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Research Article
A new species of Monstrilla (Copepoda, Monstrilloida) from the western Caribbean with comments on M. wandelii Stephensen, 1913 and M. conjunctiva Giesbrecht, 1893
expand article infoEduardo Suárez-Morales‡
‡ Unidad Chetumal, Chetumal, Mexico

Corresponding author: Eduardo Suárez-Morales ( esuarez@ecosur.mx )

Academic editor: Kai Horst George
© 2022 Eduardo Suárez-Morales.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Suárez-Morales E (2022) A new species of Monstrilla (Copepoda, Monstrilloida) from the western Caribbean with comments on M. wandelii Stephensen, 1913 and M. conjunctiva Giesbrecht, 1893. ZooKeys 1128: 33-45. https://doi.org/10.3897/zookeys.1128.84944
ZooBank: urn:lsid:zoobank.org:pub:D446044C-473C-449F-BEB9-B4CC0FCD2CFB
Open Access

Abstract

The taxonomic study of monstrilloid copepods is hampered by incomplete early descriptions, uncertain synonymies, and the difficulty of reliably matching males and females of species. A re-evaluation of male monstrilloid specimens collected from two reef areas of the Mexican Caribbean allowed me to clarify the status of Monstrilla mariaeugeniae Suárez-Morales & Islas-Landeros, 1993 and M. wandelii Stephensen, 1913 based on a comparison of males attributed to each of these species. Males from the Puerto Morelos reef system, northern Mexican Caribbean coast, were first proposed as a tropical subspecies of the subarctic M. wandelii; later on, morphologically close males collected from the Mahahual reef area, southern Mexican Caribbean coast, were designated as the males of M. mariaeugeniae. Their status is here corrected with the description of M. mahahualensis sp. nov. based on the Mahahual males; the new species shares the same type of genitalia with the antarctic M. conjunctiva Giesbrecht, 1892 and the subarctic M. wandelii; Park (1967) linked a single male from Vancouver to M. wandelii. It was realised that Park’s (1967) males from the Vancouver area and the two Mexican Caribbean groups of males represent different, undescribed species. I here reassign the males earlier attributed to M. mariaeugeniae as a new species of Monstrilla which is herein described. The new species differs from the males of M. conjunctiva and M. wandelii by details of the genitalia, length of the setae of the fifth legs, armature and integumental structures of the antennules, and size of the outer exopodal spines of legs 1–4. This is the third known species of Monstrilla with a M. conjunctiva-like male genitalia and the first one known from tropical areas. The male of both M. mariaeugeniae and M. wandelii remain unknown.

Keywords

Mexican Caribbean, Monstrillidae, morphology, protelean endoparasites, taxonomy

Introduction

Monstrilloid copepods are protelean endoparasites infecting different groups of marine invertebrates including polychaetes, molluscs, and sponges (Huys et al. 2007; Suárez-Morales 2011, 2018; Jeon et al. 2018) and are frequently recorded as adults in plankton samples. Their taxonomic and nomenclatural history is complex and uncertain (Grygier and Ohtsuka 2008; Suárez-Morales 2011, 2018; Suárez-Morales and Grygier 2021), particularly in reference to old records, early species descriptions, and the recognition and matching of males and females of the species (Grygier 1994, 1995; Suárez-Morales 2010, 2011, 2018; Suárez-Morales and Grygier 2021). The re-examination of collection specimens and the analysis of material from new sites with updated taxonomic criteria are valuable tools to solve some of these taxonomic problems (Grygier 1994; Suárez-Morales 2000a, b; Suárez-Morales and Gasca 2004).

Materials and methods

During the examination of zooplankton samples from Mahahual reef, southern coast of the Mexican Caribbean, several male monstrilloids were re-examined. They were tentatively identified by Suárez-Morales (1998) as the males of M. mariaeugeniae Suárez-Morales & Islas-Landeros, 1993, originally described from females collected at Puerto Morelos (PM) reef, on the northern coast of the Mexican Caribbean. Suárez-Morales (1996) designated PM males as a tropical subspecies of the Arctic M. wandelii Stephensen, 1913.

In order to determine the status of the northern and southern Mexican Caribbean males, which was still uncertain, I re-examined the Mahahual male specimens. I sorted five adult males closely resembling those that were previously named by me (Suárez-Morales 1996) as a subspecies (i.e., Monstrilla wandelii tropica) from the northern Mexican Caribbean reef (PM) and subsequently designated as the males of M. mariaeugeniae (Suárez-Morales 1998) to determine their status in the light of their resemblance to males from PM and to other species. As an outcome of this new evaluation using the descriptive criteria by Grygier and Ohtsuka (1995), the male individuals from Mahahual are here recognized as a new species, M. mahahualensis sp. nov., and not a M. wandelii subspecies nor the males of M. mariaeugeniae. The new species from Mahahual is fully described including examination with SEM and compared with similar species and male specimens from PM. The female of the new species remains unknown, like the true males of M. mariaeugeniae. Also, the male of M. wandelii from Vancouver, as described by Park (1967), is likely to represent an undescribed species; the male of M. wandelii from Greenland remains unknown.

Results

Taxonomy

Subclass Copepoda Milne Edwards, 1840

Order Monstrilloida Sars, 1901

Family Monstrillidae Dana, 1849

Genus Monstrilla Dana, 1849

Monstrilla mahahualensis sp. nov.

https://zoobank.org/4761E010-CB8C-4F53-AC2B-F94E18AA6E16
Figs 1, 2, 3, 4

Type material

Adult male holotype, partially dissected (ECO-CH-Z 10595); specimen mounted in glycerine, sealed with acrylic varnish; two slides, collected by L. Vásquez-Yeomans and A. González-Vera, December 31 1990, plankton sample; Paratypes: 3 adult males (ECO-CH-Z 10596), one mounted on slide, undissected, and 2 males in vial, ethanol-preserved, undissected; collection data as holotype. Additional non-type material: one adult male prepared for SEM examination following the procedures described by Silva-Briano et al. (2013).

Type locality

Reef lagoon of Mahahual (18°43'11.42"N, 87°42'11.01"W), southern coast of the Mexican Caribbean.

Etymology

The species epithet, a toponym in singular, refers to the reef system of Mahahual, the type locality of this species. The gender is feminine.

Diagnosis

Male Monstrilla with light cuticular reticulation of cephalothorax covering cephalic area and 2/3 of post-oral cephalothorax surface. Antennule 5-segmented, geniculate, segments 3 and 4 indistinctly segmented, intersegmental division marked by constriction. First antennulary segment unarmed, segments 3 and 4 each with discoid integumental structures of unknown function; armature of segments 4 and 5 reduced. Element 4d1 (Grygier and Ohtsuka 1995) robust, spinulate. Legs 1–4 with relatively long outer exopodal spines. Genital complex type II (Suárez-Morales 2000c) with short, thick shaft carrying pair of lappets subdistally. Lappets simple, conical, weakly asymmetrical, with outer surface furnished with rows of spinules. Fifth legs represented by pair of bulbous processes armed with long distal seta reaching beyond posterior margin of caudal rami.

Description of adult male

Body size of holotype 2.56 mm, of one paratype 2.72 mm measured from forehead margin to posterior end of anal somite. Body tagmosis as usual in males of Monstrilla (Suárez-Morales and Castellanos-Osorio 2019; Suárez-Morales 2000c; Suárez-Morales 2022). First pedigerous thoracic somite incorporated into cephalothorax. Cephalothorax long, cylindrical, relatively robust, representing about 60% of total body length. Oral cone located at 40% of way back along ventral surface of cephalothorax (Fig. 1B). Cephalic region anteriorly subquadrate, forehead flat (Fig. 1A). Ocelli poorly developed, almost unpigmented. Small oval hyaline bodies (see Suárez-Morales 2018; Fig. 1A) adjacent to rounded ocelli. Cuticular ornamentation of cephalothorax including light cuticular reticulation showing irregular pattern covering cephalic area and almost 2/3 of post-oral cephalothoracic surface (Fig. 1A). On ventral surface, three pairs of nipple-like processes present between oral cone and antennule bases (Fig. 1B); two foremost ventral processes with adjacent pattern of minute transverse wrinkles and small papillae (Figs 1B, 2C). Oral cone moderately produced, with adjacent field of faint, transverse striae (oc in Fig. 1B).

Figure 1. 

Monstrilla mahahualensis sp. nov., holotype adult male from Mahahual, Mexico A cephalic region showing weak reticulation pattern, dorsal view B perioral area showing oral cone (oc), cuticular processes and ornamentation, ventral view C antennule segments 2 and 3 showing setation pattern and position of integumental structure D urosome showing fifth legs and genital complex, lateral view E third and fourth antennule segments showing setation pattern and position of integumental structure (is) F fourth and fifth antennule segments showing setation pattern and position of integumental structure G fifth antennule segment showing setation pattern Scale bars: 50 μm (A–G).

Urosome consisting of five somites: fifth pedigerous somite (largest of urosome, with fifth legs), genital somite (with genital complex on ventral surface), two free somites, and short anal somite. Length ratio of urosomites (from proximal to distal) being: 44.1: 24.4: 12.6: 10.1: 8.8 = 100 (Figs 1D, 2F, 3F).

Antennules representing about 40% of total body length, and almost 65% of cephalothorax length. As usual in males of Monstrilla, antennules indistinctly five-segmented, geniculation between segments 4 and 5 (Figs 1F, 2A, B). Following setal nomenclature proposed by Grygier and Ohtsuka (1995) for female monstrilloid copepod antennules. First segment lacking setal element 1. Second segment carrying six elements, including short elements 2v1-3, and 2d1,2 plus long seta IId, with broad socket (Fig. 1C). Second and third segments separate. Third segment short, globose, with setiform flexible medial element 3 reaching about proximal 1/3 of fourth segment; inner surface of segment with discoid integumental structure of unknown function (Fig. 1C, is in Figs 1E, 3E, 4C, D) at insertion of element 3; structure present in the right antennule only. Setae IIId, IIIv absent. Fourth segment fused with third, division marked by moderate constriction; segment armed with five setal elements: 4d1, 4v1-3, IVv, and long aesthetasc (4aes) (Fig. 1F, 4D); discoid integumental structure like that on third segment present at insertion of elements 4d1 and 4v1-3 (Figs 1E, 2D, 3E). Setal element 4v1 longer than 4v2, 3; element 4d1 robust, spiniform, pinnate (Fig. 1E). Distal fifth segment geniculate, slightly longer than preceding fourth segment (Fig. 1F), with reduced armature, armed with six setal elements (sensu Huys et al. 2007) (see Table 2). Spiniform apical elements 61 and 62 (sensu Grygier and Ohtsuka 1995), or 1,2 (sensu Huys et al. 2007) present, unequally long (Figs 2A, 4A). Length ratio of antennular segments (proximal to distal): 11.4: 19.8: 11.4: 27.8: 29.6 = 100 (Fig. 2A, B).

Figure 2. 

Monstrilla mahahualensis sp. nov., SEM-prepared adult male from Mahahual, Mexico A antennules, ventral view indicating purported segments 3–5 B same, semi- lateral view indicating purported segments 1–5 C detail of wrinkled cuticular field on perioral surface, ventral view D purported fourth antennulary segment showing setation pattern and second integumental structure (is2) E legs 1–3 showing endopodal and exopodal rami general setation pattern, arrows indicate relatively long outer exopodal spines, ventral view F urosome with fifth legs armed with long setae, and genital complex, ventral view. Setal elements on antennule segments 1–4 labeled following Grygier and Ohtsuka’s (1995) nomenclature.

First incorporated pedigerous thoracic somite and succeeding three thoracic somites each bearing well-developed biramous swimming legs (Figs 2E, 3D). Swimming legs 1–4 as in M. mariaeugeniae (Suárez-Morales and Islas-Landeros 1993, fig. 1c, d, i, j), all with triarticulate endopodites and exopodites and same armament pattern except for leg 1 exopodite, bearing one seta fewer on distal segment (Fig. 1E). Exopodites longer than endopodites. Distal spiniform seta of third exopodite with denticles along outer margin, inner margin weakly setulose in all legs (* in Fig. 3D). Outer spines of exopodal segments 1 and 3 of all legs noticeably long (arrowheads in Fig. 3D). Basis of swimming legs 2–4 with short basipodal seta; basipodal seta on legs 1 and 2 not observed; seta on leg 3 long. All natatory setae lightly and biserially plumose. Huys and Boxshall (1991) was followed for general and setation nomenclature. Armament formula of legs1–4 as in Table 1.

Table 1.
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Armature of swimming legs 1–4 including basipodites, exopodites and endopodites. Roman numerals indicate spiniform elements, Arabic numbers indicate setiform elements.

Basipodite Endopodite Exopodite
Leg 1 1-0 I-1;0-1;,2.2.1 I-0;0-1; I,2,2
Legs 2–4 1-0 0-1;0-1;2,2,1 I-1;0-1; I,2,2,1
Figure 3. 

Monstrilla mahahualensis sp. nov., SEM-prepared adult male from Mahahual, Mexico, female holotype A detail of right genital lappet showing cuticle ornamentation, ventral view B same, left genital lappet, semi-lateral view C apical surface of genital complex showing genital opening and paired opercular flaps (ofl), ventral view D swimming les 2–4 showing setation pattern and long exopodal spines (arrowheads) E integumental structure on antennule segment 4, ventral view F urosome, genital complex, and caudal rami showing setation pattern (setae II–V).

Fifth legs reduced, represented by pair of small globose protuberances on ventral surface of fifth pedigerous somite armed with long single seta reaching beyond posterior margin of caudal rami (Figs 1D, l5s in Figs 2F, 3F, 4B). Genital somite carrying genital complex consisting of short, thick medially curved shaft bearing paired subdistal genital lappets (Figs 2F, 3A–C, 4B, E). Lappets divergent, conical, tapering distally, ornamented with rows of spinules on inner surface (Fig. 3A, B); genital complex carrying genital opening with pair of rounded opercular flaps in apical position (ofl in Fig. 3C).

Caudal rami subrectangular, each ramus approximately 1.3 times as long as wide, bearing four caudal setae (setae II–V; Huys and Boxshall 1991), armature incomplete in some specimens, but sockets indicate setal insertions (Figs 2F, 3F, 4B).

Figure 4. 

Monstrilla mahahualensis sp. nov., SEM-prepared adult male from Mahahual, Mexico A right antennule fifth segment showing terminal setal elements, ventral view B distal part of urosome showing genital complex, fifth legs with long setae (L5s) C integumental structure 1 (is), on third antennulary segment, semi-lateral view D integumental structure 2 (is) on fourth antennulary segment, ventral view E genital complex showing genital opening and paired genital lappets(gl) F detail of anal somite, ventral view. Setal elements on segments 1–4 labeled following Grygier and Ohtsuka’s (1995) nomenclature; armature of fifth segment labeled following Huys et al. (2007); ae2 on distal segment follows Huys et al. (2007) sequence notation of antennulary aesthetascs. Arrows indicate proximal rounded protuberance on fifth antennulary segment. Scale bars: 75 μm (A, B).

Female. Unknown.

Discussion

The male specimens studied here can be included in the genus Monstrilla by 1) the presence of two urosomites between the genital somite and the anal somite, 2) the structure of its genital complex that is typical for the genus, in this case, type II (Suárez-Morales 2000d); 3) the presence of well-developed or reduced paired fifth legs, 4) the possession of 4–6 caudal setae in both sexes, and 5) branched antennulary setae sometimes present. This combination of characters is unique among the known genera of monstrilloids (Grygier and Ohtsuka 2008; Suárez-Morales and McKinnon 2014; Jeon et al. 2018).

The males of Monstrilla conjunctiva Giesbrecht, 1902, M. wandelii sensu Park (1967), and M. mahahualensis sp. nov., share a similar structure of the genitalia. This type of male genital complex was first described and depicted for M. conjunctiva by W. Giesbrecht from a male collected in Antarctic waters (Giesbrecht 1902). There are no other known species of Monstrilla with the M. conjunctiva-like male genitalia; therefore, the comparisons in this work include the three nominal species of monstrilloids sharing this character (Table 2).The shape, structure, and ornamentation of the genital lappets, the length of the fifth leg setae, body size, and antennule setation pattern show some differences among these three taxa, as follows: the genital lappets are simple, leaf-like, distally acute in M. conjunctiva (Giesbrecht 1902, fig. 12.3) whereas they are chelate and smooth in M. wandelii (see Park 1967, fig. 2C, D); in M. mahahualensis sp. nov., the lappets are simple, conical, and ornamented with rows of spinules (Fig. 3A, B). Also, in both M. conjunctiva (Giesbrecht 1902, figs XII.3, 4) and M. wandelii sensu Park (1967, fig. 2C, D), the leg 5 setae are relatively short, not reaching the distal margin of the caudal rami. In the new species, M. mahahualensis, the fifth leg setae reach well beyond the posterior margin of the caudal rami (Figs 2F, 3F, 4B). The new species shares with M. wandelii sensu Park (1967) the relatively long outer exopodal spines of legs 1–4, and the antennule segmentation and armature pattern (see Table 2). It shares with M. conjunctiva the caudal rami setation pattern and the antennule setation (Table 2).

Table 2.
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Comparison of male antennule armature patterns of different species and populations with M. conjunctiva-like genitalia. Identification of elements on A1 segments 1–4 followed Grygier and Ohtsuka’s (1995) nomenclature; for elements on A1 segment 5, Huys et al.’s (2007) nomenclature was followed.

A1 elements (sensu Grygier and Ohtsuka 1995) M. mahahualensis sp. nov. M. conjunctiva Giesbrecht, 1902 M. wandelii Park (1967)
61/62 ratio 0.66 0.4 0.9
Element 1 absent absent absent
Elements on S2 2v1-3, 2d1,2, IId 2v1-3, 2d1,2 2v1-3, 2d1,2, IId
Elements on S3 IIIv, IIId, 3 IIIv, IIId, 3 IIIv, IIId, 3
Elements on S4 4v1-3, 4d1, 4aes, IVv 4v1-3, 4d1, 4aes, IVd 4v1-3, 4d1,2, 4aes, IVv
Elements on S5* 1,2,6, C,B AE2 1, 2, 4, 6, C, AE2 1, 2, 4, 6,C, AE2

The rounded integumental structures resembling those here observed in M. mahahualensis sp. nov. were first depicted, but not described, by Suárez-Morales (1996, fig. 2A, B) in males of PM. These structures were not reported by Giesbrecht (1902) or Park (1967) and are present in the female holotype of M. mariaeugeniae (pers. obs.), thus agreeing with Suárez-Morales’ (1996) notion that the PM males, and the Mahahual males (Suárez-Morales 1998) were linked to M. mariaeugeniae.

There are two distinct types of male genitalia among species of Monstrilla and of the related genus Caromiobenella Jeon, Lee & Soh, 2018 (McAlice 1985; Jeon et al. 2018; Suárez-Morales 2000d; Cruz Lopes da Rosa et al. 2021): the first one (type I) shows a genital shaft of variable length and thickness bearing a deep distal notch between the distally set lappets; this medial notch is frequently ornamented with spinules or teeth on its inner surface. A second type (type II) is distinguished by the presence of a smooth, rounded medial protrusion instead of a notch in the same position. The male genitalia displayed by the three species: M. wandelii sensu Park (1967), M. conjunctiva, and M. mahahualensis sp. nov. are assignable as a modified type II male genitalia, in which lappets are widely divergent, probably resulting from a strong development of the broad medial protrusion between them.

The antennule segmentation, with fused segments 3 and 4 is another character shared by the three species, M. wandelii, M. conjunctiva, and M. mahahualensis sp. nov., including their reduced setation of segments 1, 4, and 5. The setation patterns of segments 1–4 were compared following Grygier and Ohtsuka’s (1995) nomenclature for the first time. The differences among these species and Caribbean males are presented in Table 2.

The PM males share several characters with M. mahahualensis sp. nov., namely: 1) antennule segmentation pattern, 2) body proportions, cephalothorax ornamentation, 3) caudal rami armature, 4) antennule armature (see Table 1), 4) position of the oral papilla (~ 40% of way back along ventral surface of cephalothorax), and 5) the presence of integumental structures on the antennule (Suárez-Morales 1996, fig. 2A, B). There are, however, some important differences between the PM males and M. mahahualensis sp. nov. from Mahahual (southern Mexican Caribbean), as follows: In the PM males the fifth legs setae are relatively short, not reaching beyond the posterior margin of the caudal rami (Suárez-Morales 1996, figs1B, 2C), thus diverging from the clearly longer fifth leg setae of M. mahahualensis sp. nov. reaching well beyond the distal margin of the caudal rami; 2) in addition, the genital lappets of the PM males are bilobed, chela-like (Suárez-Morales 1996, figs 2C, 3A) vs. simple, conical in M. mahahualensis sp. nov. (Fig. 3A, B). Also, the PM males have two integumental structures on antennulary segment 2 (Suárez-Morales, 1996, fig. 2A, B), but not on segments 3 and 4, as observed in M. mahahualensis sp. nov. (Figs 1E, F, 2D, 3E). Antennulary segments 61 and 62 are equally long in the PM males (Suárez-Morales 1996, fig. 2A, B) and unequally long in M. mahahualensis sp. nov. (Figs 1G, 2A, 4A, Table 1). Thus, it is likely that the PM males represent a different species closely related with M. mahahualensis sp. nov. The holotype of the PM species is deposited in the USNM collection as Monstrilla wandelii tropica (USNM-259668) and is probably a different species (see Suárez-Morales 1996), whose link with M. mariaeugeniae is yet to be determined.

Acknowledgements

I thank El Colegio de la Frontera Sur (ECOSUR-Chetumal) for full support during the taxonomic analysis of these samples. Rosa Ma. Hernández and José Angel Cohuo (ECOSUR) deposited and catalogued specimens of this species in the collection of Zooplankton at ECOSUR-Chetumal. Manuel Elías-Gutiérrez kindly guided my SEM sessions with ECOSUR’s SEM equipment (JEOL JSM-6010LA) at El Colegio de la Frontera Sur (ECOSUR) Chetumal, Mexico. Part of this material was studied during my sabbatical year at the USNM in Washington, D.C. and I appreciate the continuous support from Chad Walter and for cataloguing the PM male specimens in the USNM collection. The comments and suggestions from two anonymous reviewers allowed me to improve an earlier version of this work.

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