Smacigastes pumila, Kim & Lee, 2020

Kim, Jong Guk & Lee, Jimin, 2020, A new species of the genus Smacigastes Ivanenko & Defaye, 2004 (Tegastidae, Harpacticoida, Copepoda) from the Onnuri Vent Field in the Indian Ocean, Zoosystematics and Evolution 96 (2), pp. 699-714 : 699

Smacigastes pumila sp. nov. Figs 2 , 3 , 4 , 5 , 6 , 7 , 8

Etymology.

The specific name pumila is derived from the Latin adjective pumilus meaning “dwarfish” and refers the relatively short caudal seta IV, shorter than the caudal ramus length.

Type material.

Holotype. Indian Ocean - OVF • ♀ dissected on 12 slides; the Central Indian Ridge, 11°24.88'S, 66°25.42'E; depth 2020 m; 24 Jun 2018; J. Lee leg.; MABIK CR00247427.

Allotype. Indian Ocean - OVF • ♂ dissected on 12 slides; sampling data as holotype; MABIK CR00247428.

Paratypes. Indian Ocean - OVF • 2♀♀, 1♂ dissected on 12 slides, respectively; sampling data as holotype; MABIK CR00247429-CR00247431 • 2♀♀, 2♂♂ preserved together in 95% ethanol; sampling data as holotype; MABIK CR00247432 • 4♀♀, 1♂, 4 copepodids preserved together in 95% ethanol; sampling data as holotype; MABIK CR00247433.

Other materials.

Indian Ocean - OVF • 3♀♀, 2♂♂ on a stub for SEM; sampling data as holotype.

Description.

Female. Habitus (Figs 2A , 8A ) compressed laterally, with weakly chitinised integuments covered with sensilla and pores. Total length of holotype 669.2 μm (range 625.9-703.7 μm, mean 670.7 μm, n = 9) from tip of rostrum to end of caudal rami in lateral aspect; maximum width 233.6 μm measured at distal third of cephalothorax including ventrolateral expanded area. Rostrum prominent, fused to cephalothorax, with one pair of sensilla subdistally. Prosome 4-segmented, comprising cephalothorax and three free pedigerous somites. Cephalothorax occupying about 43% of total length, slightly shorter than four succeeding somites combined; cephalic shied expanded ventrolaterally; expanded area gradually tapering ventrally. Pedigerous somites with lateral groove on epimeral area. Urosome (Figs 2A , 3A, B , 8A ) 5-segmented, composed of P5-bearing somite, genital double-somite and three abdominal somites. P5-bearing somite about 0.7 times as long as preceding somite. Genital double-somite completely fused, armed with one pair of pores dorsolaterally, one pair of sensilla dorsodistally and two pairs of pores and four sensilla ventrodistally. Genital field (Fig. 3B, C ) located at anterior fourth on ventral surface of genital double-somite; gonopore fused medially, reversed U-shaped, covered by a single plate bearing one seta, representing the P6, on each side; copulatory pore unrecognised. Second abdominal somite short, with one pair of pores ventrolaterally. Third abdominal somite short, unornamented. Anal somite 1.7 times longer than preceding one, with one pair of long sensilla and one pair of pores dorsolaterally; anal opening (Figs 3A , 8B ) wide, with triradiate membranes armed with fine setules; operculum wide, semi-rounded, unornamented.

Caudal rami (Figs 2B , 3A, B , 8B ) distinctly spaced apart from each other, about 2.9 times as long as maximum width, with slightly convex inner margin; with one posterior and two lateral pores ventrally; with seven caudal setae: seta I about 0.7 times as long as ramus; seta II longest, inserted in dorsal surface, about 2.2 times as long as ramus; seta III issuing from distal fourth of lateral margin, slightly shorter than seta II; distal seta IV slightly longer than seta VI; principal seta V well-developed, about 3.5 times as long as ramus; seta VI shortest, about half as long as ramus, inserted at distal inner corner; seta VII tri-articulated basally, close to seta III, nearly as long as ramus; all setae bare except for weakly pinnate seta V.

Antennule (Fig. 4A ) 8-segmented, elongate; first segment with one bare seta; second segment longest, with 10 setae; third segment with nine setae; fourth segment with four setae and one ae; fifth segment with two setae; sixth segment with four setae; first to sixth segments with all non-articulate and naked setae; seventh segment shortest, with two bi-articulated and two non-articulate setae; eighth segment twice as long as previous one, with four bi-articulated setae along lateral margin, one small and two long bare setae, and one ae on distal margin. Each ae fused basally to adjacent seta. Length ratio of antennular segments, 1.0: 1.40: 0.89: 0.63: 0.46: 0.48: 0.41: 0.84. Setal armature as I-[1], II-[10], III-[9], IV-[3 + (1 + ae)], V-[2], VI-[4], VII-[4], VIII-[6 + (1 + ae)].

Antenna (Fig. 5A ). Basis elongate, without abexopodal seta; outer margin armed with several rows of small spinules; inner margin armed with one row of small spinules medially and one group of long setules distally; medial surface with one row of spinules proximally and one row of tiny spinules distally. Exopod small, 2-segmented; proximal segment with one unipinnate seta distally; distal segment about half as long as preceding one, with one unipinnate seta subdistally and two weakly-pinnate setae distally. Endopod elongate, 2-segmented; proximal segment with one pinnate seta and armed with outer spinules; distal segment about 1.2 times as long as proximal one, armed with three groups of spinules along outer margin, two groups of spinules on inner margin proximally, two groups of small spinules and one surface frill on medial surface and one surface frill on distal margin; lateral armature composed of one pinnate seta proximally and set of one long pinnate seta, one small pinnate seta and one small bare seta subdistally; apical armature with seven setae consisting of three weakly-pinnate setae, one long unipinnate seta, one small spinulose seta and two long bare setae, one of which is longer than total length of endopod.

Mandible (Fig. 5B ). Coxa stout, well-developed, with one protuberance medially; gnathobase armed with five multicuspidate teeth and one unipinnate dorsal seta. Palp uniramous, consisting of basis, 1-segmented endopod and rudimental exopod; basis with two long plumose setae apically and one row of spinules laterally; exopod represented by one unipinnate seta; endopod elongate, 1-segmented, with one bare, one proximally spinulose and one plumose setae apically and one pinnate seta laterally, armed with three groups of spinules on surface.

Maxillule (Fig. 5C ). Praecoxal arthrite well-developed, with one group of spinules on dorsal margin; distal armature composed of one bare seta and one apically-pinnate seta subdistally and seven serrate spines distally, of which most ventral spine having bifid tip. Coxal endite with one spinulose seta distally. Basis broad, with two rows of small spinules anteriorly; distal endite with one stout spinulose seta subdistally, two densely-pinnate setae and one weakly-unipinnate seta, of which two dorsal setae are armed with long spinules proximally. Exopod elongate, 1-segmented, armed with one row of lateral setules, with one spinulose seta subdistally and one weakly-pinnate seta distally. Endopod incorporated in basis, represented by one small pinnate seta and one stout spinulose seta.

Maxilla (Fig. 5D ). Syncoxa elongate, with one group of long setules subdistally on outer margin; with three endites: praecoxal endite elongate, parallel to inner margin of syncoxa, with one spinulose seta and one plumose seta laterally and two plumose setae, of which the inner one elongate, longer than syncoxa, distally; proximal coxal endite with two spinulose setae distally; distal coxal endite largest, gradually widening towards distal end, with one pinnate seta subdistally and two spinulose setae distally. Basis apically with stout claw bearing one row of spinules, with one bare seta ventrally, one stout spinulose seta dorsally and one tube pore. Endopod incorporated into basis, represented by one uniplumose and two pinnate setae.

Maxilliped (Figs 5E, F , 8C, D ) subchelate, inserted on pedestal, 3-segmented, composed of syncoxa, basis and endopod. Pedestal small, unornamented. Syncoxa elongate, with one unipinnate seta subdistally, two groups of long spinules on medial surface and two groups of setules on inner margin. Basis shorter than syncoxa, with swollen outer margin with one group of long setules; palmar margin concave, distally with one predominant tongue-like process bearing numerous small papilla on its surface; proximal part swollen, with one small spine, one row of medial spinules and two rows of median spinules, of which spinules of proximal one gradually increasing in size. Endopod drawn out into serrate claw accompanying two inner and three outer accessory setae, of which the outer one very small (Fig. 8C , arrowhead).

P1 (Fig. 6A ). Intercoxal sclerite wide, unornamented. Coxa slightly longer than its width, unornamented. Basis elongate, gradually widening towards distal end, with one slender, bare outer seta, without inner element; pedestal of exopod developed, armed with one patch of anterior spinules. Exopod 1-segmented, slender, shorter than basis, armed with outer minute spinules anteriorly and long inner setules; with three pinnate outer setae and two densely pinnate apical setae, of which the outer one about 3.3 times as long as the inner. Endopod broad, slightly longer than exopod, with one plumose seta proximally and two densely pinnate setae subdistally on inner margin and three pinnate setae, of which the innermost one stout, with spinules on all sides and the central one armed with inner setules, on apical margin; with one pore on outer margin subdistally (Fig. 6A , arrowhead).

P2-P4 (Figs 6B , 7A, B ). Intercoxal sclerites wide, slightly arched, unornamented; anterior surfaces of P2-P3 with one pair of two hollows. Coxae gradually tapering towards distal end; distal tip with one group of spinules. Bases transversely elongate, with one slender, bare outer seta. Exopods shorter than endopods (distinctly shorter in P2-P3 and slightly shorter in P4), 3-segmented assumptively in P2-P3 and completely in P4; P2-P3 exp-1 and exp-2 fused, but original segmentation between them retained partially by anterior traces (incomplete sutures); each segment armed with minute outer spinules; exp-1 with one weakly-pinnate outer seta (in P2-P3) or bare seta (in P4); P2-P3 exp-1 with one plumose inner seta; exp-2 with one pinnate outer spine and one long plumose inner seta; exp-3 longest, with two pinnate outer spines, two plumose apical setae, of which outer one armed with minute outer spinules and one anterior pore distally (absent in P2); P2 exp-3 with two plumose inner setae, P3 exp-3 with three plumose inner setae and P4 exp-3 with two plumose setae and one distally-serrate seta (central). Endopod 3-segmented; each segment armed with outer setules or spinules; enp-1 with one long plumose inner seta (distally pinnate in P2 and P4); enp-2 with two long plumose inner setae (distally pinnate in P4); enp-3 longest, with one pinnate outer spine, two stout pinnate apical spines and one distal anterior pore; P2 and P4 enp-2 with two plumose (in P2) or pinnate (in P4) setae and P3 enp-3 with three plumose inner setae. Setal armature formulae of P2-P4 as follows:

Exopod Endopod

P2 2.222 1.2.221

P3 2.322 1.2.321

P4 0.1.322 1.2.221

P5 (Fig. 3D ). Baseoendopod elongate, with one long bare seta on outer margin; endopodal lobe well-developed, extending to distal third of exopod, with three long bare setae on inner margin and one stout spinulose (on all sides) and one small pinnate spine on apical margin; posterior surface with one pore subdistally. Exopod elongate, slender, unornamented, with three outer elements consisting of one long bare seta proximally and two stout spinulose (on all sides) spines subdistally and distal elements composed of two stout spinulose (on all sides) spines, of which outer one longer than inner.

Male. Body (Figs 2C , 8E ) length of allotype about 527.1 μm (range 527.1-592.6 μm, mean = 558.8 μm, n = 5) measured from anterior tip of rostrum to posterior end of caudal rami in lateral aspect. Sexual dimorphism exhibited in antennule, genital double-somite and P5.

Urosome (Figs 2C , 3D , 8F ) 5-segmented as in female by forming genital double-somite comprising genital somite and first abdominal somite. Genital double-somite with elongated, large ventral protuberance; distal tip of protuberance with asymmetrical genital flap representing P6; both genital flaps without elements, left one fused to body and right one articulated.

Antennule (Fig. 4B ) 10-segmented, haplocer, with geniculation between seventh and eighth segments. First segment elongate, with one long bare seta distally. Second segment longest, about 1.4 times as long as preceding one, with 10 setae laterally. Third segment small, slightly longer than its width, with seven setae and one ae. Fourth segment smallest, wedge-shaped, with two setae. Fifth segment elongate, about 3.5 times as long as wide, with six lateral setae and one subdistal peduncle bearing one seta and one ae. Sixth segment small, with one seta. Seventh segment slender, with one uniplumose seta proximally and one bare seta subdistally. Eighth segment as long as preceding one, with one bi-articulate seta subdistally. Ninth segment small, with two bi-articulate and two non-articulate setae. Tenth segment slightly longer than seventh segment, with four bi-articulate setae on lateral margin and one small seta, two long bare setae and one ae on distal margin. Each ae fused to adjacent seta basally. Setal armature as follows: I-[1], II-[9], III-[6 + (1 + ae)], IV-[2], V-[6 + (1 + ae)], VI-[1], VII-[2], VIII-[1], IX-[4], X-[6 + (1 + ae)].

P5 (Figs 2C , 3E ) 2-segmented, elongate, nearly as long as urosome; baseoendopod about twice as long as wide, with one outer seta; endopodal lobe undeveloped; exopod elongate, about 1.9 times as long as baseoendopod, with one weakly-pinnate seta proximally and one pinnate spine subdistally on lateral margin and two pinnate spines on distal margin.

Remarks.

Differentiation of Smacigastes species has demanded detailed attention to the setation of cephalosomal appendages, the segmentation of the P2-P3 exopods, the shape and ornamentation of P5 in both sexes and the shape of elements on P5 ( Gollner et al. 2008; Plum and Martínez Arbizu 2009). When Ivanenko and Defaye (2004) erected the genus Smacigastes , they designated that P2-P4 of the genus Smacigastes are 3-segmented, based on the characteristics of the type species, S. micheli , described from the Mid-Atlantic Ridge. However, all subsequent species ( S. barti , S. methanophilus and S. pumila sp. nov.) have 2-segmented exopods of P2-P3. In a study of the development of S. barti , Gollner et al. (2008) presumed that the 2-segmented exopods of P2-P3 resulted from the fusion of proximal and middle exopodal segments through a developmental process from copepodid phase V to phase VI (adult). A trace of the original division between these segments remains in the incomplete anterior suture of S. methanophilus and S. pumila sp. nov. and in a fissure in S. barti ( Gollner et al. 2008; Plum and Martínez Arbizu 2009). In addition, S. micheli can be clearly distinguished from other congeners by the presence of three exopodal setae on the mandibular palp and the relatively short elements on both rami of the female P5.

Smacigastes pumila sp. nov. is morphologically similar to S. methanophilus in the main characteristics of the setal armature of the antenna and maxilla, the segmentation condition of the thoracic legs and the shape of the P5 in both sexes. However, this new species is easily distinguished from the existing species by the presence of only one exopodal seta on the mandibular palp (vs. two setae in S. methanophilus ); two setae on both maxillular rami (vs. three setae on each in S. methanophilus ); the absence of a basal inner seta in the P1; the relative length of the inner seta on the P2-P3 exp-1, exceeding the end of exp-2 (vs. exceeding the end of the exopod in S. methanophilus ); and the absence of anterior ornamentation on the P5 exopod in both sexes.

This new species morphologically resembles S. barti in two robust features: (1) loss of the inner seta in the P1 basis, which is an essential element in harpacticoid copepods, except for a few taxa [i.e. the family Parastenocarididae Chappuis, 1940 and the subfamily Clytemnestrinae A. Scott, 1909 (cf. Huys and Conroy-Dalton 2000; Corgosinho et al. 2008)]; and (2) presence of two setae on the exopod of the maxillule. The morphological differences between the two species, however, are notable, including (1) the antennule in the female of S. pumila sp. nov. has eight segments, whereas it is 7-segmented in S. barti ; (2) there is no abexopodal seta on the antennary basis in S. pumila sp. nov., whereas this seta is present in S. barti ; (3) the mandibular exopod is represented by a seta in S. pumila sp. nov., whereas it is absent in S. barti ; (4) the syncoxa of the maxilla has three endites in S. pumila sp. nov., whereas the praecoxal endite is absent in S. barti ; (5) the P1 exopod has five elements in S. pumila sp. nov., whereas it has only four elements in S. barti ; (6) S. pumila sp. nov. has a broad P5 baseoendopodal lobe in the female, while it is much more slender in S. barti (L/W value is approximately 4.1 in S. barti vs. 2.8 in S. pumila sp. nov.); and (7) the exopod/baseoendopod length ratio of the male P5 is much greater (approximately 1.9) in S. pumila sp. nov. than in S. barti (approximately 1.1).

Detailed morphological differences that are sufficiently robust to justify the discrimination between Smacigastes species are given in Table 1 and the dichotomous key to species of the genus Smacigastes worldwide is provided below.

Figures

Figure 2. Smacigastes pumila sp. nov., female (A, B). A. Habitus, lateral; B. Caudal ramus, lateral. Male (C), C. Habitus, lateral.

Figure 3. Smacigastes pumila sp. nov., female (A-D). A. Urosome excluding P 5 - bearing somite, dorsal; B. Urosome excluding P 5 - bearing somite, ventral; C. Genital field; D. P 5. Male (E), E. Urosome and P 5, lateral.

Figure 4. Smacigastes pumila sp. nov., antennules. A. Female; B. Male.

Figure 5. Smacigastes pumila sp. nov., female. A. Antenna; B. Mandible; C. maxillule; D. Maxilla; E. Maxilliped, medial; F. Maxilliped, lateral.

Figure 6. Smacigastes pumila sp. nov., female. A. P 1 (arrowhead indicates a pore); B. P 2.

Figure 7. Smacigastes pumila sp. nov., female. A. P 3; B. P 4.

Figure 8. Smacigastes pumila sp. nov., SEM photographs, female (A-D). A. Habitus, lateral; B. Anal somite and caudal rami, lateral; C. Basis and endopod of maxilliped, lateral (arrowhead indicates a minute setal armature on the endopod); D. Ornamentation on the palmar margin of the maxillipedal endopod. Male (E, F), E. Habitus, lateral; F. Genital double-somite, lateral.