Monstrilla humesi Suárez-Morales, 2001

Suárez-Morales, E. & Escamilla, J. B., 2001, Taxonomic report on some monstrilloids (Copepoda, Monstrilloida) from southeast Mexico with the description of a new species of M onstrilla, Journal of Natural History 35 (10), pp. 1433-1445 : 1992-1996

Monstrilla humesi Suárez-Morales sp. nov.

( figures 3 , 4 )

Material examined

Holotype: one adult female, undissected, ethanol preserved. Laguna de Chelem , Yucatan, Mexico. 22 September 1987. Deposited in The Natural History Museum, London, registered under catalogue number 2000.1257 .

Type locality

Laguna de Chelem , northern coast of Yucatan Peninsula, Mexico (21°19.35' N; 89°49.10' W). Water column. D epth: 1.2 m.

Description

Female ( figures 3 , 4 ). Total body length of described specimen 0.75 mm; cephalothorax 0.37 mm long, representing about 50% of total ( figures 3A,C , 4A ). Oral papilla not prominent, located medially about 23% of ventral margin from anterior end of cephalothorax ( figure 3A ). Cephalic region wide in dorsal view, about as wide as posterior part of cephalothorax, moderately constricted at level of ocelli. Ventral ocellus absent. Dorsal ocelli present; pigment cups relatively large, poorly developed, relatively widely separated from each other by gap equal to diameter of ocellus; ocelli pigmented on inner parts only, rounded in dorsal view ( figure 4B ). Low, rounded protuberance with two sensilla located on forehead between antennular bases, visible in dorsal view ( figure 4B ).

Antennules relatively long, 0.18 mm, representing about 27% of total body length, and c. 53% of cephalothorax length. Antennules four-segmented, but purported segments 2–4 partially fused. Segments armed with 0–I; 1–V; 2–I; 6–IX–2aes setae (Arabic numbers), spines or spiniform setae (Roman numbers), and aesthetascs (aes), respectively. In terms of pattern described by Grygier and Ohtsuka (1995) for female monstrilloid antennular armature, expected setae and spines on first (1), second (2d,2v, IId), and third (3,IIIv,d) purported segments all present. Only 1–3 1–2 two elements absent in fourth segment (4v, b) from both antennules. Setae b 1 5 1–3 unbranched. Length ratio of antennular segments, from proximal to distal, 17.1:30.8:15.4:36.7 (=100) ( figure 3B ).

F irst pedigerous thoracic somite incorporated into cephalothorax. Pedigerous somites 2–4 accounting for 24.1% of total length in dorsal view. These four pedigers each bearing pair of well developed swimming legs, all with three-segmented rami and with same armament pattern, except for leg 1 exopod ( figures 3D,E ). Exopods longer than endopods in all cases. Coxae of each pair unarmed, joined by intercoxal sclerite as long as wide. Basis separated from coxa by diagonal articulation. Outer margin of basis of swimming legs 1, 2, and 4 with small, thin seta; seta on basis of leg 3 approximately three times larger than in the other legs, naked ( figure 3E ). Outer distal corner of first and third exopodal segments of legs 1–4 each with short spine 20–30% as long as bearing segment. All natatory setae lightly and biserially plumose except for seta on outer distal corner of third exopodal segments of legs 1–4, this being naked along inner side, but bearing row of small denticles along outer margin ( figure 3F ).

Armament formula of swimming legs as follows:

basis exopodite endopodite

leg 1 0–1 I–0;0–1;I,2,2 0–1;0–1;1,2,2 legs 2–4 0–1 I–0;0–1;I,2,3 0–1;0–1;1,2,2

Fifth leg bilobed, outer lobe about twice as long as inner one, with three terminal setae subequal in length and breadth, lightly setulated. Inner lobe slender, armed with a single terminal seta about 30% shorter than those of the outer lobe.

Urosome consisting of fifth pedigerous somite, genital compound somite, and two free abdominal somites; its length, excluding caudal rami, accounting for 31.2% of total body length. Fifth pedigerous somite relatively small, with straight lateral margins. G enital compoun d somite noticeably elongated, representing half the length of urosome ( figure 2A ), slightly wider in proximal half ( figures 3C,D ). Ratio of lengths of fifth pedigerous somite, genital somite and two free abdominal somites: 25.1:50.1:16.6:8.4 (=100). Genital double somite bearing pair of short ovigerous spines; these separate at base, thick in proximal two-thirds, and slender distally, with scattered cuticular scars on proximal one-third. Spines reaching posterior margin of anal somite, equalling about 14.2% of total body length, both equally long, moderate ventral projection anterior to spines ( figures 4D,E ). Caudal rami about twice as long as wide, not divergent, bearing four setae each, one subterminal on inner margin, one terminal, and two lateral; three latter setae subequal in length and breadth, innermost slightly longer than other two ( figure 3A ).

Male: unknown.

Remarks

Having two somites between the genital compound somite and the caudal rami in the female, this species is placed in the genus Monstrilla ( Isaac, 1975) . This new species is similar to both M. helgolandica Claus, 1863 , and M. gracilicauda G iesbrecht, 1892 (see G iesbrecht, 1892a,b; Sars, 1921; Isaac, 1975). The main point of resemblance among these three species is the proportional length of the genital compoun d somite, which is relatively long and slender. N o other species of Monstrilla has such a long genital somite, in most of them this somite constitutes less than 7.3% of the body length. In M. gracilicauda the genital somite represents about 10% of the total body length (see Sars, 1921); in M. helgolandica this figure is between 9.3 and 10.5% (see Giesbrecht, 1892b; Sars, 1921; Park, 1967). In the new species, M. humesi , this somite is relatively still longer, its proportional length being almost 13%.

In many other respects, the general aspect of these three species is quite similar, as noted by Sars (1921) for M. helgolandica and M. gracilicauda . Monstrilla humesi could easily be confused with either of these two species when considering only its habitus. Even the antennules of the three species have similar features: including quite the same proportional length, 33% of total body length in M. gracilicauda , 23.5–25% in M. helgolandica , and 26% in M. humesi . Antennules show the same segmentation pattern, in which only the 1– 2 intersegmental division is well defined, segments 2–4 appearing fused. The subterminal group of ‘b’ setae ( Grygier and Ohtsuka, 1995) are similar in the three species, all unbranched. Unfortunately, it is impossible to compare the antennular armature in more detail with the available illustrations and descriptions of either M. helgolandica or M. gracilicauda .

The new species can be separated from these other two species of Monstrilla most clearly by the structure of the fifth legs. In M. gracilicauda leg 5 is constituted by a lobe with three setae and an unarmed inner protuberance ( Sars, 1921; Isaac, 1975), and in M. helgolandica it is a long, slender lobe armed with two distal, subequal setae and no inner protuberance ( Park, 1967; Isaac, 1975). In the new species the fifth leg is diVerently built, being bilobed, with three setae on the outer lobe, and one seta on the inner lobe (see figure 4D ).

The caudal rami of both M. gracilicauda and M. helgolandica has six setae (see Sars, 1921), thus diVering from the four setae pattern present in the new species. This is, in fact, a character shared with only a few other species of Monstrilla (see Isaac, 1975).

Some other diVerences concern the ovigerous spines and the oral papilla. In M. gracilicauda the ovigerou s spines are relatively short, as they are also in M. humesi ; however, in the former species these structures are divergent from the base and barely reach the posterior margin of the bearing (genital) somite, whereas they do not diverge from the base and reach about midlength of the caudal rami in the new species. In M. helgolandica the ovigerous spines are relatively longer than in M. humesi , they reach well beyond the caudal rami (see Sars, 1921; Park, 1967), and they are moderately divergent in the proximal one-third. The oral papilla is located at 31% of way back along the cephalothorax in M. gracilicauda , at 43–51% in M. helgolandica , and at 23% of the way in M. humesi .

The body lengths of the three species are also diVerent. The known range for M. gracilicauda is 2.7–3.55 mm ( Sars, 1921; Isaac, 1975), and for M. helgolandica it is 1.4–2.31 mm ( Park, 1967; Isaac, 1975). Measuring 0.75 mm, the new species is much smaller; in fact, it is one of the smallest species of the genus.

The male of M. humesi remains unknown. The female of the related species M. gracilicauda has been suggested to be the female of M. anglica Lubbock, 1857 ( Isaac, 1975) . If this is true, it could be expected that the male of M. humesi has a close morphological resemblance to that of M. anglica . This should be taken into account during examinations of samples from this area of Mexico. However, due to the rarity of monstrilloid copepods, it might be a long time before the two sexes of this species can be found or linked. For instance, the male of M. helgolandica remained unknown for more than a century (McAlice, 1985).

The reported distributions of both M. gracilicauda and M. helgolandica are very wide. The former has been recorded mainly in the northeastern Atlantic and the Mediterranean Sea, plus an additional record in the Indian Ocean, oV Indonesia ( Isaac, 1975; Grygier, 1995). Monstrilla helgolandica has a wider purpor ted distribution, which includes the northeastern and northwestern Atlantic, the M editerranean and Black Seas, the South Atlantic, the northeastern Pacific, and parts of the Indowest Pacific ( Park, 1967; Isaac, 1975; Grygier, 1995; Razouls, 1996).

Etymology

The new species is dedicated to the memory of the late Dr Arthur G. Humes, an admired copepodologist and an extraordinary gentleman.

Figures

Fig. 3. M onstrilla humesi n.sp. Adult female holotype. Laguna de Chelem: (A) habitus, ventral view; (B) left antennule, dorsal view; (C) habitus, dorsal view; (D) left first swimming leg; (E) left third swimming leg showing long basipodal outer seta; (F) outer apical exopodal seta of third leg.

Fig. 4. M onstrilla humesi n.sp. Adult female holotype. Launa de Chelem: (A) habitus, lateral view; (B) anterior end of head, dorsal view; (C) caudal rami, ventral view; (D) fifth pedigerous somite with legs 5, and genital double somite with ovigerous spines, ventral view; (E) same structures, lateral view.

Fig. 2. Cymbasoma quintanarooense Suárez-Morales. Laguna de Chelem. Adult female: (A) urosome showing dorsal cuticular striations; (B) right antennule, dorsal view; (C) second swimming leg, showing armature of outer distal exopodal natatory seta; (D) fifth pedigerous somite with right leg 5 and anterior part of compound genital somite, lateral view; (E) fifth pedigerous somite with legs 5, and compound genital somite, dorsal view.