Amblyopsoides ohlinii (W.M. Tattersall, 1951 )

Hernández-Payán, José Carlos & Hendrickx, Michel E., 2020, Amblyopsoides ohlinii (W. M. Tattersall, 1951) (Peracarida, Mysida, Mysidae) in western Mexico, Zootaxa 4878 (1), pp. 169-181 : 170-180

publication ID	https://doi.org/ 10.11646/zootaxa.4878.1.8
publication LSID	lsid:zoobank.org:pub:3D074640-435E-4087-A107-D22558BE69DD
DOI	https://doi.org/10.5281/zenodo.4565807
persistent identifier	https://treatment.plazi.org/id/03AF2D26-8A7F-7A67-FF41-FA20FD7F6E35
treatment provided by	Plazi
scientific name	Amblyopsoides ohlinii (W.M. Tattersall, 1951 )
status

Treatment

Amblyopsoides ohlinii (W.M. Tattersall, 1951)

( Figures 2‒9 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 )

Amblyops crozetii. — Ohlin, 1901a: 371; Zimmer, 1904: 451 (distribution only); 1905: 149 (passim); Stephensen, 1913: 73.

Amblyops Crozetii. — Ohlin, 1901b: 74.

Amblyops crozeti .— Zimmer, 1909: 114 (distribution only).

Amblyops ohlinii W.M. Tattersall, 1951: 130‒132 , fig. 45.— Gordan, 1957: 338 (list).— Birstein & Tchindonova, 1958: 334 (table 2), 352 (passim).

Amblyopsoides ohlini .— Birstein & Tchindonova, 1962: 66‒67; 1970: 285‒287, fig. 5.

Amblyopsoides ohlinii .—O.S. Tattersall, 1955: 108.— Mauchline & Murano, 1977: 46, tab. 1 (list).—Müller, 1993: 76‒77 (cata- logue).— Fukuoka, 2009: 421.

Material examined. TALUD XVI-B. Station 22 (30°49.78”N, 116°35.9”W), May 28, 2014, 1 male (CL 8.26 mm, TL 23.50 mm), benthic trawl operated at 1480‒1560 m (ICML-EMU-12635) ( Fig. 1 View FIGURE 1 ) .

Description. Single male specimen available. Carapace ( Fig. 2A, C View FIGURE 2 ) elongated, frontal margin uniformly arched, without rostrum, cervical groove well marked anteriorly and laterally.

Eyes ( Fig. 2A, B View FIGURE 2 ) large, sub-rectangular, a small median, subtriangular projection or papilla on dorsal surface, small spines on antero-lateral rim.

Antennular peduncle ( Fig. 2D View FIGURE 2 ) short, with 3 articles; first rectangular, longer than second, 1 single seta on outer margin; second article much reduced, 2 long, simple setae on inner margin; third article squarish, with tuft of long, simple, ventral seta (lobus masculinus).

Antennal peduncle ( Fig. 2E View FIGURE 2 ) slightly longer than 1/2 scale length; first article short, without spines or setae; second and third articles of approximately same length, 6 long, simple setae on inner margin of second article, 4 short, simple setae on third article. Antennal scale ( Fig. 2E View FIGURE 2 ) extended well beyond distal edge of antennular peduncle, approximately 3 times as long as broad, spine on external margin robust, located at over half length of scale, scale with setae along whole inner margin and part of outer margin, short distal suture present, distal segment very small.

Labrum ( Fig. 3D View FIGURE 3 ) asymmetrical, anterior margin slightly bilobed, with numerous short, simple setae.

Mandible ( Fig. 3A View FIGURE 3 ) palp long, wide, 3-articulated; first article short, without spines; second longer than third, proximally broad, margins setose; third segment approximately 4 times as long as broad, comb-shaped series of short setae on inner margin, 7 long, simple seta on outer margin, 2 long setae on distal edge. Right mandible ( Fig. 3A View FIGURE 3 ) well developed; incisive process composed of a chitinous ridge with 4 teeth; lacinia mobilis reduced; par centralis represented by a row of 4 small setae; molar process small. Left mandible ( Figs. 3A View FIGURE 3 , 4 View FIGURE 4 A‒D) with strong incisive process, consisting of a chitinous ridge with 6 teeth, 4 large and well marked, 2 reduced; lacinia mobilis present, well developed, consisting of 5 teeth, one bifid, four large and well marked; par centralis represented by 10 similar flattened elements, each with a row of long setae, united at base; molar process oval-shaped, grinding surface with about 12 parallel lamellae.

Maxilla ( Fig. 3B View FIGURE 3 ) exopodite narrow, sharp, extending to proximal margin of distal segment of endopodite, 34 marginal setae without setulas; distal segment of sub-rectangular endopod with long, simple setae on margins; endites armed with long, simple setae on distal part of internal margin.

Maxillula ( Fig. 3C View FIGURE 3 ) external lobe armed with 11 strong, apically serrated setae, 3 long plumose setae on ventral surface; inner lobe with 1 long, robust, apically serrated seta and 2 slender, plumose setae on distal part, 2 small, plumose setae on inner margin, and 2‒4 slender, plumose setae on outer margin.

First thoracopod ( Fig. 5A View FIGURE 5 ) small, thin; inner margin of pre-ischium with 4 long, simple setae; ischium with 10 long, simple setae on inner margin, 6 short setae on ventral surface; merus elongated, with 13 long, simple setae on internal margin, 3 short setae on ventral surface; carpopropodus with 4 long, simple setae on inner margin, 5 thin, simple setae on outer margin; dactyl short, triangular, with several long, thin setae on the inner and outer margins; terminal nail long; exopod longer than endopod, 9-articulated, each article with long, plumose setae.

Second thoracopod ( Fig. 5B View FIGURE 5 ) longer than first; endopod elongated, not subchelated; basis with 7 long, simple setae on inner margin; pre-ischium with 1 seta on ventral surface; ischium with 6 long, simple setae on inner margin, 2 on the outer margin; merus longer than broad, with 6 long, simple setae on inner margin; carpopropodus with 7 long and short, simple setae on inner margin, and 8 simple setae on outer margin; dactyl short, sub-triangular, with 7 simple setae on outer margin, 5 long, serrated setae on inner margin, terminal nail robust (exopod not illustrated).

Thoracopods 3‒8 missing. Penis ( Fig. 5C View FIGURE 5 ) at base of eighth thoracopod, elongated, with a series of long, simple setae on ventral surface, a tuft of 8 long, plumose setae at distal end.

Abdominal somites 1‒6 without spines. Pleopods ( Fig. 6 View FIGURE 6 A‒E) biramous, well developed; endopods with a semi-circular lobe on proximal edge, armed with long and simple setae; endopodite of first pair entire, exopodite multi-articulated; pleopods 2‒5 with multi-articulated endopod and exopod, sub-equal in length, all covered with long, plumose setae on both margins.

Uropods ( Fig. 7A View FIGURE 7 ) long, robust; exopod approximately 4 times as long as broad, extending beyond tip of telson by approximately 1/3 of its length; endopod about 2/3 length of exopod, statocyst at its base, armed with 1 proximal spine on inner margin.

Telson ( Fig. 7B View FIGURE 7 ) wide, approximately twice as long as maximum width at most-proximal part, 1/3 of proximal margins unarmed, remaining 2/3 armed with 20 spines increasing in size posteriorly, posterior margin wide, truncated, with a very shallow depression in middle, armed with 6 (2 x 3) strong spines increasing in length towards apex, a pair of small, median spines, with 2 long, plumose setae in between ( Fig. 7C View FIGURE 7 ).

Geographic distribution. ( Fig. 1 View FIGURE 1 ). Known from the type locality, ”Albatross” station 2550 (39°44’30”N, 70°30’45”W), and “Albatross” station 2192 (39°46’30”N, 70°14’45”W), both on the east coast of the United States (W.M. Tattersall 1951). Amblyopsoides ohlinii has also been reported in the North Pacific (44°35’N, 149°12”E) ( Birstein & Tchindonova 1970) and in three localities off Japan (42°27’36”N, 144°47’24”E; 40°N, 143°31’24”E; 39°20’06”N, 142°52’12”E) ( Fukuoka 2009). The material examined herein is the known southernmost limit for the Pacific Ocean (30°49.78”N, 116°35.9”W). The material reported by Ohlin (1901a) as Amblyops crozetii (considered by W.M Tattersall 1951 to belong to A. ohlinii ) came from 72°42’N, 14°49’W, off Greenland. The material reported by W.M. Tattersall (1951) is the only one available in the USNM collections (two lots, two adult and one immature females) (supra).

Bathymetry. The record by Ohlin (1901 a, 1901b; identified by him as A. crozetii ) is from 2000 m depth (four males and one female). In the original description of A. ohlinii, W.M. Tattersall (1951) indicated that the three specimens he examined, two adult females (type material, USNM-81262) and one immature female (USNM-82405), were collected at 1060 and 1081 fathoms (about 1939 and 1977 m, respectively) in beam trawl samples. Because opening-closing gear were most probably not used, specimens could have been collected near bottom or in the water column and the accurate depth at which they were captured is therefore unknown. Birstein & Tchindonova (1958) reported the same depth range as W.M. Tattersall (1951), with a small variation due to transformation of fathoms into meters (i.e., 1940‒1980 m). Birstein & Tchindonova (1970) indicated that the three specimens they examined, two females (13 and 14 mm TL) and 1 male (13 mm TL) were collected at a depth of 2165‒2265 m. This depth range is actually the range reported in WoRMS for this species (WoRMS Editorial Board 2020). The three samples reported by Fukuoka (2009) were all collected with a benthic beam trawl (probably non closing) operating between 1709 and 2055 m, and accurate depth at which their material was caught is also uncertain. According to Müller (1993), A. ohlinii is bathypelagic (i.e., 1000‒4000 m) and, although this is likely to be correct, there is no absolute evidence for this. The gear used during the TALUD survey sampling, a benthic sledge, remained open during the descent and the ascent in the water column, thus acting as a mid-water net. Consequently, there is no precise information related to the depth at which the only specimen available was caught in the water column. Only using opening-closing sampling gear operated throughout the water column, including very deep water, will it be possible to establish the bathymetric range at which A. ohlinii occurs.

Remarks on synonymy. As correctly stated by W.M. Tattersall (1951: 131), the illustrations and text provided by Zimmer (1904: figs. 117‒118, 1909: figs. 229‒231) when referring to Amblyops crozetii “... quote the description of the real A. crozetii from the Challenger report and reproduce Sars’ figures of that species, but the recorded distribution refers to Ohlin’s paper [1901a, 1901b] “. Consequently, the contributions by Zimmer (1904, 1909) should not be included in the synonymy of A. ohlinii as stated by W.M. Tattersall (1951: 130) except for the distribution information.

General remarks. As stated by W.M. Tattersall (1951), A. ohlinii is very close to A. crozetii (G.O. Sars, 1883) with which it has been confused earlier (see synonymy). While describing A. ohlinii, W.M. Tattersall (1951) put emphasis on two diagnostic characters observed in the type material: 1) the presence of “... only 6 spines arming the broad truncate or slightly emarginate apex of the telson” (vs. 14, more slender spines in the type material of A. crozetii ; but see note related to this below); 2) “The anterolateral angle of the ocular plate is microscopically spinulose or denticulate” (vs. smooth eye in the description of A. crozetii , but see note related to this below). The material examined from the Mexican Pacific coincides with the characteristics observed or illustrated by W.M. Tattersall (1951: fig. 45) (see Fig. 8 View FIGURE 8 ), such as the shape, proportion (length/width ratio), and number of spines on the lateral margins of the telson, the relative size of the antennular peduncle and of the antennal scale, and the length/width ratio of the antennular peduncle articles. However, the type material used and illustrated by W.M. Tattersall (1951: fig. 45) was somewhat damaged, especially the armature of the apex of the telson, and the number of terminal spines was therefore probably estimated based on number of sockets: “... Unfortunately, the material is very defective, and I have shown only those spines that are actually still present on the specimen” (W.M. Tattersall, 1951: 130). The material from Western Mexico, however, possess an intact telson, with the apex bearing a series of 6 long median spines (3 on each side) gradually decreasing in size towards the lateral corner, and a pair of much shorter middle spines and two middle, long, plumose setae between this pair. This feature perfectly match the illustration of the telson apex of a male specimen reported by Birstein & Tchindonova (1970: fig. 5; reproduced herein as Fig. 9 View FIGURE 9 for comparative purposes) (see Fig. 9K View FIGURE 9 ).

According to W.M. Tattersall (1951), the other distinctive character of A. ohlinii is the presence of small microscopic spines in the anterolateral angle of the eye plate. This character was observed and illustrated by him (W.M. Tattersall 1951; fig. 45) (see Fig. 8A, B View FIGURE 8 ) and by Birstein & Tchindonova (1970: Fig. 5 View FIGURE 5 ) (see Fig. 9A, B View FIGURE 9 ). Because of their very small size, these spines are inconspicuous under low magnification, but can be clearly observed in the Mexican specimen ( Fig. 1B View FIGURE 1 ). It should be noted, however, that similar small spines on part of the eye plates can be found in all other species of Amblyopsoides (e.g., A. crozetii , A. halleyi , A. obtusa ) (O.S. Tattersall, 1955; Birstein & Tchindonova, 1970; Ledoyer, 1990) and in several species of Amblyopsis (e.g., Amblyopsis australiensis Murano, 2012 ; A. izuensis Murano, 2012 ; A. okinawensis Murano, 2012 ; A. pacificus Murano, 2012 ; A. sagamiensis Murano, 2012 ). It would therefore appear that the presence-absence of the small spines or denticles on the eye plates does not appear to be a valid character that separate A. ohlinii from other species of Amblyopsoides or, for that matter, from several species of Amblyopsis . On the other hand, the outer rim of the eye in the material examined herein is more pronunciated and rounded than in the material reported by Birstein & Tchindonova (1970) (see Fig. 9A, B View FIGURE 9 ), but this might be an artefact due to the illustration process.

It should be noted, however, that Birstein & Tchindonova (1970) formulated some reserves to their observations, and while referring to A. ohlinii and A. crozetii (both species referred to with a single final “ i ”) they stated: “... [considering the] two forms [that] appeared in the collection, one of which practically does not differ from A. ohlini , and the other from A. crozeti [...] it should be borned in mind that the insufficient completeness of the original descriptions and drawings of both species prevent detailed comparison with our former [ A. ohlinii ] specimen. The specimens we relate to this species [ A. ohlinii ] differ from the one depicted by [W.A.] Tattersall A. ohlini only in a slightly narrower telson”. They further comment on the structure of the apex of the telson in material previously identified as A. ohlinii , noting that: “O. Tattersall (O. Tattersall, 1955) suggests that this species [ A. ohlinii ] lacks a pair of small median spines of the posterior margin of the telson (the only studied specimens of A. ohlini has most of the spines of the posterior margin of the telson broken off), but in our specimen they are available. The relative length of the two pairs of large spines closest to the middle varies: in one of our specimens they end at the same level, which O.S. Tattersall attaches great importance to; in the other two, the inner ones are somewhat longer than the outer ones.” Birstein & Tchindonova (1970: fig. 5) provide illustrations of the telson apex for both sexes which are reproduced herein (see Figure 9J, K View FIGURE 9 ).

There is little doubt that our male specimen fits well with the material reported by Birstein & Tchindonova (1970), with only very subtle variations (all observed on the illustrations provided by Birstein & Tchindonova (1970) and in this contribution) like in the proportion of the inner to outer uropods (exopod 1.25 times the length of endopod and exopod 4.2 times as long as wide in our material; vs. 1.72 times and 5.6 times, respectively, in Birstein & Tchindonova (1970) (compare Fig. 7A View FIGURE 7 with Fig. 9H View FIGURE 9 ). Unfortunately, the type material’s uropods were not described or illustrated by W.M. Tattersall (1951). Birstein & Tchindonova (1970) compared their specimens with the illustrations of A. ohlinii provided by W.M. Tattersall (1951) and found that they only differ in the shape of the telson, which is slightly narrower in the material they reported (see Fig. 9I View FIGURE 9 ). Compared with the illustrations in W.M. Tattersall (1951) and in Birstein & Tchindonova (1970), the apex of the telson appears less strongly emarginated in our specimen (compare Fig. 7B View FIGURE 7 with Figs. 8D View FIGURE 8 , 9 View FIGURE 9 I‒K). The apex of the telson, which is intact in our specimen and considered a diagnostic character by W.M. Tattersall (1951), possesses 6 long median spines (gradually decreasing in size from the inner to the outer), a pair of much shorter middle spines and two middle setae, as in Birstein & Tchindonova’s (1970) illustration.

Neither W.M. Tattersall (1951) nor Birstein & Tchindonova (1970) have noted the presence of a small, distal segment in the antennal scale of A. ohlinii . However, the antennal scale of our specimen is delimitated by a short, transversal suture (see Fig. 2E View FIGURE 2 ). O.S. Tattersall (1955) considered this character as facultative (present or not) in her definition of Amblyopsoides and included the illustration of the distal segment in the antennal scale of A. obtusa (O.S. Tattersall 1955: fig. 24B). The presence of such a short distal suture has not been reported in A. crozetii by Birstein & Tchindonova (1970) or in A. halleyi by Ledoyer (1990). G.O. Sars (1885: pl. 33, fig. 13) did not represent a distal suture in his illustration of A. crozetii , with the apex of the scale being clearly narrow and rounded; but in the description of this appendage he stated that “... the apex being very obliquely truncate ...”, which might indicate that when observed by G.O. Sars (1885) the apex of the scale was folded, precisely at the level of the distal suture, thus giving a “truncate” appearance to the scale. Ledoyer (1990: fig. 2A) reported a distal section (“lobe distal”) in the antennal scale (distal to the lateral spine) of A. halleyi , which is about 1/3 the length of the scale (as in A. obtusa vs. about 1/ 2 in A. ohlinii and A. crozetii ), but without referring to the presence of a distal suture in any of these species.

The diagnostic characters frequently used by authors to separate the four known species of Amblyopsoides are not very convincing (see Table 1 View TABLE 1 ). The shape of the frontal portion of the carapace might varied according to the angle at which it is observed and the posterior margin (apex) of the telson is often damaged, thus making it difficult to count the posterior spines. In addition, sometimes part of the postero-lateral spines might be regarded as posterior spines. For example, the diagnosis proposed by G.O. (Sars 1885) for A. crozetii stated that there are about fourteen posterior spines, while in the illustration there seems to be only 5‒6 pairs of spines (G.O. Sars 1885, pl. 33, fig. 16). The presence-absence of spinules on the eye plates, which were considered diagnostic for A.ohlinii by W.M. Tattersall (1951) has been proved to be a character common to all the species of Amblyopsoides . In contrast, some other characters (see Table 1 View TABLE 1 ) might be more convincing (e.g., position of the lateral spine in the antennal scale; length-width ratio of distal antennular article) but remain to be tested. In future, a close examination of large series of mature specimens of both sexes is need in order to confirm the validity of each species.

The scarcity of A. ohlinii in the samples taken during the entire TALUD survey of western Mexico (a single male) is in contrast with the numerous specimens of certain species of pelagic Lophogastrida (e.g., Eucopia unguiculata (Willemoes-Suhm, 1875) ; unp. data) and some other species of Mysida (e.g., Hansenomysis lucifugus (Faxon, 1893) and Petalophthalmus armiger Willemoes-Suhm, 1875 ; see Hendrickx & Hernández-Payán 2018, 2020) captured during the same survey in the same area. We speculate that the scarcity of A. ohlinii might be due to the great depths at which this species inhabits.