Mysidetes unam, Hernández-Payán & Hendrickx, 2025

Hernández-Payán, José Carlos & Hendrickx, Michel E., 2025, A new species of the genus Mysidetes Holt & Tattersall, 1906 (Peracarida, Mysida, Mysidae) from western Mexico, Zootaxa 5723 (3), pp. 425-436 : 426-435

publication ID	https://doi.org/10.11646/zootaxa.5723.3.7
publication LSID	lsid:zoobank.org:pub:9AC958A1-1710-4AF5-ACAC-6F29EB32BA12
persistent identifier	https://treatment.plazi.org/id/038E879A-FFBD-1769-FF27-69A05B839E28
treatment provided by	Plazi
scientific name	Mysidetes unam
status	sp. nov.

Treatment

Mysidetes unam sp. nov.

( Figures 1‒5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 )

Material examined. Holotype, female (CL 6.90 mm; TL, 21.42 mm), TALUD VII cruise, St. 18 ( 24°14’30” N, 108°16’24” W), SE Gulf of California, Mexico, June 7, 2001, BS operated at 950‒1010 m (ICML-EMU-12634). GoogleMaps

Description. Carapace ( Fig. 1A, B View FIGURE 1 ) elongate, slightly longer than half of the abdomen length, slightly produced anteriorly in small, triangular, obtuse rostrum, anterolateral margins rounded, posterior edge deeply emarginated, with well-marked cervical groove anteriorly.

Eyes ( Fig. 1A, B View FIGURE 1 ) large, globular, cornea reaching to distal margin of first segment of the antennular peduncle, pigments brown (fixed material).

Antennular peduncle ( Fig. 1A, B, D View FIGURE 1 ) robust, 3-segmented; first segment rectangular, longer than second, almost the same length as third, with 4 short distolateral, plumose setae; second segment shortest, with 1 long distomesial, plumose seta; third segment rectangular, more robust than first, with 3 long plumose setae on mesial margin, small protuberance in middle of distolateral margin, bearing 1 short seta.

Antennal peduncle ( Fig. 1A, C View FIGURE 1 ) slender, about 6 times as long as wide, a little more than 2/3 of scale length; first segment shortest; second segment longest, with 2 long distomesial plumose setae. Antennal scale ( Fig. 1A‒C View FIGURE 1 ) long, narrow, extending well beyond distal edge of antennal peduncle, about 7 times as long as wide, inner margin slightly convex, setose, distal suture present, distal segment very small, about 1/30 of scale total length.

Mandibles ( Fig. 2A View FIGURE 2 ) palp long, robust, first of three segments shortest, naked; second segment about 2.2 times as long as third, widen in middle, with series of short, simple setae on outer margin, 3 long, plumose setae on distolateral corner; third segment about 3 times as long as wide, with short comb-like setae on inner margin, apex with 3 long, plumose setae. Right mandible well-developed, incisive process composed of a chitinous ridge with 7 teeth; pars centralis reduced, represented by 3 small spine-like setae between incisive process and molar process; molar process small, rectangular, concave in center. Left mandible well developed, incisive process composed of a 4-teeth chitinous ridge; pars centralis composed of 3 small hook-like setae; lacinia mobilis present, well-developed, with 4 teeth; molar process slightly more robust than in right mandible.

Maxillula ( Fig. 2B View FIGURE 2 ) with outer lobe armed with 11 stout apical setae, 11 long plumose setae on ventral surface; inner lobe with 7 long, stout, plumose apical setae, 4 long, plumose setae on inner margin, 10 (partly broken) setae on ventral surface.

Maxilla ( Fig. 2C View FIGURE 2 ) exopod broader than long, extending slightly beyond proximal margin of second segment of endopodite, with 42 marginal, moderately long, plumose setae; distal segment of endopod oval, densely setose on both margins, setae long, plumose, longer and more robust along inner margin; 2 basis endites, distal smaller than proximal, both armed with long, plumose setae on distal part of inner margin.

Labrum ( Fig. 2D View FIGURE 2 ) generally symmetrical, posterior margin with simple, short setae.

First thoracopod ( Fig. 3A View FIGURE 3 ) endopod short, slender; basis rectangular, with 4 short plumose setae on inner margin; gnathobase with a small, poorly developed lobe, covered with 14 long, plumose setae on inner margin; pre-ischium with 2 long, plumose setae; ischium with 9 long, plumose setae on inner margin; merus elongate, with 24 long, plumose setae on inner margin; carpopropodus with several long, slender, plumose setae on inner and outer margins; dactylus short, triangular, ending in long serrated nail, slightly longer than dactylus; exopod almost twice as long as endopod, with 8 articles, 1st to 7th with one long, distolateral seta, 8th with 2 long, distal, plumose setae.

Second thoracopod ( Fig. 3B View FIGURE 3 ) endopod longer than first, elongate, without subchelate ending; basis broad, with 3 long, plumose setae on distal margin; pre-ischium short, with long plumose setae on both margins; ischium about 2 times as long as pre-ischium, armed with 16 long, plumose setae on inner margin; merus about 4 times as long as wide, armed with 13 long, plumose setae on inner margin; carpopropodus unsegmented, shorter than merus, armed with 6 long, plumose setae on inner margin, 10 long, plumose setae on outer margin; dactylus densely covered with long, plumose setae on both margins, terminal nail missing (in both thoracopods 2); exopod longer than endopod, with 10 articles, 1st to 9th with long, distolateral plumose seta, 10th with 2 long, distal plumose setae.

Third thoracopod ( Fig. 3C View FIGURE 3 ) with long, slender endopod; basis broad, with 2 long simple setae on both margins; pre-ischium short, without setae; ischium armed with 10 long, simple setae on inner margin; merus about same length of ischium, armed with about 25 simple setae on inner margin, four long and 2 short, simple setae on distal margin; carpopropodus 3-segmented, first slightly longer than combined length of second and third, latter two segments almost equal in length, each segment with numerous long and short setae on inner margin; dactylus about as long as third article of carpopropodus, terminal nail about 2/3 of dactylus length; exopod longer than endopod, with 9 articles, 1st naked, 2nd to 8th with one long, distolateral plumose seta, 9th with 2 long, distal plumose setae.

Thoracopods 4‒8 missing. Marsupium composed of 3 pairs of oostegites.

Pleopods ( Figs. 1A View FIGURE 1 , 4A‒E View FIGURE 4 ) uniramous, well developed, increasing in size posteriorly, covered with long, plumose setae on inner margin, proximal part with several short, simple setae on inner margin, 1 or 2 short, simple setae on outer margin.

Uropods ( Figs. 1A View FIGURE 1 , 5A View FIGURE 5 ) long, slender; exopod approximately 2.5 as long as telson, about 10 times as long as wide, margins completely setose; endopod approximately 2 times as long as telson, with well-developed statocyst, inner margin with 23 robust, equally spaced, mobile setae, extending from middle region of statocyst to distal 2/3 of the endopod, slightly increasing in length distally.

Telson ( Fig. 1A View FIGURE 1 , 5B, C View FIGURE 5 ) short, about 2/3 of last abdominal somite length, linguiform, broader at base, maximum width about 0.6 times of telson length, lateral margins unarmed on proximal 2/3, remaining 1/3 armed with 8 robust setae, similar in size, apex truncated with shallow, semi-elliptical cleft, depth about 1/12 of telson length, width about twice as wide as apical lobe, armed with 3 (left) and 5 (right) small setae, apical lobes armed with 2 robust setae, outer one about twice as long as inner one.

Size. Female ( 1 specimen), CL 6.90 mm, TL 21.42 mm.

Etymology. The species name, “ unam ” is in honor of the Universidad Nacional Autónoma de México (UNAM), the leading institution in higher education and scientific research in Mexico.

Distribution. So far only known from type locality, in the SE Gulf of California, Mexico ( 24°14’30” N, 108°16’24” W).

Depth range. Precise residence depth unknown. The only available female was captured in a non-closing net operating between the surface and 950‒1010 m depth, about 55 km offshore.

Remarks. The genus Mysidetes Holt & W.M. Tattersall, 1906, currently contains 17 species formally described between 1906 and 1995 ( Mees et al. 2025). The material examined from the Mexican Pacific matches the characteristics of the genus Mysidetes ( type species, M. farrani ) originally proposed by Holt & W.M. Tattersall (1906), particularly: the lanceolate antennal scale; the mandibles with a distinct and well-developed molar process; the pleopods of female rudimentary; the telson lateral margins armed with robust setae, no median setae; the uropodal endopod armed with a row of setae along the greater part (>0.5) of the inner edge.

It also matched the diagnostic characteristics proposed by Wittman & Chevaldonné (2021) (female only): eyes well developed, cornea large, globular; eyestalk well developed; antennula without modified setae; antennal scale setose all around, without spines or teeth; maxilla without spines; thoracic endopod 3 (4‒8 missing in the new species) slender, not prehensile, carpopropodus multi-segmented (3 articles in thoracopod 3), pleopods nondimorphic, reduced to bifid setose plates, no modified setae or spines; uropodal endopod with row of spine-like setae on inner margin; telson apical cleft present, cleft with short setae (laminae); telson lateral margin with spine-like setae in distal third.

The absence of males in the material examined did not allow for comparison with the other characters proposed for males by Wittmann & Chevaldonné (2021), including the shape of the appendix masculina, the absence of notches on the outer margin of thoracic endopod 2, the size (long) and shape (slender, stiff) of the pene, and the shape of pleopods (similar to females pleopods).

A new genus, Muscamysis Kou, Meland & Xinzheng Li, 2025 , very similar to Mysidetes , was recently added to the Mysidetini (Kou et al. 2025) . Again, the lack of male specimens does not allow for a comparison of their genital appendage, i.e., Muscamysis features a well developed, rod-like male genital organ (Kou et al. 2025: fig. 4j), but not similar to Mysidetes that features an extremely enlarged pene (Kou et al. 2025). Also, Muscamysis does not feature a distal suture on the antennal scale vs. a suture present in Mysidetes unam sp. nov.; in Mysidetes unam sp. nov. the endopod of thoracopod 2 does not end in 2 long, spine-like setae as in Muscamysis . The presence of a 3-segmented carpopropodus on the thoracic endopod in Mysidetes unam sp. nov. is also seen in M. farrani and M. halope , which is uncommon for the genus, where most species have a carpopropodus with 6‒8 articles. It is also worth noting that a similar three-segmented carpopropodus in thoracopods 3–7 is seen in Muscamysis (Kou et al. 2025) . Based on these observations, the new species is assigned to Mysidetes ; however, to determine the potential affinity of Mysidetes unam sp. nov. with Muscamysis examination of adult males is necessary.

Major characteristics used to distinguish among the species of Mysidetes are (1) the sharpness of the rostrum, (2) the shape and relative size of the eyes, (3) the proportional size of the antennular peduncle articles, (4) the shape and relative size of the antennal peduncle and scale, and (5) the shape and proportions of the telson and uropodal appendages and their armature. Characters 4 and 5, in particular, permit to distinguish most species.

Among the 17 species of Mysidetes , 13 are readily distinguished from M. unam sp. nov. by the armature of the lateral margins of the telson, consisting of robust setae in the distal 1/3 of the telson length in the new species. Three of these 13 species feature a series of robust setae present on the entire lateral margins of the telson: M. dimorpha O.S. Tattersall, 1955 , M. hanseni Zimmer, 1914 , and M. posthon Holt & W.M. Tattersall, 1906 (senior synonym of M. similis Zimmer, 1914 ). Another five species feature marginal setae located in a large distal section of the telson (>1/2 to 2/3 of telson total length) and in a shorter, proximal section, separated from the distal and medial setae by a narrow fringe devoid of marginal setae or armed with sparse setae: M. antarctica O.S. Tattersall, 1965 , M. illigi Zimmer, 1914 , M. kerguelensis (Illig, 1906) , M. morbihanensis Ledoyer, 1995 , and M. microps O.S. Tattersall, 1955 . Six other species feature a telson with marginal, distal setae in at least 1/2 of telson length, but without proximal setae: M. anomala O.S. Tattersall, 1955 , M. brachylepis W.M. Tattersall, 1923, M. farrani Holt & W.M. Tattersall, 1905 (but see text below), M. intermedia O.S. Tattersall, 1955 , M. macrops O.S. Tattersall, 1955 , and M. patagonica O.S. Tattersall, 1955 . In addition, both M. kerguelensis and M. morbihanensis feature a distinctive pattern of hexagonal figures on the telson ( Ledoyer 1995), absent in M. unam sp. nov.

Some of these 13 species are also distinguished from M. unam sp. nov. by the shape and depth of the telson cleft, where in some species is much deeper and armed with a much larger number of setae: M. anomala , M. brachylepis , M. intermedia , M. macrops , M. morbihanensis , M. patagonica , and M. posthon . Two species, M. anomala and M. patagonica , possess numerous robust setae on the apical lobes of telson vs. apical lobes armed with only two robust setae, one short inner and one long outer, as in M. unam sp. nov.

The remaining three species, M. crassa Hansen, 1913 , M. halope O’Brien, 1986 , and M. peruana Bacescu, 1967 , as well as M. farrani , are distinguished from the new species by the following characters.

Mysidetes crassa features a much shorter, less projecting rostrum and a short, proportionally wide antennal scale, about twice as long as wide, slightly overreaching the antennular peduncle vs. a very slender and long scale in M. unam sp. nov., about seven times as long as wide, extending well beyond the distal edge of the antennal peduncle; M. crassa possesses nine marginal setae on the distal part of the telson vs. eight in M. unam sp. nov., and a deep, V-shaped telson cleft (about 1/4 of telson length) armed with 14 minute setae in about the deeper half on each side of the cleft vs. a semi-elliptical cleft (less than 1/10 of telson length) armed with 3‒6 small setae on each side in M. unam sp. nov.

In M. halope the antennal scale is less than 4 times as long as wide vs. 7 times in M. unam sp. nov.; in M. halope the lateral margins of the telson possesses only 3 weak setae vs. 8 robust setae in M. unam sp. nov.; in M halope the uropodal exopod is about 1.6 times as long as the endopod vs. about 1.3 times in M. unam sp. nov. and the endopod and exopod are more slender in M. unam sp. nov. (e.g., uropodal exopod about 6 times as long as wide in M. halope vs. 10 times in M. unam sp. nov.); in M. halope , the ventral face of the uropodal endopodite is armed with 12 or 13, widely spaced, robust, spine-like setae covering about half the endopod length vs. 23 equally spaced, robust setae, extending along the mesial margin from the middle region of the statocyst to about 60% of the endopod length in M. unam sp. nov.

In M. peruana the rostrum is very weakly projected, almost straight, vs. projecting, obtusely triangular in M. unam sp. nov.; the antennal scale of M. peruana is about 4 times as long as wide vs. 7 times in M. unam sp. nov.; the uropodal endopod of M. peruana possesses 7 irregularly spaced robust setae, with proximal five closely set, vs. 23 equally spaced, robust setae in M. unam sp. nov.; in M peruana the telson marginal setae are similar to those of M. unam sp. nov. but with larger numbers (11‒12 vs. 8) and are located in the distal 1/2 of the telson vs. in about 1/3 of the telson length in M. unam sp. nov.; the telson cleft is very narrow in M. peruana (less than 1/2 the width of apical lobe) vs. wide (about twice as wide as each apical lobe), and semi-elliptical in M. unam sp. nov.

Mysidetes unam sp. nov. bears a close resemblance to M. farrani View in CoL , which is the type species of the genus, originally collected in deep water off western Ireland (three damaged specimens, all females). The first account of M. farrani View in CoL , where it was briefly described, was in Holt & W.M. Tattersall (1905), without illustrations, and was tentatively included in the genus Mysideis View in CoL . They concluded, however, that due to the characters of the mouthparts, it would be necessary to include this species in a new genus, a nomenclatural act which they formalized one year later (Holt & W.M. Tattersall 1906) with a more detailed description and illustrations of Mysidetes farrani View in CoL (dorsal view of female; antenna, antennal scale, and thoracopods 1‒3 of same) based on freshly collected material of both sexes. They emphasized, however, that the material they examined presented a “variation in the depth of the cleft of the telson [which is] rather surprising” (i.e., from 1/10 to 1/5 of telson length), adding that “the armature of the cleft is not always the same”. In their review of “British Mysidacea”, W.M. Tattersall & O.S. Tattersall (1951) provided additional information and illustrations of M. farrani View in CoL based on the original, northern material, and more recently obtained material from the Mediterranean. They distinguished two “races”, a small race occurring in the Mediterranean (W.M. Tattersall & O.S. Tattersall 1951: figs. 76C, 77) and a large race from northern waters from where the species was originally described (W.M. Tattersall & O.S. Tattersall 1951: figs. 76A, B, 78). The Mediterranean specimens had a “shorter, more obtuse rostrum, less pronounced antero-lateral angles with respect to the carapace, a broader antennal scale, uropods with fewer spines on the endopods, fewer spines on the telson, and a much shallower telson notch compared with that in Irish specimens” (W.M. Tattersall & O.S. Tattersall 1951: p. 310). However, they still considered that, giving more weight to similarities, there was no justification for recognizing two different species “simply because of the differences in the relative proportions of the antennal scale and the telson” (W.M. Tattersall & O.S. Tattersall 1951: p. 310).

While examining material from the Bay of Biscay (Golfe de Gascogne), in the northeastern Atlantic, Lagardère & Nouvel (1980) studied a series of specimens of both sexes of Mysidetes farrani View in CoL . They elaborated a comparative table with some diagnostic characteristics such as the number of ventral spines in the cleft and on the lateral margins of the telson, the proportion of the antennal peduncle articles, and the number of spines on the uropods. They compared these characteristics with the figures available in Holt & W.M. Tattersall (1906: pl. V) and in W.M. Tattersall & O.S. Tattersall (1951: figs. 76‒78), concluding that Mysidetes farrani View in CoL , as referred to by these authors, seems to comprise two distinct species, diagnosed by the telson armature and the relative proportions of the antennal scale. Based on these observations, Lagardère & Nouvel (1980) suggested that M. farrani View in CoL as originally described corresponds to the “large” form (or race) designated by W.M. Tattersall & O.S. Tattersall (1951). However, Lagardère & Nouvel (1980) came short of recognizing the “small race” as a different species pending the availability of more information related to the relative proportions of the antennal scale between the less ornamented specimens reported by Holt & W.M. Tattersall (1906) and the Mediterranean material reported by W.M. Tattersall & O.S. Tattersall (1951). We agree with the suggestion of two morphological variants of M. farrani View in CoL and have compared M. unam sp. nov. with both “forms”.

The Mediterranean form of M. farrani “small race”, as illustrated by W.M. Tattersall & O.S. Tattersall (1951) features a short, wide antennal scale, vs. a long, narrow scale in M. unam sp. nov; the exopod of thoracopod 3 in M. farrani “small race” comprises 11 articles vs. 9 articles in M. unam sp. nov.; the telson cleft is deeper and proportionally narrower in M. farrani “small race” than in M. unam sp. nov. These differences alone exclude the possibility for the western Mexico material to belong to M. farrani “small race”.

There is a doubtful record by Fukuoka (2009) of a damaged adult female identified as “? Mysidetes farrani ”, from the northwest Pacific off Japan. The illustrations by Fukuoka (2009: fig. 20) of a broken uropod broken and telson missing most setae both along the lateral margins and inside the cleft are not conclusive. And the author did not provide any convincing arguments in favor of a possible identification with M. farrani , limiting his statement to “... specimen is damaged but is similar to the illustrations of the large race of M. farrani ”.

In the North Atlantic M. farrani “large race” the combined length of the second and third antennal peduncle articles is about 3.5 times as long as their width vs. almost 5 times as long in M. unam sp. nov. In addition, the tubercle observed at the base of the second antennular peduncle article by Lagardère & Nouvel (1980: fig. 43) is absent in M. unam sp. nov. The posterior margin notch of the carapace is shallow, circular in M. farrani (Holt & W.M. Tattersall 1906: pl. 5, fig. 1) vs. deep, V-shaped in M. unam sp. nov.

In M. farrani the dactylus of thoracopod 3 is shorter than third carpopropodus article and the nail is about as long as the combined length of articles 2 and 3 vs. dactylus as long as third carpopropodus article with a nail slightly shorter than this third article in M. unam sp. nov.

According to W.M. Tattersall & O.S. Tattersall (1951), the telson of M. farrani is equal in length to the last abdominal somite vs. about 2/3 of this somite length in M. unam sp. nov. In both species, the telson maximum width is about 0.6 times the telson length, but in M. farrani the telson cleft is deep, oblong, varying in depth from 1/5 to 1/10 the telson length, vs. shallow, semi-elliptical, about 1/12 of telson length in M. unam sp. nov.; the telson cleft aperture is also proportionally narrower in M. farrani , measuring less than half the telson width measured transversally at the cleft deeper point vs. more than half this width in M. unam sp. nov. In both species, the inner margins of the telson cleft are armed with short setae, 3‒10 in M. farrani vs. 3‒5 in M. unam sp. nov., but setae in M. farrani are minute; in both species apical lobes are armed with 2 robust setae, one short inner and one long outer, but telson margins in M. farrani are armed along the 3/5 of their length with 10‒26 robust setae, increasing in size distally, vs. unarmed on proximal 2/3 and remaining 1/3 armed with 8 similar in size, robust setae in M. unam sp. nov. In M. farrani the uropodal endopod and exopod are approximately 1.5 times and twice as long as telson, respectively, vs. twice and 2.5 times as long as telson, respectively, in M. unam sp. nov.; both species feature a long series of robust endopod setae extending from mid-statocyst to about 2/3 of endopod length: 13‒30 in M. farrani vs. 23 in M. unam sp. nov.

Wittmann & Chevaldonné (2021) provided a revised diagnostic for Mysidetes and a key to the 17 species known at that time, essentially based on the armature and shape of the telson, the uropodal endopod, and the shape and size of the antennal scale, eye, and rostrum. They redescribed two species, M. illigi Zimmer, 1914 (reinstalled) and M. hanseni Zimmer, 1914 , and provided COI and 18S sequences for the first time for the genus Mysidetes . Their study emphasized the need for a more detailed description for many species of Mysidetes , imprecisions in original descriptions, and complexity of the genus.

As stated by Kou et al. (2024) the history of the genera Mysideis (2 species) and Mysidetes (18 species including the one described herein) is a complex one, even more considering the addition of Muscamysis (so far comprising a simple species), which is closely related to these two genera ( Kou et al. 2024). The more specious genus, Mysidetes , is mostly neritic or oceanic, thus favoring a potential for very wide dispersion making its comprehensive study more complex due to the difficulty to gather material from across the globe.

Kou et al. (2024) concluded that characters such as the cleft telson and the reduced male pleopods are the results of parallel evolution, taking place multiple times across Mysida evolution. In the specific case of Mysidetes , Mysideis and Muscamysis , the availability of fresh material of a larger number of species might certainly help to elucidate their relationship within the Mysida .