Hyalella cenotensis, Marrón-Becerra, Aurora, Hermoso-Salazar, Margarita & Solís-Weiss, Vivianne, 2014

Marrón-Becerra, Aurora, Hermoso-Salazar, Margarita & Solís-Weiss, Vivianne, 2014, Hyalella cenotensis, a new species of Hyalellidae (Crustacea: Amphipoda) from the Yucatán Peninsula, Mexico, Zootaxa 3811 (2), pp. 262-270 : 263-268

Hyalella cenotensis n. sp.

( Figs. 1 , 2)

Material examined. Holotype female, size 5.3 mm, UNAM Cat. No. CNCR 28870, from algae in the outer area of Cenote Aktun-Ha, Quintana Roo, Mexico (20°16.48´N, 87°29.20´W) in April 2008; paratypes: 1 ovigerous female, UNAM Cat. No. CNCR 8871, locality same as holotype, collectors Vivianne Solís-Weiss and Sarita Frontana.

Type locality. Cenote Aktun-Ha, Quintana Roo, Mexico (20°16.48´N, 87°29.20´W)

Etymology. The name is derived from the habitat where it was found, a cenote or sinkhole.

Habitat. In algae, freshwater.

Diagnosis. Eyes absent. Tergites of somites 1 and 2 with dorsoposterior carina. Head length approximately equal to pereonites 1 and 2 combined. Antenna 1 shorter than antenna 2. Antenna 2 almost 0.2 times longer than antenna 1. Maxilla 1, vestigial palp, uni-articulated, apical pointed; inner plate with three strong and pappose apical setae. Lower lip, distal lobes subtriangular. Telson wider than longer, distal margin rounded, with three short submarginal setae in both sides left and right, without apical setae.

Description. Coxae 1–4 subrectangular, longer than wide, inferior margin rounded with small setules, coxa 4 with a posterior excavation; coxae 5–7 shorter than coxae 1–4, coxa 5 with two subequal lobes, coxa 6 with two unequal lobes, coxa 7 with a single lobe.

Pleon: pleonites 1 and 2 with dorsoposterior carina, ventral margin of epimeral plates 2 and 3 slightly pointed. Cuticle with numerous minute setae present all over the surface of the pereopod propodus.

Head: typically gammaridean, smooth surface, length approximately subequal to pereonites 1 and 2 combined; rostrum absent; eyes absent.

Antenna 1: shorter than antenna 2, length reaching beyond third pereonite; peduncle slightly longer than head, but not surpassing half the length of first pereonite. Articles of peduncle becoming smaller gradually in length and width towards their distal portion; close to the middle of the length of the ventral margin of the first article of the peduncle, a short cuspidate seta at the distal end; flagellum with seven articles, longer than peduncle, basal article of flagellum not elongate; no accessory flagellum.

Antenna 2: almost 0.2 times longer than antenna 1, length close to a third of the total length of body, reaching the first third of the length of fourth pereonite; peduncle: articles increasing gradually in length and decreasing in width towards their distal end; flagellum with eight articles (left antenna with nine articles), length 1.35 times the length of peduncle.

Buccal parts: upper lip, distal margin of upper lip rounded with numerous simple setae, longer and more distant towards the lateral ends, distal surface of the outer surface with two rows of short setae, very close towards the middle of the row. Lower lip, distal lobes subtriangular, rounded distally, apices relatively separated from one another; no inner lobes; mandibular lobes rounded, distal ends slightly directed outwards.

Mandibles: incisor, distal tooth stronger than the proximal four; molar strong and triturative; left lacinia mobilis more developed than right, with five teeth; length of the external tooth subequal to the distal tooth of the incisive process; right lacinia mobilis with three teeth, each half-moon shaped and with distal margin denticulate; left mandible raker row of the left jaw with three pappose setae two on the right side and a large accesory pappose seta at the end of the molar process; palp lacking.

FIGURE 2. Hyalella cenotensis , holotype, female, 5.3 mm. G1–2, gnathopods 1–2; P1–7, pereopods 1–7; U1–3, uropods 1–3; T, telson; E, egg. Scale bars = 100 microns.

Maxilla 1: outer plate of maxilla 1 longer than wide, slightly longer than 2.5 times the width of the inner plate, distal margin with nine serrate setae; vestigial palp uni-articulated, longer than wide, with pointed apex, palp length slightly shorter than a third of the distance of the palp base to the end of the longest seta of the outer plate, inner plate slender, shorter than outer one, almost reaching the palp base, distal margin with three pappose setae.

Maxilla 2: plates subequal in length and shape; inner plate at almost half the length of the inner margin with two plumose setae, anterior margin of both plates with simple setae.

Maxilliped: inner plate shorter than outer one, distal margin with three cuspidate setae of equal size and with plumose setae, inner margin with plumose setae; outer plate elongate, distal and inner margins with numerous simple setae; palp composed by four articles. First article with three simple setae at the farther anterior end of the inner margin; second article with numerous simple setae in the inner margin and two in the outer margin; third article with five simple setae at the distal inner margin and eight simple setae at the apicolateral margin; fourth article ungiform, distal end with three simple setae shorter than the nail; nail slightly shorter than the length of the fourth article.

Gnathopod 1: subchelate, hammer shaped, palm transverse, shorter than gnathopod 2; basis elongate, almost four times longer than wide; a simple seta almost at midway of the posterior margin; ischium short, maximum width and length subequal to the length of the merus, distal posterior end with three simple setae; merus longer than wide, shorter than the carpus, distal posterior margin with simple setae, comb scales on posterior surface of both sides; carpus longer than wide, slightly longer than the propodus, posterior margin of lobe with 11 simple setae, posterior surface of both sides with comb scales near the margin, distal anterior margin with four simple setae; propodus longer than wide, distal end of anterior margin with five simple setae, anterodistal and posterodistal margin with comb scales, inner surface with three stout pappose setae; palm oblique, with long setae, posterior end with a robust seta and cup for dactylus; dactylus claw-like, nail present, anterior end with a plumose seta, posterior margin with two simple setae, apex with comb scales.

Gnathopod 2: subchelate, palm inversely oblique, elongate basis, posterior margin at almost half its length with two simple setae; distal end of the anterior end with one short simple seta, distal end of posterior margin with two long simple setae; ischium short, subquadrate, shorter than merus, posterodistal end with four simple setae; merus short, distal end of the posterior margin with five simple setae, distal half of the posterior surface of the inner and outer surfaces with several comb scales; carpus shorter than propodus, distal end of the anterior margin with four simple setae, posterior lobe with several submarginal simple setae, surface of both the inner and outer surfaces of lobe with comb scales; propodus elongate, approximately 2.5 times longer than wide, close to 1.2 times the length of the propodus of gnathopod 1, inner surface towards the posterior margin with a row of three simple setae diagonally placed, distal surface of both sides with comb scales close to the anterior and posterior margins, basis of dactylus with three simple setae in its posterior margin; palm inversely oblique with several simple setae; dactylus claw-like, nail present, shorter than the propodus, anterior margin proximal with a short plumose seta, posterior margin with two submarginal setae, basis of the nail with a plumose seta.

Pereopods: simple, gradually longer posteriorly, pereopod 5 shorter than fourth and sixth. Pereopod 3 basis elongate with a simple seta at half the length of posterior margin, anterior distal end with a short simple seta, distal posterior end with two simple setae, one longer than the other; ischium subquadrate, distal end of posterior margin with three simple setae, merus longer than ischium, posterior margin with four simple setae, the third longer than the others and close to the fourth; carpus longer than ischium, posterior margin of the carpus with four cuspidate setae, the fourth longer and close to the distal end of the posterior margin with several simple setae, distal end of anterior margin with three simple setae; posterior margin of the propodus with four cuspidate setae, second and third cuspidate setae with a simple seta at its base, distal end of anterior margin with three simple setae; dactylus claw-like, nail present, first proximal third of the anterior margin with a plumose seta, posterior margin with a simple seta close to the nail, a simple seta at half length of the proximal margin of the outer side of the nail. Pereopod 4 similar in shape to pereopod 3 but longer; coxa wider than coxa 3 with a posterior excavation; basis: almost at half length of the posterior margin with one seta. Pereopods 5–7 similar in shape; coxa of pereopod 5 wider than long with two unequal lobes, posterior lobe longer; coxa of pereopod 6 with anterior lobe much reduced, posterior lobe not reaching half the length of the basis; coxa of pereopod 7 short, with no anterior lobe. Basis of pereopods 5–7 with a posterior lobe rounded and denticulate; lobe of basis of pereopod 7 widely expanded, posterior margin with 11 setae.

Pleopods: not modified, biramous, elongate, with numerous long plumose setae.

Uropods: uropod 1 longer than uropod 2; length of peduncle longer than rami, anterior half of dorsal margin with stout simple setae; rami subequal in length with two dorsal and four distal setae. Uropod 2 subrectangular peduncle, length subequal to the rami, distal half of the dorsal margin with two stout simple setae; inner ramus slightly longer than outer one, dorsal margin with two stout setae and three distal ones. Uropod 3, peduncle, distal end with four robust setae, ramus styliform, total length subequal to the length of peduncle, apex truncate with three apical simple setae and one connate seta.

Telson: slightly wider than long, entire, fleshy, smooth dorsal surface with three short submarginal plumose setae on both sides (left and right); they do not reach the distal margin; no apical setae present.

Gills. Coxal gills small, simple, saclike, on segments 2–7.

Sternal gills tubular, on pereonites 3–7.

Oostegites foliaceous, with setae ending in a curl on the margin, on pereonites 2–7.

Remarks. The absence of eyes in the specimens described could indicate that they usually live in a subterranean area; therefore their presence in open waters could mean that they were carried away from their normal habitat accidentally.

In this study, it was noticed that the number of articles of the flagellum of antenna 2 can vary from one side to the other in one specimen. In the holotype, eight articles were present on the right side and nine on the left. The holotype appendages are more slender those of the paratype.

The most important morphological variations in both specimens were observed in the setation of the different appendages. In the holotype, five slender setae are present on the palp of maxilla 1 in the apex on the right side and only one is found on the left side, while in the paratype, apparently no setae were present. Articles 3 and 4 of the maxilliped bore different numbers of setae: 11–13 and 6–8, respectively. The uropods also varied in their number of setae: the smaller specimen had one seta less than the larger one; in the peduncle of uropod 1, two or three setae could be observed and one or two setae in the rami of uropod 2. The same pattern could be seen in the carpus and propodus of gnathopod 1 and on the basis and ischium of gnathopod 2: in the holotype, one additional seta was observed than in the paratype in these segments.

In the literature, we could see that the number of setae and the number of articles in the antennae are reported to increase with size in the organisms ( Stevenson & Peden 1973; Geisler 1994; García-Schroeder & Araujo 2009). It is important to indicate the variation ranges and handle this character with caution to avoid taxonomic confusion; anyway, it is common to include the number of setae in the taxonomic keys.

Ruffo (1957), in his description of H. anophthalma , mentioned that in the subterranean species, occasionally, a certain “accentuation” of some characters can be noticed, or that they could be anomalies. This author detected morphological variations similar to those found in this study.

This new species can be distinguished from the other five species with no eyes in the genus Hyalella by the presence of a dorsoposterior carina on pleonite 1 and 2. This character is not present in any of the eyeless species of the genus (Table 1). In addition, the lower lip is subtriangular, while in the case of H. spelaea it is subquadrangular and in H. anophthalma , H. caeca , H. muerta and H. imbya it is rounded. On the other hand, in both cenote specimens, three pappose setae were observed on the inner plate of maxilla 1, while in other species only two such setae are present although it is important to remark that the number of setae in some appendages is quite variable and increase with the length of the organism ( Garcia-Schroeder & Araujo 2009). In general, the species described in this study is morphologically closer to H. anophthalma , because of the absence of apical setae on the telson and the number of articles of some appendages. Both species have comb scales on the propodus of gnathopod 1, same as in H. spelaea , while in the other species of the genus they are absent (Table 1). Zimmer et al. (2009) have questioned the taxonomic value of the comb scales because of the variability observed between species. However, the nomenclature varies between authors. That is why it would be advisable to designate only by one name these setae to avoid further taxonomic confusions.

Figures

FIGURE 1. Hyalella cenotensis, holotype, female, 5.3 mm. UL, upper lip; LL, lower lip; LMd, left mandible; RMd, right mandible; LaM, lacinia mobilis; Mx 1 – 2, maxilla 1 – 2; Mxp, maxilliped; A 1 – 2, antenna 1 – 2. Scale bars = 100 microns.