded process protruding between antennulary bases (arrowed in Fig. 2B ); nipple-like cuticular processes surrounded by striated surface ( Fig. 3A ). Figure 2. Monstrilla chetumalensis sp. nov., male holotype A habitus dorsal view B habitus lateral view, arrow indicates medial ventral protuberance C urosome ventral view; arrow indicates ventral globular processes on fifth pedigerous somite D urosome lateral view; arrow indicates ventral globular processes on fifth pedigerous somite E genital complex with lappets, ventral view. Scale bars: 100 μm . ( A-C ) Figure 3. Monstrilla chetumalensis sp. nov., male holotype ( A-E , C ) A anterior part of cephalosome ventral view; arrow indicates nipple-like processes; s = sensilla B antennule segments 1-4 in dorsal view showing setal elements (sensu Grygier and Ohtsuka 1995 ) C fifth antennulary segment with setal elements (sensu Huys et al. 2007 ) D urosome lateral view E genital complex with lappets ventral view. Scale bars: 200 μm ( A, B ), 100 μm ( C, D ), 50 μm ( E ). Table 1. Armature of legs 1-4 including basis, exopods, and endopods. (Roman numerals indicate spiniform elements, Arabic numbers indicate setiform elements, set from inner to outer positions).

-	Basis	Exopod	Endopod
Leg 1	1-0	0 -I;0- 1;2,2,I	1-0;1-0;2,2,I
Legs 2-4	1-0	0-I;1-0;2,2,1, I	1-0;1-0; 2,2,1, I

Antennule relatively robust ( Fig. 3B ), 5-segmented; length = 0.53 mm, representing 38% of total body length, and 75% of cephalothorax length, with segments 1-4 separated by complete sutures. Intersegmental division between segments 3 and 4 lacking suture, division marked by constriction; segment 4 being longest: geniculation between segments 4 and 5 ( Fig. 3B ). Armature, using terminology of Grygier and Ohtsuka (1995) for female monstrilloid antennular armature of segments 1-4 complemented with nomenclature by Huys et al. (2007) for elements on male fifth antennule segment, antennulary segment (1) element 1 present on first; element setiform, setulated, distinctively long, reaching well beyond distal margin of second segment. (2) elements 2d1, 2, 2v1-3, and IId expressed on second segment. (3) third segment with elements 3, IIId, and IIIv with setal element 3 setiform, pinnate, remarkably long, reaching beyond proximal half of succeeding fourth segment. (4) Segment four bearing normally developed elements 4d1, 2 and 4v1-3 as well as setae IVd, IVm, and IVv; elements of group 4v1-3 short, spiniform, except for long, setiform, spinulose element 4v3. Slender aesthetasc 4aes in ventral position. (5) terminal segment armed as follows (sensu Huys et al. 2007 ): elements 1-7 present on anterior margin, with three branched setal elements A-C ( Fig. 3C ); segment with small apical aesthetasc (element 2 in Fig. 3C ). Terminal segment lacking unusual features or ornamentation ( Fig. 3C ). Legs 1-4 with smooth intercoxal sclerites of rectangular, smooth. Bases with straight inner margins; outer basal setal sparsely setulose on legs 1-4; on leg 3, outer basal seta about twice as long as and slightly thicker than in other legs. Endopods and exopods of triarticulated, outer margins of exopods smooth. All elements setiform and biserially plumose except for outer spines on first segments and outer apical spiniform seta on third exopodal segments displaying and third exopodal segments displaying sparsely spinulose inner margin and smooth outer margin ( Fig. 4 A-D ). Armature of legs 1-4 as: Figure 4. Monstrilla chetumalensis sp. nov., male holotype A leg 1 B leg 2 C leg 3 D leg 4. Scale bars: 100 μm . Urosome consisting of fifth pedigerous, genital somite (carrying genital complex), two short, free postgenital somites divided by incomplete dorsal suture, and short anal somite ( Fig. 2 B-D ). Fifth pedigerous somite with ventrally produced proximal half; dorsal surface smooth. Distal half of fifth pedigerous somite with pair of small medial rounded processes visible in lateral view (arrowed in Fig. 2C, D ). Posterolateral margins of fifth pedigerous somite produced, partially overlapping succeeding genital somite both dorsally and laterally ( "ple" in Fig. 2A , arrow in Fig. 2B ). Genital somite slightly shorter than fifth pedigerous somite; genital complex of type I ( Suarez-Morales and McKinnon 2014 ), represented by short, robust ventrally expanded shaft; complex with short, widely divergent lappets tapering distally into apical subtriangular opercular process ( Fig. 2 C-E ). Lappets with rugose anterior surface, branches connected medially by wide dentate margin. Anal somite 1.3 times as long as genital somite. Caudal rami subquadrate, approximately 1.1 times as long as wide and about 0.7 times as long as anal somite. Each ramus armed with four subequally long caudal setae ( Fig. 2A ). Remarks. The new species differs from the males of other known congeners in several respects. Firstly, there are only a few other male Monstrilla with divergent genital lappets that point backwards and end in a subtriangular or spiniform opercular process. The male of Monstrilla chetumalensis sp. nov. most closely resembles the Indian species Monstrilla lata Desai & Bal, 1962. Both have similar body proportions, cephalothorax ornamentation, and paired, divergent genital lappets, each with a short distal opercular structure. However, in M. chetumalensis , the lappets are strongly curved and have an inverted U-shape ( Desai and Bal 1962 , figs 4, 5). Moreover, the antennulary armature differs strongly between these two species, particularly in the size and number of setal elements of segments 1-4 ( Desai and Bal 1962 : fig.3). In addition, M. lata has six caudal setae ( Desai and Bal 1962 : fig. 5) vs four in the new species ( Fig. 2A ). In M. lata the fifth pedigerous somite has a weak concavity in its proximal half ( Desai and Bal 1962 : fig.4), whereas the same structure is ventrally produced in M. chetumalensis ( Fig. 2D ). The genital complex of the new species shares some features with M. papilliremis Isaac, 1975 from South Africa. Both have divergent lappets with distal subtriangular opercular structures pointing backwards ( Isaac 1975 ); however, in M. papilliremis , the lappets are medially connected by a smooth margin with a medial notch and also have an inverted U-shape, thus diverging from the conditions observed in the genital complex of M. chetumalensis . In two other well-known species of the genus, M. longicornis Thompson, 1890 and M. longiremis Giesbrecht, 1893, the genital lappets are also divergent, connected medially by a smooth, straight margin ( Sars 1921 ; Huys and Boxshall 1991 ; Suarez-Morales 2010 ) and thus diverging from the dentate condition observed in M. chetumalensis sp. nov. In addition, both M. longicornis and M. longiremis have a 1-segmented leg 5 (see Sars 1921 ; Suarez-Morales 2010 ), which is absent in the new species. Backwardly directed genital lappets, as those observed in M. chetumalensis sp. nov., were reported in male M. longicornis by Huys and Boxshall (1991) and also in male M. longiremis by Suarez-Morales (2010) . Secondly, the presence of small rounded ventral processes on the fifth pedigerous somite have been reported previously only in the Caribbean M. marioi Suarez-Morales , 2003 ( Suarez-Morales 2003 : fig. 4), but in this species the process involves three small lobes ( Suarez-Morales 2003 : fig. 4) instead of two observed in M. chetumalensis sp. nov. ( Fig. 2D ). The distinctive characters observed in our male specimen appear to be enough evidence to support its assignment as a new species. Also, we considered the resemblance of the described male with males of the recently described genus Caromiobenella Jeon, Lee & Soh, 2018, which is known from males only. It has been recognized ( Grygier and Ohtsuka 2008 ) that the type species of Monstrilla , M. viridis Dana, 1849 should be redescribed from a neotype in order to clearly define the genus delimitation and clarify the status of related genera. Morphological comparison shows that M. chetumalensis sp. nov. is assignable to Monstrilla , as it clearly diverges from Caromiobenella in the following characters: In male Caromiobenella branched antennulary setae are absent from the fifth antennulary segment ( Jeon et al. 2018 : fig. 2C), but in the new species setae A and B of thee fifth antennulary segment are branched ( Fig. 3C ). The cephalothoracic ornamentation in Caromiobenella spp. includes two pairs of large dorsal crater-like depressions and pitted sensilla ( Jeon et al. 2018 : fig. 1A); these structures are absent in M. chetumalensis in which the cephalothoracic ornamentation is represented mostly by a conspicuous pattern of striations on its ventral surface and a few scattered dorsal pores ( Fig. 2A, B ). In addition, the two outermost setae on the third exopodal segments of legs 1-4 are serrate along the outer margin and smooth along the inner margin in Caromiobenella ( Jeon et al. 2018 : fig. 3 C-E ). In M. chetumalensis a distinct condition was observed: these exopodal setae are either smooth or serrate along the inner margin ( Fig. 4 A-D ). Also, species of Caromiobenella have five or six caudal setae ( Jeon et al. 2018 ), whereas only four caudal seta are present in the new species ( Fig. 2A ). According to Jeon et al. (2018) , the presence of a type 3 male antennule (see Huys and Boxshall 1991 ; Suarez-Morales 2011 ), with a modified fifth segment bearing distal transverse serrate ridges on the inner distal margin, is a diagnostic character to species of Caromiobenella . The new species has an unmodified (type I) antennule. In Caromiobenella the genital complex is represented by a robust shaft and short, subtriangular non-divergent distal lappets with a medial smooth protrusion ( Jeon et al. 2018 : fig.7C) and paired medial opercular flaps. In M. chetumalensis the genital complex is also robust and short, but lappets are strongly divergent and are medially joined by a dentate margin. No such medial opercular flaps were observed in the new species; these are probably represented by the terminal structures on the tip of each lappet. Currently, the are eight species of Monstrilla recorded from different coastal or reef areas of the Mexican Caribbean: M. reidae Suarez-Morales , 1993a (male) from Bahia de la Ascension , M. mariaeugeniae Suarez-Morales and Islas-Landeros, 1993 (female) from off Puerto Morelos reef zone, M. ciqroi ( Suarez-Morals , 1993b) (female) from Bahia de la Ascension, M. barbata Suarez-Morales & Gasca, 1992 (female) (see Suarez-Morales et al. 2006 ), M. elongata Suarez-Morales , 1994 (both sexes) (see Suarez-Morales 1996 ) from Puerto Morelos reef zone, M. globosa Suarez-Morales , 2003 (male) and M. marioi Suarez-Morales , 2003 (male), and M. rebis Suarez-Morales , 1993b (female) from Bahia de la Ascension (see Suarez-Morales and Gasca 1992 ; Suarez-Morales 1993a , 1993b , 1996 , 2003 ; Suarez-Morales and Islas-Landeros 1993 ; Suarez-Morales et al. 2006 ).