A new deep-sea species of the genus Gracilimesus (Isopoda: Asellota: Ischnomesidae) from the Bay of Campeche, southwestern Gulf of Mexico

Una especie nueva de aguas profundas del género Gracilimesus
(Isopoda: Asellota: Ischnomesidae) de la bahía de Campeche en el suroeste del golfo de México

The family Ischnomesidae comprises deep-sea asellote isopodes (Crustacea), which are brittle in nature and distributed from 213 to 9,000 m depth in the bathyal and abyssal zones of the Atlantic Ocean (Kavanagh et al., 2006; Kussakin, 1988; Menzies, 1962; Wilson, 2008; Wolff, 1962). The described diversity of these isopods has been increasing due to recent reports of several new species (Brökeland & Brandt, 2004; Merrin & Poore, 2003; Schotte et al., 2009).

In a revision of the genus Haplomesus s. l., Kavanagh and Wilson (2007) created 4 new genera: Contrarimesus, Cornuamesus, Fortimesus, and Gracilimesus, in addition to Haplomesus s. str. The species of Gracilimesus are unusual because they bear an elongate pereonite 5 and several species lack pereopod 7 in adult specimens, which also occurs in Stylomesus hexapodus Brökeland & Brandt, 2004 (Kavanagh & Wilson, 2007). Prior to this study, the genus Gracilimesus included 11 nominal species: G. angustus (Hansen, 1916), G. insignis (Hansen, 1916), G. modestus (Hansen, 1916), G. tenuispinis (Hansen, 1916), G. gorbunovi (Gurjanova, 1946), G. orientalis (Birstein, 1960), G. tropicalis (Menzies, 1962), G. modestatenuis (Menzies & George, 1972), G. corniculatus (Brökeland & Brandt, 2004), G. hanseni (Kavanagh, Wilson & Power, 2006), and G. celticensis (Kavanagh, Wilson & Power, 2006).

Literature on the deep-sea isopods of the Gulf of Mexico is scarce (Schotte et al., 2009). Wilson (2008) documented 22 families and 95 genera associated with soft bottoms in the NE Gulf of Mexico at depths between 213 and 3,732 m. Ischnomesidae included the Haplomesus s.l., Heteromesus, and Ischnomesus genera; this family was distributed across the continental slope (542-3,409 m depth) at a frequency of 49% across the samples.    

During an expedition along the Bay of Campeche, SW Gulf of Mexico, several lots of peracarids were collected in the bottom sediment samples. Within these samples, we recognized a member of the family Ischnomesidae, which represents a new species of the genus Gracilimesus. Here we compare the morphology of this new species to the 11 known species. This is the first described specimen of this genus from the Gulf of Mexico.

Materials and methods

The samples examined in this study were collected during the oceanographic cruise Xcambó-4, carried out jointly by PEMEX Exploration and Production and Cinvestav-Mérida (Bay of Campeche, SW Gulf of Mexico from September 4 to October 10, 2009) aboard the research vessel “Justo Sierra” of the Universidad Nacional Autónoma de México (UNAM). Samples were collected using a benthic box corer (Hessler-Sandia MK II) in a sampling grid oriented between 50 and 3,300 m depth. Four subsamples were taken from each sample using a 20-cm diameter PVC corer. Material was sieved at a mesh size of 500 µm, relaxed with MgCl₂, and fixed with 4% formalin. At the Cinvestav Benthos Laboratory, organisms were sorted, washed, taxonomically identified, and preserved in 70% alcohol.

Images were captured with the aid of a Motic microscope BA210 equipped with a camera lucida, and figures were illustrated with CorelDRAW 12 software. Terminology and taxonomic classification were taken from Kavanagh et al. (2006), Kavanagh and Wilson (2007), and Boyko et al. (2008). The specimen is deposited in the Colección Nacional de Crustáceos, Universidad Nacional Autónoma de México (CNCR-IB-UNAM).

Description

Of all the biological material collected during the sampling effort, the new species of Gracilimesus was observed at only 1 station (Fig. 1) at a depth of 2,548 m, where it was represented by a single female adult.

Order Isopoda Latreille, 1817

Suborder Asellota Latreille, 1802

Superfamily Janiroidea Sars, 1897

Family Ischnomesidae Hansen, 1916

Genus Gracilimesus Kavanagh & Wilson, 2007

Gracilimesus tzasnae new species

(Figs. 2-5)

Material examined. Holotype. Adult female, 3.3 mm total length (CNCR29115), collected from the type locality Bay of Campeche, SW Gulf of Mexico; PEMEX station number: 81_49; 21°00’00” N, 93°00’00” W; soft bottom at 2,548 m depth; September 13, 2009; coll., María Teresa Herrera-Dorantes.

Diagnosis. Antennula with flagellum 5 articulated; maxilliped with 2 coupling hooks. Pereonites 1 and 4 each with 1 lateral subequal spine; spines of pereonite 1, 0.4× as long as pereonite 1; pereonites 2 and 3 without spines. Pereopods 1-6 with neck; pereopod 1 ischium ventral margin with 1 distal robust seta. Merus lacking robust setae; carpus dorsal margin without setae, ventral margin with 1 proximal robust seta; propodus with 1 short subdistal robust seta, ventrally; pereopods 2 and 4 posterior margin of basis and merus devoid of robust setae. Pereonite 7 length not reduced; pereopod 7 absent; pleotelson posterolateral margins with pedestal spines; pedestal spines 0.5× length of uropods. Uropods surpassing posterior margin of pleotelson.

Description. Holotype. Adult female 3.3 mm total length. Body (Fig. 2A, B), head 0.7× as long as wide; frontal margin curved, lobe on ventrolateral margin poorly developed, dorsal cuticle covered with sparse setae. Pereonites (Fig. 2A, B, C), 2 and 4 bearing anterolateral conical simple spines; pereonite 5 length 6× width, 0.31× total body length. Antennula (Fig. 2A, B, C), 5 articulated; article 2 length 1.2× width of head, with 2 short ventromedial setae; 3 distal articles combined half-length of article 2; aesthetascs absent. Antenna (Fig. 2A, B), article 2, 1.6× longer than article 1; article 3, 0.3× shorter than 2; remaining articles missing; cuticle smooth.

Mouthparts: maxilliped (Fig. 3A), basis 2.6× broader than palp; with 2 coupling hooks with rounded tips on inner margin; palp article 2, 1.3× wider than 3; articles 4-5 tapering distally. Maxillula (Fig. 3B), outer lobe with 9 robust distal setae and 1 subdistal; inner lobe length 0.3× of outer; with 3 distal and 2 subdistal robust setae, 1 midway on inner margin. Maxilla (Fig. 3C), outer lobe with 3 long distal setae: medial lobe with 2 long and 3 short distal setae; inner lobe the widest, with 2 setae on inner margin and 13 short and 2 long on upper and outer margins. Mandible (Fig. 3D, E), without palp, molar big and triturative, with 3 racket robust setae; left mandible (Fig. 3D), bearing 6 incisors; lacinia mobilis linguiform; right mandible (Fig. 3E), with 3 incisors; lacinia mobilis missing. Paragnath (Fig. 3F), upper margins of left lobe with 6 setae and 9 on right.

Pereopods (Fig. 4A-F), with neck; long and slender; paucisetose: pereopod 1 (Fig. 4A), poorly prehensile; basis as long as ischium-merus-carpus combined; ischium and carpus, both bearing 1 robust seta on posterior margin; carpus with 2 long robust setae on distal half of anterior margin; with small robust seta on posterior margin; dactylus as long as propodus. Pereopod 2 (Fig. 4B), basis as long as ischium-merus-carpus combined; carpus with 2 robust setae on posterior margin; propodus, with 1 robust setae on posterior margin, dactylus as long as propodus. Pereopod 3 (Fig. 4C), basis 1.7× as long as ischium and merus combined; carpus with 1 seta on posterior margin; propodus with 2 robust setae on posterior margin, dactylus as long as propodus. Pereopod 4 (Fig. 4D), basis 1.6× as long as ischium and merus combined; carpus posterior margin with 1 robust seta; dactylus 0.7× as short as propodus. Pereopod 5 (Fig. 4E), basis 1.3× as short as ischium; ischium with 1 robust seta on anterior margin; merus with 1 seta on anterior margin; propodus with 1 robust seta on posterior margin; dactylus 0.6× as short as propodus. Pereopod 6 (Fig. 4F) basis 1.6× longer than ischium; basis with 3 setae on anterior margin and 1 on the posterior; ischium with 1 posterior robust seta; merus with 2 robust setae on posterior margin, 1 on the anterior; carpus with 2 robust setae on posterior margin; dactylus 0.6× as long as propodus. Pereopod 7 absent.

Pleopods (Fig. 5A, B, C): operculum (pleopod 2), ovoidal, without proximal neck; as wide as long, with 1-3 simple lateral and 4 distal plumose setae. Pleopod 3 inner lobe wider and longer than outer. Pleopod 4 ovoid.

Uropods (Fig. 2D), longer than wide, length 0.3× length of pleotelson; surpassing posterior margin of pleotelson, with 5 long distal setae.

Pleotelson (Fig. 2A), length 1.2× width; dorsal surface axial ridge poorly vaulted, not separated from lateral fields, lacking dorsal spines on pleonite 1 of pleotelson.

Male unknown.

Etymology. This new species is named to honor Dr. Claudia Tzasná Hernández Delgado, Facultad de Estudios Superiores Iztacala, UNAM.

Associated Fauna at PEMEX Station 81_49. The fauna associated with the isopod Gracilimesus tzasnae sp. nov. at this station was scarce. The Polychaeta class was the most abundant with 4 individuals belonging to 4 families (Syllidae, Sabellidae, Spionidae, and Fauveliopsidae) and 4 genera (Streptosyllis, Sabellonga, Spiophanes, and Fauveliopsis).

Discussion

Cinvestav-Mérida has conducted oceanographic expeditions in the Gulf of Mexico from 1999 to the present, sampling a range of depths from 50 to 3,500 m. However, after more than twenty expeditions, this is the first appearance of this new species of isopod. Even so, the species was represented by a single adult female, suggesting that it is infrequently found and occurs in low abundance. It is clear that more intense fieldwork is needed to better document the presence, abundance, and distribution of this species in the area. Yet it is worth mentioning that recording a new species in deep-sea waters is expensive in terms of funding, time, and effort, such that even a single individual is highly valuable from the taxonomic point of view.

It is not easy to conduct a comparative study among the species of this genus, especially since data are not available for all the corporal structures of each sex. There is no doubt that the presence of peoreonite 7 and perepopod 7, as well as the presence of the anterolateral projection in pereonite 4, should be considered the most salient characteristics in the taxonomy of this genus. Taking into account the above, it can be established that Gracilimesus tzasnae sp. nov. is taxonomically very close to G. modestus and G. tenuispinis, all of which have these corporal structures. Both of these other species are from the Davis Strait (northern Canada), very far from the sampling site where G. tzasnae sp. nov. was collected.

 

As the genus Gracilimesus has not been previously reported in the Gulf of Mexico, it becomes necessary to distinguish Gracilimesus tzasnae sp. nov. from all other known species. The new species differs based on 2 maxilliped coupling spines; well-developed lateral spine on pereonal segment 4; posterodistal part of ischium on pereopod 1 and posteromedial margin of carpus, each with a long robust seta; propodus on pereopod 4 with 1 robust seta midway on posterior margin; pedestal spines well developed and directed backward; dorsal surface of pleotelson smooth. Finally, all other known Gracillimesus species have a distribution outside the Gulf of Mexico and the Caribbean Sea. The major differences between G. tzasnae sp. nov. and all other Gracilimesus species are summarized in table 1.

Table 1

Morphological differences among Gracilimesus tzasnae, a new species, and all other known species of Gracilimesus (* female unknown; ** male unknown; – not documented).

Table 1 Continuation
Gracilimesus species	Pereonite 7	Pleotelson dorsal surface	Antero-lateral projection on pereonite 4 (female)	Articles antennula	Articles antenna	Pereopod 7	Uropod length vs. pleotelson	Distribution
Gracilimesus species	Pereonite 7	Pleotelson dorsal surface	Antero-lateral projection on pereonite 4 (female)	Articles antennula	Articles antenna	Pereopod 7	Uropod length vs. pleotelson	Distribution
angustus *	absent	weakly vaulted, separated from lateral fields by shallow elongate concavities	–	5	–	absent	0.2×	Denmark Strait
celticensis	absent	weakly vaulted, separated from lateral fields by shallow elongate concavities	absent	5	–	absent	0.11×	Celtic Sea
corniculatus *	not reduced	weakly vaulted, separated from lateral fields by shallow elongate concavities	–	6	26	absent	0.2×	Drake Passage
gorbunovi**	not reduced	smooth, not separated from lateral fields	absent	6	12	present	0.11×	Arctic Ocean
hanseni	absent	weakly vaulted, separated from lateral fields by shallow elongate concavities	present	6	26	absent	0.1×	Argentina Basin
insignis *	reduced, lesser  pereonite 6	weakly vaulted, separated from lateral fields by shallow elongate concavities by weak indentations	present	6	25	present	0.27×	Davis Strait
modestatenuis *	not reduced	smooth, not separated from lateral fields	–	6	16	present	–	Peru-Chile Trench
modestus**	not reduced	smooth, not separated from lateral fields	present	–	–	present	–	Davis Strait
orientalis *	—	weakly vaulted, separated from lateral fields by shallow elongate concavities by weak indentations	–	6	–	present	0.23×	North Pacific
tenuispinis**	not reduced	smooth, not separated from lateral fields	present	6	–	present	2.6×	Davis Strait
tropicalis	absent	weakly vaulted, separated from lateral fields by shallow elongate concavities	absent	–	–	absent	0.2×	Mediterranean Sea
tzasnae sp. nov. **	not reduced	unvaulted, smooth, not separated from lateral fields	present	5	–	absent	0.3×	Campeche Bay, Gulf of Mexico

Acknowledgements

To the Environmental Monitoring Program of the Southern Gulf of Mexico of ‘PEMEX Exploración y Producción, Regiones Marinas (Campaña Oceanográfica Xcambó-4, 2009)’, which permitted us to obtain the biological material described in this study. We also thank Sandra G. Patiño Espinosa and David Romero (Cinvestav, Mérida) for their assistance in field sampling and map preparation, respectively. The authors also acknowledge the Project PAPCA, 2014, FESI-DIP-PAPCA-2014-20.

References

Boyko, C. B., Bruce, N. L., Hadfield, K. A., Merrin, K. L., Ota, Y., Poore, G. C. B. et al. (Eds.). (2008). World marine, freshwater and terrestrial isopod crustaceans database. Retrieved on 2019-03-02, from: http://www.marineespecies.org/isopoda

Brökeland, W., & Brandt, A. (2004). Two new species of Ischnomesidae (Crustacea: Isopoda) from the Southern Ocean displaying neoteny. Deep-Sea Research, 51, 1769–1785.

Kavanagh, F. A., Wilson, G. D. F., & Power, A. M. (2006).  Heterochrony in Haplomesus (Crustacea: Isopoda: Ischnomesidae): revision of two species and description of two new species. Zootaxa, 1120, 1–33.

Kavanagh, F. A., & Wilson, G. D. F. (2007).  Revision of the genus Haplomesus (Isopoda: Asellota: Ischnomesidae) with erection of four new genera. Invertebrate Systematics, 21, 487–535.

Kussakin, O. G. (1988). Marine and brackish-water (Isopoda) of cold and temperate waters of the Northern Hemisphere.  Suborder Asellota I. Janiridae, Santiidae, Desmosomatidae, Munnidae, Hap-omunnidae, Mesosignidae, Haploniscidae, Mictosomatidae, Ischnomesidae. Opredeliteli po Faune SSR. Akademiya Nauk SSR, 152, 1–501.

Menzies, R. J. (1962).  The isopods of abyssal depths in the Atlantic Ocean. Vema Research, Series I, 79–206.

Merrin, K. L., & Poore, G. C. B. (2003).  Four new species of Ischnomesidae (Crustacea: Isopoda: Asellota) from off south-eastern Australia. Memories of the Museum of Victoria, 60, 285–307.

Schotte, M., Markham, J. C., & Wilson, G. D. F. (2009). Isopoda (Crustacea) of the Gulf of Mexico. In D. L. Felder, D. K.Camp (Eds.), Gulf of Mexico origins, waters and biota, Vol. 1. Biodiversity (pp. 973–986). College Station: Texas A&M University Press.

Wilson, G. D. F. (2008). Local and regional species diversity of benthic Isopoda (crustacean) in the deep Gulf of Mexico. Deep-Sea Research, Part II, 55, 2634–2649.

Wolff, T. (1962). The systematic and biology of bathyal and abyssal Isopoda Asellota. Galathea Report, 6, 7–320.