The Caribbean basin hosts high copepod richness, registering between 355 (^{Suárez-Morales et al., 2009}) and approximately 700 species, and the number of records is increasing according to the database maintained by the Banyuls Oceanographic Observatory (^{Razouls et al., 2005}-2015). However, few studies have been published on the specific composition of the zooplankton of the Colombian Caribbean, the most complete being that by ^{Medellín-Mora and Navas (2010)}, who listed 217 species. Recently, ^{Fuentes-Reines and Suárez- Morales (2014)} added 14 species of the Order Harpacticoidea to the list for the northern Colombian Caribbean, but there was no record for the Order Monstrilloida Sars, 1903. The present work constitutes the first recognition of a species belonging to this order in the Colombian Caribbean.

The order Monstrilloida is one of the least known orders of Copepoda since the members are parasites during the juvenile phase and part of the adult stage (^{Raibaut and Trilles, 1993}), and during the early nauplii stages, they are endoparasites of primarily benthic marine invertebrates such as free-living polychaetes of the families Capitellidae, Serpullidae, Spionidae and Syllidae. In gastropod mollusks and bivalves, monstrillids enter their host and remain in the bloodstream, where they develop until they reach the preadult stage (^{Caullery and Mesnil, 1914}; ^{Davis, 1984}; ^{Boxshall and Halsey, 2004}). In the adult stage, these copepods are free-living planktonic organisms lacking a mouth and anus, and they are commonly captured in neritic and coastal zones from the tropics to higher latitudes (^{Suárez-Morales and Días, 2001}, Suárez-Morales e Ivanenko, 2004).

This order comprises approximately 125 species that are grouped into five genera; the European waters of the North Atlantic are the most diverse (32 species) followed by the Caribbean Sea and the Gulf of Mexico (24), the Mediterranean and Black Seas (20), Indonesia, Malaysia and the Philippines (17), Japan (17), Brazil and Argentina (16) and finally Australia with eight species (Suárez- Morales, 2015).

The genus Cymbasoma comprises 66 species, including a complex of five species associated with “Cymbasoma longispinosum” that shares subtle but consistent differences, making species identification difficult (^{Suárez-Morales and McKinnon, 2016}). This genus is distributed in different geographical areas of Europe and Brazil (C. longispinosum Bourne, 1890), the Gulf of Mexico (Cymbasoma chelemense Suárez-Morales and Escamilla, 1997), the Gulf of California (Cymbasoma californiense Suárez-Morales and Palomares-García, 1999), Japan, Vietnam and India (Cymbasoma morii Tokioka, 1949) and the Red Sea and Egypt (Cymbasoma janetae Mageed, 2010), evidencing its wide distribution (Suárez-Morales, 2011).

The Bay of Cartagena is located north of the department of Bolívar in the central zone of the Colombian Caribbean (Figure 1). Although it is geologically a bay, due to the significant contribution of river water from the Canal del Dique (Dique Channel), which discharges between 470.4 and 825.1 m3s-1 of fresh water, it is currently considered an estuary (CIOH - CARDIQUE, 1997; ^{Garay and Giraldo, 1998}; ^{Lonin et al., 2004}; ^{Restrepo et al., 2013}). The only recorded specimen, identified as C. chelemense, was found in a zooplankton sample from the area known as Ciénaga de las Quintas in the interior of the Bay of Cartagena during the low-water period (February 2016) at the station designated P01.

Figure 1 Map of the study area indicating the sampling station where C. chelemense was recorded (P01*) in the Ciénaga de las Quintas, Bay of Cartagena, Bolívar. Source: LABSIS, INVEMAR. 

For each sample, 80 L of water was collected using a 20 L plastic bucket that was filtered using a 150-µm mesh plankton net. The collected material was spiked with magnesium chloride (MgCl2) for 10 minutes, fixed with 4% formalin neutralized with borax and placed in 500-ml flasks. In addition, in situ parameters were measured including water salinity and temperature (WTW 3210 parametric probe), depth (Depthmate SM-5 Portable sounder), pH (WTW 3110 parametric probe) and dissolved oxygen (WTW Oxi 3205 parametric probe).

The samples were then transported to the laboratories of the Museum of Natural Marine History of Colombia (Museo de Historia Natural Marina de Colombia - MAKURIWA), where they were homogenized and fractionated with a Folsom separator (according to the density of the sample) in 1/2, 1/4, 1/8 or 1/16 parts following the recommendations of ^{De Oliveira-Díaz et al. (2010)}. To confirm the identification, photographs were taken with a Leica DFC-450 series stereo-microscope. Once the C. chelemense sample was separated, it was taken to the Scanning Electron Microscopy Laboratory of the National University of Colombia (Universidad Nacional de Colombia) to photograph the diagnostic morphological structures to confirm the taxonomic identity of the species using critical point drying with a Quorum K850 dryer (^{Lux, 1984}; ^{Dykstra and Reuss, 2003}).

The sample and the specimen are deposited as reference material in the plankton collection of MAKURIWA of the Institute of Marine and Coastal Research (Instituto de Investigaciones Marinas y Costeras - INVEMAR) with the catalog number INV CRU 8403.

Phylum Arthropoda Subphylum Crustacea Brünnich, 1772 Subclass Copepoda Milne Edwards, 1840 Order Monstrilloida Sars G.O., 1901 Family Monstrillidae Dana, 1849 Genus Cymbasoma Thompson I.C., 1888 Species Cymbasoma chelemense ^{Suárez-Morales and Escamilla, 1997}.

Identification references: ^{Giesbrecht, 1892}, p. 37; ^{Björberg, 1981}, p. 615; ^{Suárez-Morales and Escamilla, 1997}, p. 539; Suárez-Morales, 1999, p. 71; ^{Boxshall and Halsey, 2004}, p. 837

Material examined: One specimen, female, adult, dry preserved, dehydrated, on a stub for electronic microscopy, entered as a control in the collection of the Museum of Natural Marine History of Colombia with code INV CRU 8403. Bay of Cartagena, Bolívar, Colombia, 2016 (10°24.364’ N; 75°31.505’ W).

Diagnosis: Cephalothorax almost 68% of the total length of the body. Oral papillae less than 20% of the length of the ventral surface of the cephalothorax (Figure 2 A, B). Cuticular ridge on the front smooth and irregular with a swirl. Cuticular striae present in the urosome (Figure 2 E, F). Posterior margin of the double genital somite, rectum in dorsal view. Ventral process between antennules absent (Figure 2 C, E). Relative length between the antennule/cephalothorax 21%. Relative length of the last antennal segment 51%. Eye present (Figure 2 D), medially conjugated, well-developed pigment-cups, intensely pigmented only in the central portion, round in dorsal view. Two large groups of simple sensory cells in the lateral portions of the cephalic zone. Genital complex with ovigerous spines, joined at the base, almost 14% longer than the total length of the body and approximately ten times longer than the urosome.

Figure 2 Detailed morphology of C. chelemense found in ciénaga de las Quintas. (A) Dorsal and lateral view showing the mandibular pore. (B) Photograph of typical C. chelemense. Red arrow in figures A-B indicate the oral papilla. (C) A1 right, A1 left. (D) A1 left side of the identified organism. Red arrow in figures C-D indicate three aligned distal setae. (E) Urosome, dorsal. (F) Urosome, ventral view. Red arrow in figures D-E show the almost square caudal rami. (G) P1 female. (H) P1 magnified; red arrow in figures G-H show terminal spines. Figures modified from ^{Suárez-Morales and Escamilla, (1997)}. 

Distribution: Bay of Cartagena (this work) and Chelem Lagoon (Laguna de Chelem), a shallow coastal lagoon located northeast of the Yucatan Peninsula, Mexico. The area is influenced by warm waters of the Caribbean and the Gulf of Mexico; the coastal lagoon is partially isolated from the marine waters by sand banks formed during the dry season (^{Suárez-Morales and Escamilla, 1997}).

Comments: Organism collected in the inland waters of Ciénaga de las Quintas, the interior of the Bay of Cartagena (Figure 1), at a salinity of 35.5, water temperature of 29°C, depth <1.90 m, pH 8.09, dissolved oxygen 4.72 mg L-1. ^{Suárez-Morales and Escamilla (1997)} note that the Chelem Lagoon (Yucatan - Mexico), where C. chelemense was first found, presented a salinity of 32.9, a temperature of 27.5°C, a depth between 0.9 and 1.2 m, and a pH of 8.2.