Biodiversity Data Journal :
Taxonomy & Inventories
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Corresponding author: Torben Riehl (torben.riehl@senckenberg.de)
Academic editor: Diana Galassi
Received: 23 Feb 2024 | Accepted: 08 Apr 2024 | Published: 21 May 2024
© 2024 Torben Riehl, Katharina Schienbein, Constantine Mifsud
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Riehl T, Schienbein K, Mifsud C (2024) A new species of Mexicope Hooker, 1985 (Crustacea, Isopoda) — the first record of Acanthaspidiidae Menzies, 1962 from the Mediterranean Sea. Biodiversity Data Journal 12: e121508. https://doi.org/10.3897/BDJ.12.e121508
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The marine isopod family Acanthaspidiidae Menzies, 1962 (Asellota, Janiroidea) has global distribution from shelf to hadal depth. The majority of species has been recorded from relatively deep waters and the Southern Hemisphere. To date, 36 species have been described in the family belonging to three genera: Ianthopsis Beddard, 1886; Iolanthe Beddard, 1886; and Mexicope Hooker, 1985.
Here, a new species of Mexicope is described from Maltese waters, adding a fourth species to the genus. It is the first member of the family reported from the Mediterranean Sea. The new species can be recognised by the unique combination of the following characters: cephalothorax with pre-ocular spine large and pointed anterolaterally, rostral projection blunt, eyes reduced; pereonal sternites each with one ventral spine; pereonite two lateral margins with single projection; pleotelson posterior apex long, projecting to approximately half of the length of the uropod protopod; pleopods I distolateral lobes projecting beyond distomedial lobes, apices curved and pointed laterally; uropod exopod length approximately 0.5 endopod length. An identification key to the species of Mexicope is presented and the generic diagnoses of Ianthopsis and Mexicope are compared, discussed and revised.
Crustacea, sp. nov., taxonomy, marine invertebrates, benthos, isopod
The Mediterranean Sea is often considered a marine biodiversity hot spot (
In the Mediterranean Sea, 295 species of isopods have presently been recorded (
Asellote isopods are represented by the superfamilies Aselloidea, Gnathostenetroidoidea, Janiroidea and Stenetrioidea. The family Acanthaspidiidae (Janiroidea) has not yet been reported from the Mediterranean Sea. The main distribution of this relatively small family (in terms of the number of known genera and species), established by
In its taxonomic history, Acanthaspidiidae has undergone several revisions (
Here, a new isopod species of Mexicope is described from off the coast of Malta. It represents the first record of Mexicope and the family Acanthaspidiidae in the Mediterranean Sea and raises the number of species for the genus to four. The diagnoses of the rather similar acanthaspidiid genera Mexicope and Ianthopsis are compared, discussed and revised and an identification key to the species of Mexicope is presented.
A single specimen was collected in Maltese waters, central Mediterranean Sea, off Ġnejna Bay (Fig.
For the examination, dissection and voucher imaging of the specimen, a Leica M125 C stereomicroscope with motorised z-drive was used. Taxonomic drawings were made with a Leica DM2500 LED compound microscope, equipped with DIC and with a camera lucida. For the drawings, dissections and confocal laser scanning microscopy (cLSM), the specimen was transferred to glycerine and stained. To avoid shrinking, it was placed in a 1:1 solution of 75% ethanol and glycerine with a droplet of saturated, ethanol-based solution of Congo Red and Acid Fuchsin added (
The cLSM scans were made using a Leica TCS SPE including a Leica DM 2500 microscope and the Leica LAS X software (Leica Application Suite X; Version 3.3.3.16958). For all scans, the 405 nm, 488 nm and 561 nm lasers were used (see Suppl. material
For the description, terminology follows previous works on Acanthaspidiidae and other Janiroidea (
The holotype is deposited at the crustacean collection of the Senckenberg Research Institute and Natural History Museum Frankfurt, Frankfurt am Main, Germany, with the collection prefix “SMF”.
Throughout the descriptive part of this article, the following abbreviations have been used:
AI - antennula; AII - antenna; Ceph - cephalothorax; Md - mandible; MxI - maxillula; MxII - maxilla; Mxp - maxilliped; P - pereopod; Plp - pleopod; Plt - pleotelson; Prn - pereonite; UB - unequally bifid (seta); Urp - uropod.
Composition: Mexicope westralia Just, 2001, Mexicope sushara Bruce, 2004 and Mexicope maletensis Riehl & Schienbein, sp. nov.
Mexicope Hooker, 1985: 261. – Just (2001: 913); Bruce (2004: 2) (
Cehalothorax laterally with eyestalks and developed or reduced eyes; eyestalks directed laterally or anterolaterally, with preocular spine. Pereonites 1 and 5–7 with single lateral projection; pereonites 3–4 with bifid lateral projections; pereonite 2 lateral projection single or bifid; lateral projections with apical short setae; pereonites 2–4 and 6–7 posterior projections with long setae recurving dorsally over tergites, pereonite 5 without long setae. Pereonite 7 with sternal spine. Pleotelson margins finely serrated, with spine-like, short, curved setae. Antennula length ca. 0.33 antenna length, article 1 and 2 subequal in length. Mouthparts prognathous; mandibular molar tapering or pointed with apical long setae; mandibular palp present or absent; maxilliped epipod short, reaching half way between coxobasal articulation and palp insertion. Pleopod II fringed with long setae, longest apically; stylet coiled. Uropod length equal or longer than pleotelson, peduncle and rami long and slender; rami length equal or longer than peduncle length.
This diagnosis has been completely revised to overcome the inconsistencies and overlaps of previous diagnoses.
A new diagnosis for Mexicope is presented here because previous diagnoses of the genera Ianthopsis and Mexicope (
Character states that support classification as either Ianthopsis or Mexicope are the absence of dorsal tubercles on the cephalothorax, the presence of eyes on eyestalks, single lateral projections on pereonite 1, the absence of tergal spines or humps, a fringe of long setae laterally on the second pleopod basis which are longest apically, finely serrated pleotelson margins and uropod rami that are at least as long as the peduncle. The presence of a simple, bulge-shaped rostrum, the antennula length of 0.25 times antenna length, the absence of a triangular lobe on antenna article 6 and the short, curved, robust setae on the pleotelson on the one hand speak for a classification of the new species as Ianthopsis according to the previous diagnosis.
The classification of the new species as Mexicope, on the other hand, is supported by the presence of a pre-ocular spine, rows of elongate, simple setae on the posterolateral projections of pereonites 2–4 and 6–7, the presence of a sternal spine on pereonite 7, prognathous mouthparts, a strongly tapering, pointed mandibular molar with long apical setae, the lack of a mandibular palp, a short maxilliped epipod reaching only half way between the coxo-basal articulation and the palp articulation, a coiled pleopod II stylet and uropods that are as long as the pleotelson.
In summary, over ten, partially complex characters suggest the classification as Mexicope as opposed to only four, relatively weak characters in favour of Ianthopsis (see Table
Comparison of diagnostic characters of Ianthopsis (
Character |
Character states Ianthopsis Beddard, 1886 |
Character states Mexicope Hooker, 1985 |
Character states Mexicope malitensis sp. nov. |
cephalothorax frontal projection (rostrum) |
present |
absent |
present |
cephalothorax rostrum shape |
simple (bulge) / complex (spine-like rostrum) |
spine-like rostrum in M. sushara |
simple (bulge) |
cephalothorax dorsal tubercles presence |
present / absent |
present / absent |
absent |
cephalothorax eyestalks |
present / absent |
present |
present |
cephalothorax eyestalks shape |
long stalks / low bulges |
long stalks / low bulges |
low bulges |
cephalothorax eyes presence |
present / absent |
present |
present |
cephalon preocular spine |
absent |
present |
present |
pereonite 1 lateral projection |
single / bifid |
single |
single |
pereonite 2 lateral projection |
bifid |
single / bifid |
single |
pereonites 2-4, 6-7 posterolateral projections rows of elongate, simple setae |
absent |
present |
present |
pereonites dorsal spines/humps |
present / absent |
absent |
absent |
pereonite 7 sternal spine |
absent |
present |
present |
antenna article 6 distal triangular lobe |
absent |
present / absent |
absent |
mouthparts orientation |
ventral / moderately prognathous |
prognathous |
prognathous |
mandibular molar shape |
cylindrical, triturating / rarely tapering |
strongly tapering / pointed |
strongly tapering / pointed |
mandibular molar apical long setae presence |
absent |
present |
present |
mandibular palp presence |
present |
present / absent |
absent |
maxilliped epipodite length versus maxilliped basis length |
long (reaching to or beyond palp articulation) |
short (reaching half way to palp articulation) |
short (reaching half way to palp articulation) |
pleopod 2 stylet shape |
straight |
coiled |
coiled |
pleotelson setation |
short curved robust setae |
absent |
short curved robust setae |
uropods length versus pleotelson length |
shorter than pleotelson / longer than pleotelson |
as long as / longer than pleotelson |
as long as pleotelson |
uropod rami length versus peduncle length |
shorter than / similar peduncle length |
similar / longer than peduncle |
similar / longer than peduncle |
Body (Fig.
Mexicope maletensis sp. nov. male holotype (SMF 62000) relative pereonite lengths. L = length; L/W = length-width ratio.
Proportion |
Pereonite 1 |
Pereonite 2 |
Pereonite 3 |
Pereonite 4 |
Pereonite 5 |
Pereonite 6 |
Pereonite 7 |
L/W |
0.21 |
0.23 |
0.20 |
0.21 |
0.15 |
0.22 |
0.20 |
L/body L |
0.09 |
0.09 |
0.08 |
0.08 |
0.06 |
0.09 |
0.07 |
Prn (Fig.
Plt (Fig.
AI (Fig.
Mexicope maletensis sp. nov. male holotype (SMF 62000) antennula relative article lengths. L = length; L/W = length-width ratio.
Proportion |
Article 1 |
Article 2 |
Article 3 |
Article 4 |
Article 5 |
Article 6 |
Article 7 |
Article 8 |
L/W |
2.4 |
3.8 |
1.3 |
1.3 |
5.0 |
3.0 |
1.0 |
2.0 |
L/article 2 L |
0.9 |
1.0 |
0.3 |
0.2 |
0.5 |
0.3 |
0.1 |
0.1 |
AII (Fig.
Md (Fig.
Mxp (Fig.
P (Figs
Mexicope maletensis sp. nov. male holotype (SMF 62000), digitised pencil drawings of seventh pereopods in lateral view: A right pereopod VII; B left pereopod VII; C simple (asetulate) seta; D whip seta; E robust seta; F unequally bifid (sensillate) seta; G broom seta (distal half of shaft setulate, pappose); H broom seta (apically setulate, pappose); I simple sensilla (shaft subcylindrical, tip rounded); J fringed sensilla (shaft subcylindrical, distally densely covered by small scales, tip rounded). Scale bar (A–B) = 250 µm (C–I not to scale).
PI (Fig.
PII (Fig.
PIII (Fig.
PIV (Fig.
PV (Fig.
PVI (Fig.
PVII (Fig.
PlpI (Fig.
Urp (Figs
Body elongate, widest at pereonite 1. Cephalothorax rostrum bulge-shaped; eyes reduced; preocular spine pointed anterolaterally, posterolateral margin serrated. Pereonites 1–2 and 5–7 lateral projection single; pereonites 3–4 lateral projections bifid; pereonites 1–2 projections posterolateral margin serrated, pereonites 5–7 projections anterolateral margin serrated; pereonites 3–4 lateral projections anterolaterally and posterolaterally serrated; pereonites sternal spines present. Pleotelson proximal quarter narrow, gradually wider towards posterior, lateral margins in medial 2/4 concave, distal quarter narrow, tapering lateral margins convex with 4 spine-like setae on each side; posterior apex protruding. Antenna flagellomere 1 length similar to combined length of remaining 11–12 flagellomeres. Mandibular palp absent. Pleopods I lateral lobes projecting beyond medial lobes; apices pointed laterally. Uropods articulated at posterolateral pleotelson margin, extending beyond pleotelson apex; posterolateral margin concave at insertion of uropods. Uropod endopod length subsimilar protopod length, exopod length 0.50 endopod length.
The name “maletensis” is related to the geographic location of the type locality in Malta. According to one theory about the etymology of “Malta”, the name is derived from the Phoenician “malet” (also spelled “maleth”) and means “place of refuge”, “haven” or “port”. So, “Mexicope maletensis” translates into “Maltese Mexicope” or “Mexicope from Malta”.
Only known from type locality. The known depth distribution of the genus Mexicope is extended, based on the newly-presented record. Previously ranging from 2 m to 80 m depth, Mexicope now has been reported from 2 m to 120 m depth. Furthermore, the distribution of the family Acanthaspidiidae is extended to include the Mediterranean Sea.
A comparisons between the species of Mexicope were based on male specimens, because only a single male specimen of Mexicope maletensis sp. nov. was available for this study. While similarities to some congeners are evident in various characters, M. maletensis sp. nov. shows a unique combination of morphological features rendering this species overall distinct.
The assignment of the new species to the Acanthaspidiidae is based on typical family characters, such as the pointed body outline, single lateral projections on pereonites 1 and 5–7, bifid lateral projections on pereonites 3–4, antenna with a long flagellum and article 3 with exopod and cylindrical uropods (
Similarities between M. westralia and M. maletensis sp. nov. are: Ceph inter-antennal margin (frons) curved anteriorly; Prn dorsally with robust setae, Prn 3–4 lateral projections bifid, subequal in length, Prn 6–7 mid-sternal spine present; Plt margins finely serrate, anteriorly to Urp insertion concave, finely serrate; AII peduncle article 6 slightly longer than 5; left Md with 6 denticulate spine-like setae, molar strongly tapering; PlpI proximal 1/3 broadest, abruptly narrowing, slightly tapering towards apex; PlpII basis lateral margin mid-1/3 broadly convex with fringe of long slender setae, distal 1/3 concavely tapering to blunt point, endopod stylet long, whip-like, coiled extension; PlpIII exopod distal margin with 6 plumose setae; PlpIV exopod about half length of endopod, slender, naked; uropods about as long as Plt.
Mexicope maletensis sp. nov. differs from M. westralia as follows: Ceph rostrum present (absent), Prn mid-sternal spine present (presence on Prn 5–7 only), Prn 2 lateral projection single (bifid), Plt proximal quarter narrow, lateral margin convex, becoming wider, lateral margins medial 1/2 concave, distal quarter narrowly tapering, lateral margins convex (widening distally, in proximal 1/3 with faintly concave margins, middle 1/3 broadly convex), Plt apical lobe twice as long as wide (half as long as wide), AII flagellum article 1 conjoint, length as long as remaining 13 short articles (half-length of 14 short articles), Md palp absent (present), right Md with 6 denticulate spine-like setae, length progressively increasing, distal to proximal denticles number increasing (with 8 such setae), MxI outer lobe with 13 spines (12 denticulate spine-like setae), Mxp basis length 1.5 width (2.5 width), PlpI dorsally without setules, without tabs (dorsal mid-surface with field of setules, more distally with 2 triangular locking tabs), PlpIII endopod with 7 plumose setae (with 6 plumose setae), Urp outer ramus length 0.50 protopod length (exopod and protopod of similar length), exopod length ca. 0.50 endopod length (ca. 0.80).
Similarities between M. sushara and M. maletensis sp. nov. are: body length 2.0 width; inter-antennal margin (frons) more or less convex, rostral spine short, pre‑ocular spines prominent, eyes laterally, stalked; Prn 1 anterolateral projections acute, curved towards anteriorly, Prn 3–4 lateral margins distinctly bifid, projections subequal in size, Prn 5–6 lateral projections directed posteriorly, Prn 2–3 and Prn 6–7 projections with anteriorly directed long dorsal setae, Prn 4 setae only on posterior lobe; Prn 7 mid-sternal spine present; AII flagellum conjoint article 1 length subequal length of remaining 12 articles; left Md lacinia mobilis with 4 cusps, molar tapering; MxII lateral and middle lobes each with 2 long and 2 short strongly serrate setae; Mxp epipodite linguiform; pereopods similar in shape, setation subequal, PI–III proportionally shorter than PIV–VII, length differences mostly caused by length variation of propodus; PlpI proximally widest, lateral margin narrowing abruptly, tapering smoothly to bilobed apex; PlpII lateral margin medially strongly convex, with dense row of simple setae, distolateral margin concave, endopod stylet long, distally coiled; PlpIII exopod distal margin with 6 plumose setae, exopod lateral margin with continuous fringe of setae; PlpIV exopod half as long as endopod, apically acute.
Mexicope maletensis sp. nov. differs from M. sushara as follows: Ceph rostral projection blunt, barely projecting (rostrum acute, projecting and curved upwards); Plt anterior quarter narrow, lateral margin convex, becoming wider posteriorly, lateral margins medially concave, distally narrowly tapering, convex (proximally narrow, widening abruptly); Prn 2 lateral projection single (bifid); AII article 6 distodorsal triangular lobe absent (present), Md incisors with 5 cusps (with 4 cusps), right and left Md with 6 denticulate spine-like setae, length and number of denticles progressively increasing from distal to proximal (left spine row of 5 denticulate spine-like setae, 8 simple setae; right spine row with 8 denticulate spine-like setae), palp absent (present); Mxp basis length 1.5 width (2.1 width); PlpI length 2.1 width (3.9 width); PlpII length 1.7 width (2.6 width), apex with 3 long simple setae (with 6 long plumose setae), exopod apex unilobed (bilobed); PlpIII endopod distal margin with 7 plumose setae (with 6 plumose setae).
Similarities between M. kensleyi and the new species are: preocular spines orientated anterolaterally; eyes on stalks; Prns laterally with few setae; AI heavily setose, flagellum with 13 articles, first article longest; Md palp absent, molar process conical setiferous, left lacinia quadridentate, one distal simple spine; MxI inner ramus shorter than outer one; Mxp palp 5-segmented, segment width subequal, distal segment setose, endite broad, outer margin rounded, inner margin 2 coupling hooks, distal third with numerous setules, epipodite short, apically narrowly rounded; P propodus longest segment, merus bulbous, dactylus with two claws; PVII longest; PlpI laterally setose; PlpII protopod outer margin with long and short setae, endopod exceeding length of protopod, distally coiled; Urp elongate, biramous, peduncle length similar exopod length.
Mexicope maletensis sp. nov. differs from M. kensleyi as follows: body length almost 2.0 width (length almost 3.0 width), eyes ventrolaterally (dorsolaterally), Prn 1 widest (Prn 3 and 4 widest); left Md distally with 5 dentate spines (with 6 spines); PlpII endopod distally coiled at least 10 times (stylet coiled once).
The initial difficulties we had in assigning the new species to a genus are emblematic of the currently inadequate and vague diagnoses of all three genera of Acanthaspidiidae (see also
A new diagnosis for Mexicope is presented here because previous diagnoses of the genera Ianthopsis and Mexicope (
Composition: Ianthopsis beddardi Kussakin & Vasina, 1982, Ianthopsis beddardi Kussakin & Vasina, 1982, Ianthopsis certus Kussakin & Vasina, 1982, Ianthopsis franklinae Brandt, 1994, Ianthopsis kimblae Brandt, 1994, Ianthopsis laevis Menzies, 1962, Ianthopsis monodi Nordenstam, 1933, Ianthopsis multispinosa Vanhöffen, 1914, Ianthopsis nasicornis Vanhöffen, 1914, Ianthopsis nodosa Vanhöffen, 1914, Ianthopsis ruseri Vanhöffen, 1914, Ianthopsis studeri Kussakin & Vasina, 1982, Ianthopsis vanhoeffeni Just, 2001.
Ianthopsis Beddard, 1886: 15. – Nordenstam (1933: 180); Brandt (1994: 224); Just (2001: 918-919); Janthopsis Menzies (1962b: 83) (unjustified emendation) (
Cephalothorax pre-ocular spine present; frontal projection (rostrum) present, rostrum shape simple (bulge-shaped) to complex (spine-like rostrum); dorsal tubercles present or absent; eyestalks present or absent. If present, eyestalk shape variable, from long stalks to low bulges; exes present or absent. Pereonites dorsal (tergal) projections absent or present, variable in shape if present (hump-shaped to spine-shaped). Pereonite 1 lateral projections either single or bifid. Pereonite 2 lateral projections bifid. Pereonites 2-4, 6-7 posterolateral projections without rows of elongate, simple setae. Pereonite 7 without sternal spine. Pleotelson with short curved robust setae.
Antenna article 6 without distal triangular lobe. Mouthparts orientation ventral or moderately prognathous. Mandibular molar shape variable, cylindrical, triturating or (rarely) tapering, without apical long setae; palp present. axilliped epipodite long, reaching to or beyond palp articulation.
Pleopod 2 stylet straight. Uropod length variable, from shorter than pleotelson to longer than pleotelson; uropodal rami shorter than or peduncle length.
Character states that were once considered diagnostic for Ianthopsis became irrelevant over time with more species being described, which is expressed in the fact that "either-or" statements survived as quasi testimonies of this development in the genus diagnosis: “cephalothorax dorsal tubercules present or absent”, “eyestalks present or absent”, “eyes present or absent”, “pereonite 1 lateral projection single or bifid”, “dorsal spines or humps on pereonites present or absent” (Table 1). This made it necessary to examine both genera in detail and to critically revise their diagnoses, which we have accomplished here.
Besides strictly diagnostic (unique) character states, the revised diagnosis includes also variable characters which are diagnostic in the genus Mexicope or the new species Mexicope maletensis, sp. nov.
Key to the species of Mexicope Hooker, 1985 |
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1 | All pereonites with mid-sternal spines; pereonite 1 widest part of body; pereonite 2 lateral projection single | Mexicope maletensis sp. nov. |
– | Mid-sternal spines absent or only on few pereonites; pereonite 1 narrower than some of the following segments; pereonite 2 lateral projection bifid | 2 |
2 | Pleotelson length subsimilar width; pleotelson posterior apex length exceeding width; mandibular palp absent | M. kensleyi Hooker, 1985 |
– | Pleotelson length smaller width; pleotelson posterior apex length smaller width; mandibular palp present | 3 |
3 | Cephalothorax rostrum absent; pereonite 1 narrower than pereonite 2; pereonites 5–7 mid-sternal spine present | M. westralia Just, 2001 |
– | Cephalothorax rostrum present; pereonite 1 wider than pereonite 2; pereonites 5–7 mid-sternal spine absent | M. sushara Bruce, 2004 |
Initial difficulties in assigning the new species to a genus reflect the inadequate and vague diagnoses of Acanthaspidiidae genera (see also
As more new species of Acanthaspidiidae are discovered, further family revisions will be needed for clear genus distinctions. Though not delving into a full family revision here, certain characteristics are earmarked for future systematic analyses: rostrum (presence, shape, length), eyes (presence, orientation, ommatidia count), eyestalks (length, width, orientation, association with pre-ocular spine), pereonite 1 lateral projection (shape, length, orientation, serration, setation), mid-sternal spines (presence, size, shape), male pleopod II stylet (shape) and uropods (relative lengths, widths, protopod, rami orientation, setation).
For future analyses of Mexicope, attention should be paid to rostrum presence and shape, eyestalks, antenna II, mandibular palp, pereonite 2 lateral projection and male pleopod II stylet shape. Additionally, phylogenetics could aid in species and genus differentiation by shedding light on character evolution and illuminate family origins. Expanding the sparse molecular data currently available (
The authors would like to thank the working group “Section Crustacea” at the Senckenberg Department of Marine Zoology, as well as members of the SOSA project for the support and, especially, Angelika Brandt, Ruth Wasmund, Andreas Kelch, Stefanie Kaiser, Henry Knauber and Katharina Kohlenbach, who have supported this work with practical help and insightful discussions.
Overview of the specimen examined by cLSM with information on the laser line, laser intensity, detection range, detector gain, lens, frames and FiJi ImageJ projections type for respective figures. The lens used was an ACS APO 10x/0.30 DRY and ACS APO 20x/0.60 IMM. The number of frames was set to 1.5 times the suggested system optimised number. Scan speed was 400 Hz, the scan direction was unidirectional. The pinhole aperture was 94.3 µm. The specimen was stained with Acid Fuchsin and Congo Red. PMT = photomultiplier tube; CH1–CH3 = detection channels 1–3; STD = Standard Deviation; MI = Max Intensity. All objects were embedded in glycerine.
Mexicope maletensis sp. nov. male holotype (SMF 62000): A habitus, lateral; B habitus, dorsal; C habitus, ventral. Scale bar = 1 mm.