Euchaetomera plebeja Hansen, 1912

Hernández-Payán, J. C. & Hendrickx, M. E., 2024, Euchaetomera plebeja Hansen, 1912 (Mysida; Mysidae) in western Mexico, Zootaxa 5519 (2), pp. 279-295 : 281-294

Euchaetomera plebeja Hansen, 1912

Euchaetomera plebeja Hansen, 1912: 202 , pl. 3, figs. 1a, b.— Illig 1930: 578 (key).— Tattersall 1943: 67; 1951: 112.— Gordan 1957: 348 (list).— Murano, 1977: 142 (key), 145‒146.—Buji et al. 2010: 43 (table 2).— Ortiz et al. 2016: 113 (species attributed to G. Sars, 1884 by error).—Wittmann 2024: 154 (key).

A total of 109 specimens of Euchaetomera plebeja , 35 males and 74 females were collected in 20 localities during the present study. There were no ovigerous females in the samples. This represents by far the largest collection of E. plebeja ever made available. Based on this abundant material, an almost complete description of E. plebeja is provided herein (Thoracopods 4‒8 are lacking in our material), allowing for a better understanding of this species morphology.

Material examined. TALUD I. St. 5 (22°00’22’’ N, 106°49’18’’ W), December 11, 1989, 1 female (CL 1.40 mm) , MN between surface and 500 m ( TD 1970 m) (ICML-EMU-12636).

TALUD III. St.19-B (25°18’24’’ N, 109°18’36’’ W), August 20, 1991, 2 males ( CL 1.30 and 1.98 mm) and 1 female ( CL 1.00 mm), IK between surface and 600 m ( TD 1890 m) (ICML-EMU-12637- A); St. 25 A-1 (25°51’00” N, 109°57’00” W), August 21, 1991, 9 males ( CL 1.00‒ 2.37 mm) and 18 females ( CL 1.16‒2.32 mm), IK between surface and 200 m ( TD 2000 m) (ICML-EMU-12637- B); St. 25 A-2 (25°50’54’’ N, 109°56’54’’ W), August 21, 1991, 3 males ( CL 1.19‒1.78 mm) and 7 females ( CL 1.69‒2.20 mm), IK between surface and 230 m ( TD not recorded) (ICML-EMU-12637- C) .

TALUD IV. St. 22 (24°17’20’’ N, 108°50’30’’ W), August 26, 2000, 3 females (CL 1.11‒1.81 mm) , MN between surface and 1325 m ( TD 1500 m) (ICML-EMU-12638-A); St. 29 (24°57’48’’ N, 109°37’00’’ W), August 27, 2000, 1 female (CL 2.14 mm) , MN between surface and 1280 m ( TD 2080 m) (ICML-EMU-12638-B); St. 36 (25°51’59’’ N, 110°11’00’’ W), August 27, 2000, 2 males (CL 1.14 and 1.37 mm) and 1 female (CL 2.22 mm) , MN between surface and 1000 m ( TD 2100 m) (ICML-EMU-12638-C) .

TALUD V. St. 15 (23°21’30’’ N, 107°48’12’’ W), December 14, 2000, 1 female (CL 1.53 mm) , MN between surface and 1350 m ( TD 2384 m) (ICML-EMU-12639-A); St. 29 (25°14’36’’ N, 109°24’15’’ W), December 17, 2000, 1 male (CL 1.95 mm) and 2 females (CL 1.01 and 1.39 mm) , MN between surface and 1290 m ( TD 2040 m) (ICML-EMU-12639-B); St. 36 (25°54’30’’ N, 110°11’24’’ W), December 17, 2000, 3 males (CL 1.54‒1.70 mm) and 5 females (CL 1.39‒2.12 mm) , MN between surface and 1340 m ( TD 1990 m) (ICML-EMU-12640) .

TALUD VI. St. 7 (22°21’39’’ N, 107°01’42’’ W), March 14, 2001, 1 male (CL 1.14 mm) , MN between surface and 1305 m ( TD 2100 m) (ICML-EMU-12641-A); St. 22 (24°17’26” N, 108°50’05” W), March 15, 2001, 3 females (CL 1.63‒1.93 mm) , MN between surface and 1600 m ( TD 1760 m) (ICML-EMU-12641-B); St. 29 (25°16’24” N, 109°24’54” W), March 16, 2001, 1 male (CL 1.44 mm) and 1 female (CL 1.99 mm) , MN between surface and 1440 m ( TD 2080 m) (ICML-EMU-12641-C); St. 36 (25°53’15’’ N, 110°10’08’’ W), March 17, 2001, 3 males (CL 1.70‒1.80 mm) and 9 females (CL 1.61‒2.34 mm) , MN between surface and 1360 m ( TD 2000 m) (ICML-EMU-12642) .

TALUD VII. St. 29 (25°17’31” N, 109°24’30” W), June 8, 2001, 5 males (CL 1.05‒1.70 mm) and 7 females (CL 1.42‒2.26 mm) , MN between surface and 1500 m ( TD 2080 m) (ICML-EMU-12643); St. 36 (25°42’37” N, 110°04’35” W), June 9, 2001, 2 males (CL 1.81 and 2.08 mm) and 5 females (CL 1.63‒2.48 mm) , MN between surface and 1500 m ( TD 2400 m) (ICML-EMU-12644) .

TALUD X. St. 7 (27°53’09’’ N, 112°16’42’’ W), February 10, 2007, 2 females (CL 1.01 and 1.18 mm) , MN between surface and 900 m ( TD 1191 m) (ICML-EMU-12645-A); St. 11 (27°34’16’’ N, 111°40’30’’ W), February 11, 2007, 2 males (CL 1.43 and 1.96 mm) and 4 females (CL 1.00‒ 1.82 mm) , MN between surface and 1220 m ( TD 1800 m) (ICML-EMU-12645-B); St. 20 (27°14’41’’ N, 111°36’15’’ W), February 13, 2007, 1 female (CL 2.01 mm) , MN between surface and ca. 1250 m ( TD 1785 m) (ICML-EMU-12646-A); St. 23 (27°00’30’’ N, 111°12’00’’ W), February 14, 2007, 1 male (CL 1.07 mm) and 2 females (CL 1.59 and 2.03 mm) , MN between surface and ca. 1250 m ( TD 1770 m) (ICML-EMU-12646-B) .

Diagnosis. Body without spines. Rostrum triangle-shaped, apically blunt. Antennal scale about 5 times as long as broad, subterminal spine not overreaching terminal lobe, distal suture present. Posterior cornea sub-rectangular, about same size or slightly larger than anterior cornea in dorsal view, long, tubular ocular papilla present ventrally. Uropodal endopod without spine on medial margin. Telson slightly shorter than broad, without spines along lateral margin, apex with 1 pair of lateral setae.

Description. (Male except indicated) Body thin, elongated. Carapace short, produced anteriorly to form subtriangular rostrum ending in blunt point, not reaching base of eyestalks ( Fig. 1B ).

Eye ( Fig. 1A, B ) enlarged, bilobed, lobes well developed, separated, each with well developed cornea, distal margin of anterior cornea reaching to proximal margin of second antennal article; in dorsal view anterior cornea crescent-shape, extending along entire anterior margin, broader than long, posterior cornea sub-rectangular, postero-lateral, extending on about half the eye width, outer margin protruding; ocular papilla present on ventral face of the eye, long, tubular.

Antennular peduncle ( Fig. 1C ) slender, first and third articles about the same length, first much slender than third, second about 1/3 length of third; first and second with one simple setae on inner distal corner, third slightly broader than other two, one subterminal simple setae on inner margin and two setae on outer distal corner.

Antennal peduncle ( Fig. 1E ). Second article of antennal peduncle about 2.5 times as long as first, about 1.7 times as long as third; first and second articles naked, a simple setae on inner distal corner of third article. Antennal scale ( Fig. 1E ) about 5 times as long as broad, overreaching anterior margin of antennular peduncle, outer margin almost straight, distal suture present, distal lobe about as long as broad, outer subterminal spine short, located at about the length of the distal lobe beyond the antennal scale distal suture.

Labrum ( Fig. 2C ) subcircular, semi-symmetrical, posterior margin semi-bilobed, with short and simple setae (left) and finely jagged (right).

Mandibles ( Fig. 2A , 3A‒F ). Palp long; article 1 short, without setae; article 2 little less than twice as long as article 3, wider at about mid-length, outer and inner margins with simple setae; article 3 with one long, submarginal simple setae near proximal margin, a series of 8 subdistal, short setae, and one long seta, outer margin armed with 7 serrated setae, one partly serrated curved setae, and one long seta. Mandibles both with well developed incisor process, lacinia mobilis, spine row, and molar process. Right mandible ( Fig. 2A , 3A‒C ) incisor process well developed, robust, composed of chitinous ridge with 6 strong teeth; lacinia mobilis well developed between incisor process and pars centralis, represented by 2 rows of small spinal setae, both row almost the same size, first row ending at proximal edge with one long teeth, second row with one large, proximal bifid teeth; pars centralis formed by a series of 9 strong apical setae united at their base; molar process oval, with a series of scale-like lamellae on crushing surface. Left mandible ( Figs. 2A , 3D‒F ) incisor process composed of chitinous ridge with 8 teeth; lacinia mobilis very well developed, consisting of one bifid chitinous teeth ridge; pars centralis represented by series of long lamellae with broad base, spiny on their margins; molar process similar to right mandible.

Maxillula ( Fig. 2D ) external lobe (protopod) armed with 9 robust apical setae, some of them barbed, 3 long simple setae on ventral surface; inner lobe with 5 long, simple apical setae, 2 short, simple setae on the inner margin, and 2 longer, simple setae on ventral surface.

Maxilla ( Fig. 2B ) exopod small, armed with 8 simple marginal setae; second article of endopod with 5 simple, long setae on outer margin, inner margin with 5 long, simple setae and 5 strong, serrated setae; 3 subrectangular endites, distal margins with simple long setae; protopod with a dense cover of long, simple setae, and one serrated seta.

Thoracopod 1 ( Fig. 4A ) (female) short, robust; gnathobase (basis) of endopod with cluster of plumose setae on ventral surface, subterminal endite elongate, with distal cluster of plumose setae; ischium, merus, carpus and propodus with one or several plumose setae on inner margin, and simple setae on ventral surface; dactylus short, with setae on margins and ventral surface; nail slightly curved, stout, longer than dactylus; exopod about 2.3 times as long as endopod, 8-articulated, each article with one long plumose setae, except last article with 2 long plumose setae.

Thoracopod 2 ( Fig. 4B ) endopod longer than endopod of first thoracopod, shorter than third, elongated; pre-ischium short, naked; ischium armed with one short simple setae on both margins; merus with a pair of subterminal setae; carpus and propodus of similar length, armed with a few simple setae on both margins; dactylus short, triangular, several setae on margins and ventral surface; nail slightly curved, shorter than dactylus; exopod longer than endopod, 9-articulated, each article with one long, plumose setae, except last article with 2 long plumose setae.

Thoracopod 3 ( Fig. 4C ) endopod elongated, longer than exopod; pre-ischium naked; ischium with one long simple setae on inner distal margin and 2 short setae on outer margin; merus with long, simple setae on both margins; carpus twice as long as propodus, both with some long, simple setae on outer margin and 1‒2 seta on inner margin; dactylus short, with 2 setae on distal margin; exopod 9-articulated, each article with one long seta, except last article with 2 long plumose setae.

Thoracopods 4‒8 missing.

Penis ( Fig. 4D ) elongated, about 3 times as long as wide, distal edge with a small circular lobe, one simple setae on ventral surface.

Abdominal somites 1‒6 without spines ( Fig. 1A ).

Male pleopods ( Fig. 5A‒E ) biramous, well developed; exopod and endopod similar in size except in pleopod 1; exopods 7-articulated, with simple distal seta on both margins, except distal article with two simple setae; first pleopod endopod uniarticulated, about 1/3 length of corresponding exopod, proximal pseudobranchial lobe with 3 plumose setae, rest tapering distally with one short, simple setae on distal margin. Endopod of pleopods 2 to 5 7- articulate, with simple distal seta on both margins, except distal article with two simple setae, pseudobranchial lobes subrectangular, armed with 4-5 plumose setae.

Uropods ( Fig. 6A ). Exopod long and slender, about 3 times telson length, about 9 times as long as wide, about 1.2 times as long as endopod; endopod about 2.5 times as long as telson, notably wider proximally near statocyst, maximum width about 0.3 times endopod length, without spines on inner margin.

Telson cordiform ( Fig. 6B ) short, slightly broader than long, distal margin truncate, about 0.27 maximum width of telson, with one short lateral seta on each side and a pair of very long, plumose, apical setae about 0.8 times telson length.

Female description. Similar to male.

Antennular peduncle ( Fig. 1D ). Peduncle slender than in males; first article as long as second and third combined length, with one simple setae on outer distal edge; second about 1/2 length of third; second and third with two simple setae on inner distal edge.

Marsupium of female comprised of two pairs of oostegites. Female pleopods ( Fig. 5F‒J ) uniramous, well developed, increasing in size posteriorly with some long, simple setae on distal edge and on both margins (some setae missing); pleopod 5 about 3 times as long as first.

Size (CL). Males, 1.00‒ 2.37 mm; females, 1.00‒ 2.48 mm. No ovigerous females.

Distribution. Off Peru (14°28.9’ S, 81°24’ W) and off the southern tip of the Baja California Peninsula (22°45’ N, 110° W) ( Hansen 1912); off Colombia (02°54’ N 80°02’ W) and Chile (27°04’ S, 84°01’ W) (W.M. Tattersall 1943). Throughout the Gulf of California, Mexico, to about 27°53’ N (present study). Possibly off Hawaii ( Murano 1977).

The material of E. plebeja collected in the Gulf of California during this study indicates that this species is abundant and well distributed in this enclosed sea ( Fig. 8 ). The absence of previous reports for this area is certainly linked to the fact that it lives in deep waters, not adequately sampled in the past.

Depth distribution. Not very precise due to lack of records and discrete samples. From surface to 300 fathoms (ca. 550 m) ( Hansen 1912). At 100 m depth (W.M. Tattersall 1943). Collected with an opening-closing device from depths of 95 to 110 m ( Murano 1977). Between 95 and 110 m depth ( Price 2004). In depths of 95‒100 m (WoRMS editorial board 2024). The material examined was collected between surface and 200‒600 m (IK) and between surface and 1600 m (MN), in localities where the total depth was comprised between 1191 and 2400 m. According to Murano (1977), E. plebeja is an oceanic species.

Remarks. As noted above, the description of E. plebeja by Hansen (1912) is incomplete and rather superficial. Hansen (1912: 202, pl. 3, figs. 1a, b) provided a dorsal view of the forehead, including the cephalic appendages, and a dorsal view of the telson and the uropodal appendages. These illustrations were reproduced by H. Nouvel and uploaded in WoRMS (WoRMS Editorial Board 2024) by J.P. Lagardère. A better reproduction of these illustrations was obtained during our study (see Fig. 9 ). Hansen (1912) considered E. plebeja to be closely allied to E. tenuis and E. oculata Hansen, 1910 . In his description (text and figures) of E. plebeja he focused on a few characters:

1. Shape of the frontal plate. Broadly triangular, with sides little concave, tip moderately broadly rounded in E. plebeja vs. narrow frontal plate in E. oculata .

2. Size and shape of corneas. Corneas of the eyes similar in size in E. plebeja , with posterior cornea longer and broader than in E. tenuis , separated from the anterior part of cornea by a space less than half the length of the posterior cornea vs. posterior cornea smaller than anterior, space between anterior and posterior cornea as long as margin of posterior cornea in E. tenuis .

3. Shape of the antennal scale. Five times as long as broad in E. plebeja vs. 3.5 times as long as broad in E. oculata ; outer margin almost straight in E. plebeja vs. regularly concave in E. tenuis .

4. Rami of the uropods. In E. plebeja broader in proportion to length than in E. tenuis .

5. The telson somewhat broader than long in E. plebeja compared with E. tenuis .

Hansen’s (1912) illustration of the anterior part of the cephalothorax and of telson also clearly permits to note the absence of small spines. The material examined herein fits well with Hansen’s (1912) description and figures, except for a few details: the antennal scale does have a subdistal, small spine, and is a little more than five times as long as broad.

In their comparative table of characters in species of Euchaetomera, Biju et al. (2020) used most of Hansen’s (1912) characters except for the absence of the lateral tooth on the antennal scale, which they did not consider; however, they considered a few characters illustrated by Hansen (1912), but not emphasized in his text:

1. Body. Smooth.

2. Rostrum. Blunt.

3. Antennal scale. Five times as long as broad.

4. Outer margin of antennal scale. Straight.

5. Eye postero-lateral cornea. Expanded laterally, much larger than anterior one.

6. Ocular papilla. Absent (but see text below).

7. Uropodal endopod. Without spine.

8. Telson. Shorter than broad.

9. Lateral margin of telson. Without spines.

10. Telson apex. With one pair of spines.

In his identification key to species of Euchaetomera, Wittmann (2024) differentiated E. plebeja from all other formally described species by the absence of lateral spine (robust setae) on the telson, the lateral cornea larger than the frontal cornea, and the rounded rostrum.

As expected, our material also fits well with these characters except for the length-width proportion of the antennal scale and the presence of a well developed ocular papilla (see below). Also, the rostrum in our material is rather acute vs. rounded in Wittmann’s (2014) key.

In addition to E. plebeja , two other species of Euchaetomera occur in the eastern Pacific. Euchaetomera typica , the type species of the genus, was described from the North Pacific, without a specific locality, and is known from 37°52’ N, 160°17’ W (about 3100 km off the USA west coast), in the Gulf of Panama, and south to off the Galapagos Islands (also recorded in the tropical and temperate Atlantic and Indian Oceans); E. tenuis is known from off Chile (type locality), the Galapagos Islands, and British Colombia (also widely distributed in the Atlantic, Pacific and Indian Oceans) (W.M. Tattersall 1943, 1951; Murano 1977; Price 2004). Comparatively with our material of E. plebeja , E. typica is easy to distinguish for its spiny body (smooth body in E. plebeja ), the uropodal endopod with one spine (no spine in E. plebeja ), and for the lateral margin of the telson armed with 5‒6 spines (unarmed in E. plebeja ).

Euchaetomera tenuis was described by G.O. Sars (1883) without providing illustrations. The same author provided some illustrations two years later (G.O. Sars 1885: pl. XXXVII, figs. 21‒24), including a dorsal view of a female entire specimen, the right antenna and scale, the endopod of “a leg”, and telson. This original description of E. tenuis , however, is unsatisfactory and causes a problem. Of particular interest is the shape of the eyes of the female type (G.O. Sars 1885: pl. XXXVII, fig. 21). G.O. Sars’ description is rather imprecise: “The eyes are greatly expanded towards the apex, being broader than long, and originate close together, so as not to project laterally. The cornea exhibits, somewhat anterior to the middle, a transverse constriction similar to that in the preceding species [= E. typica ]”. As represented, the eyes are bilobulate, with a triangular peduncle, with the “dark” area (presumably the corneas) set obliquely and extending in a continuous manner from the tip of the eye to its posterior margin, without any intermediate area devoid of facets. As noted by Hansen (1912: 199‒200) in his description of E. typica (a species also described by G.O. Sars in 1983, and illustrated by him in 1885), some of G.O. Sars’ illustrations based on the type material of E. typica are not reliable. In the case of E. tenuis , the situation appears to be similar. Nor the description in the text neither the figure (i.e., fig. 21) provided by G.O. Sars (1885) fit with the notes provided by Hansen (1912). Indeed, while describing E. plebeja, Hansen (1912: 202) provided a comparative evaluation of the eyes between E. tenuis and E. plebeja : “The eyes [of E. plebeja are] about as thick as in E. tenuis , but the posterior area [cornea] with acting facets is much larger, being longer than broad and the distance between the anterior and the posterior area [cornea] at the outer margin [is] distinctly less than half as long as the outer margin of the posterior area [cornea], while in E. tenuis that distance is about as long as the whole outer margin of the posterior area [cornea] which is shorter than broad”. This precise description does not match G.O. Sars’ (1885) illustration, but fits well with illustrations of E. tenuis eyes provided by successive authors ( Illig 1930; W.M. Tattersall & O.S. Tattersall 1951; Ii 1964) and with our material.

Based on the comments by Hansen (1912) and subsequent illustrations of E. tenuis , in E. tenuis the postero-lateral cornea is much smaller than the anterior cornea, triangular in shape in dorsal view vs. postero-lateral cornea slightly larger than the anterior one, oval-shape in E. plebeja ( Hansen, 1912: fig. 1a). The specimens collected off western Mexico possess an eye similar to the one of E. plebeja as illustrated by Hansen (1912), and its shape remains unchanged with growth of the specimens ( Fig. 7 ). The eye in our material, however, possesses a well developed, tubular-shaped ocular papilla, visible in ventral view near the postero-lateral margin of the anterior cornea ( Fig. 7 ). This papilla is similar to the one observed in E. richardi ( Nouvel 1945; Nouvel et al. 1999), and both appear to be located in the same position. Hansen (1912) did not illustrate an ocular papilla and did not signal its presence in his short description. The type material of E. plebeja (USNM 45372), however, was located and examined (courtesy of R. Lemaitre). According to Dr. Lemaitre comments, the specimen is small and in bad shape, but there is an ocular papilla on the eye, visible only ventrally, which explains why it was overlooked by Hansen (1912). This ocular papilla is similar in all respects to the one illustrated herein (see Figs. 7A‒D ) for the Mexican material of E. plebeja (R. Lemaitre, pers. comm. Nov 2023).

It is worth noting that, according to Biju et al. (2010), E. tenuis lacks an ocular papilla, but a papilla is clearly present according to Illig (1930: figs. 93, 94), W.M. Tattersall & O.S. Tattersall (1951: fig. 66C), and Ii (1964: fig. 94D).

Another character differentiating E. plebeja from E. tenuis is the presence of a single pair of short setae at the telson apex in E. plebeja vs. two pairs of setae in E. tenuis ; these short setae, however, are sometimes lost during sampling operations. All specimens collected during this survey possess a single, slender seta on each postero-lateral corner of telson apex ( Figs. 6B , 7A‒D ), similar among specimens of different size.

In our material of E. plebeja , uropodal appendages are similar in specimens of different size, but there is a slight variation in the exopod-endopod length ratio ( Fig. 7 ), which could be attributed to intra-specific variations.

As stated earlier, E. plebeja is a rare species, with material reported only three times: two specimens in the original description by Hansen (1912), two by Tattersall (1943), and presumably six by Murano (1977). As stated above, the type specimen examined by Hansen (1912) is deposited at the Smithsonian Institution (USNM 45372); the second (immature) specimen reported by Hansen (1912) is deposited in the Museum of Comparative Zoology, Harvard (MCZ CRU-7478). One of the two specimens reported by W.M. Tattersall (1943) from off Chile (adult male) and deposited at the Smithsonian Institution (USNM 86035) could not be located; the whereabouts of the other specimen from off Colombia (immature female) remains unknown. Review of databases and inquiries made with staff of the United States National Museum in New York, the Museum of Comparative Zoology at Harvard, the London Museum of Natural History, and the Natural Museum Cardiff, Wales were unsuccessful.

Material of E. plebeja from near Hawaii reported by Murano (1977; six specimens, including adult males and females) was partly illustrated. Murano (1977) observed a couple of differences between his material and the original description by Hansen (1912): 1. In the material from off Hawaii, the antennal scale features an external tooth in the mature male ( Murano 1977: fig. 2a), tooth absent in Hansen’s description (but present in our material). 2. In the material from Hawaii, the eye is proportionally broader than in Hansen’s description when compared to the breadth of the anterior margin of the carapace. This Hawaiian material, if still available, probably needs further study to confirm its identity.

In addition to the three species occurring in the eastern Pacific, the genus Euchaetomera comprises of another six formally described species. Biju et al. (2010) described one of these species, E. spinosa, from the southern Indian Ocean, and provided a comparative table for the nine species known at that time. Two of these, E. spinosa and E. zurstrasseni feature a spiny body, a character not found in E. plebeja . As in E. tenuis , E. richardi has the postero-lateral cornea much smaller than the anterior one vs. being larger than the anterior one in E. plebeja . Euchaetomera oculata is distinguished from E. plebeja by a much less slender antennal scale, only 3.5 x as long as wide vs. about 5 x in E. plebeja . The eye of E. oculata , as illustrated by Hansen (1910), features a semi-circular anterior cornea and a triangle-shaped postero-lateral cornea, of about the same size, with no trace of an ocular papilla; in subsequent contributions, however, the postero-lateral cornea of E. oculata is oval ( Coifmann 1937: fig. 25b) or angular ( Illig 1930), in both cases larger than the anterior cornea and without an ocular papilla. It is not clear why Biju et al. (2010) included the presence of an ocular papilla in E. oculata in their comparative table. The last two species are E. glyphidophthalmica and E. intermedia. In both species, the rostrum is acute vs. obtusely triangular in E. plebeja , the antennal scale is less slender than in E. plebeja , and there is no ocular papilla vs. present in E. plebeja ( Biju et al. 2010) .

Shape of the eye, particularly form and extension of corneas, appears to be a critical, distinctive character for separating species of Euchaetomera . Ideally, precise dorsal and lateral views of the eye should be provided. In some cases, the presence or absence of an ocular papilla appears to be another character that has not been adequately described and that should be checked carefully.

Figures

FIGURE 1. Euchaetomera plebeja Hansen, 1912. A‒C, E, male (CL 2.07 mm; TL 5.67 mm) (ICML-EMU-12643), D, female (CL 1.57 mm; TL 4.39 mm) (ICML-EMU-12638-A).A, lateral view; B, anterior part of cephalothorax with appendages, dorsal view, and magnification of left eye enlarged; C, antennula of male, dorsal view with distal portion of appendix masculina (setae omitted) (arrow); D, antennula of female, dorsal view; E, antennal peduncle with scale, dorsal view.

FIGURE 2. Euchaetomera plebeja Hansen, 1912. Male (CL 2.07 mm; TL 5.67 mm) (ICML-EMU-12643). A, Left and right mandibles with mandibular palp, enlarged distal article of left palp (arrow); B, maxilla, with enlarged distal seta of endopodite distal article; C, labrum, with magnification of posterior margin (arrow); D, maxillula, with enlarged distal seta of distal article (arrow).

FIGURE 3. Euchaetomera plebeja Hansen, 1912. Male (CL 2.07 mm; TL 5.67 mm) (ICML-EMU-12643). SEM images of mandibles. A‒C, left mandible; D‒F, right mandible

FIGURE 4. Euchaetomera plebeja Hansen, 1912. A, female (CL 2.40 mm; TL, 6.83 mm) (ICML-EMU-12637). B, C, D, male (CL 2.07 mm; TL 5.67 mm) (ICML-EMU-12643). A, thoracopod 1, with endite (arrow); B, thoracopod 2, with enlarged endopodal dactylus and claw (arrow); C, thoracopod 3, with enlarged endopodal distal articles (arrow); D, penis.

FIGURE 5. Euchaetomera plebeja Hansen, 1912. A‒E, male (CL 2.07 mm; TL 5.67 mm) (ICML-EMU-12643), F‒J, female (CL 1.57 mm; TL 4.39 mm) (ICML-EMU-12638-A). A‒E, pleopods 1 to 5; F‒J, pleopods 1 to 5.

FIGURE 6. Euchaetomera plebeja Hansen, 1912. Male (CL 2.07 mm; TL 5.67 mm) (ICML-EMU-12643).A, uropodal exopod and endopod, dorsal vie; B, telson, dorsal view, with magnification of distal part (arrow).

FIGURE 7. Euchaetomera plebeja Hansen, 1912. A‒D, eyes in dorsal view, sketch of left eye in ventral view showing ocular papilla, uropodal appendages, telson, and tip of same in four specimens of different size. A, male (CL 1.04 mm) (ICML-EMU- 12643); B, male (CL 1.29 mm) (ICML-EMU-12643); C, male (CL 1.39 mm) (ICML-EMU-12640). D, male (CL 1.74 mm) (ICML-EMU-12640).

FIGURE 8. Distribution of sampling localities in western Mexico, where Euchaetomera plebeja Hansen, 1912 was collected during this study.

FIGURE 9. Euchaetomera plebeja Hansen, 1912. Type (USNM 45372). A, anterior part of cephalothorax with appendages, dorsal view; B, telson and left uropodal appendages, dorsal view (from Hansen, 1912); C, telson and left uropodal appendages, dorsal view (redraw from Hansen, 1912).