Acartia (Odontacartia) edentata Srinui, Ohtsuka & Metillo

Srinui, Khwanruan, Ohtsuka, Susumu, Metillo, Ephrime B. & Nishibori, Masahide, 2019, A new species of Acartia (Copepoda, Calanoida) from the Philippines, based on morphological and molecular analyses, ZooKeys 814, pp. 71-94 : 74-82

Acartia (Odontacartia) edentata Srinui, Ohtsuka & Metillo sp. n. Figures 2, 3, 4, 5

Material.

Type locality: Carigara Bay, Off Leyte Island, the Philippines (11°30'70"N; 124°69'01"E) (Fig. 1), August 23, 2013 (10 ♀, 10 ♂).

Types.

Holotype: ♀, dissected and mounted on 2 glass slides ( BIMS–Zoo– 0267); paratype (allotype): 1 ♂, dissected and mounted on 2 glass slides ( BIMS–Zoo– 0268); additional paratypes: 4 ♀, 3 ♂ partially dissected and mounted on 3 glass slides ( BIMS–Zoo– 0269).

Measurements.

Female. Total length, 1.19-1.23 mm (mean ± SD = 1.21 ± 0.01 mm, N = 10; holotype, 1.19 mm); prosome length, 0.42-0.46 mm (0.44 ± 0.01 mm; holotype, 0.44 mm); prosome width, 0.24-0.29 mm (0.26 ± 0.01 mm; holotype, 0.25 mm). Male. Total length 1.08-1.15 mm (mean ± SD = 1.10 ± 0.02 mm, N = 10; allotype, 1.10 mm); prosome length, 0.39-0.41 mm (0.40 ± 0.00 mm; allotype, 0.40 mm); prosome width, 0.23-0.26 mm (0.24 ± 0.01 mm; allotype, 0.25 mm).

Descriptions.

Female. Body (Fig. 2A, B) elongate; cephalosome completely separate from first pedigerous somite; anterior margin of cephalosome triangular in dorsal view; rostrum with pair of thick, strong and sharp filaments (Figs 4F, 5F); fourth and fifth pedigerous somites fused. Posterior prosome symmetrical with pair of acute processes on each side: large ventrolateral, pointed processes with pair of small prominences between and pair of smaller, pointed processes dorsally (Fig. 2A). Urosome composed of three free somites; genital double-somite symmetrical with ratio of width–length ratio approximately 1:1, lacking posterodorsal pointed processes (Figs 2A, 5A); second urosomite with pair of strong, posterodorsal, pointed processes; anal somite as wide as long, without lateral rows of fine setules. Proportional lengths of urosomites and caudal ramus 41:22:15:22 (= 100). Caudal rami with setules along lateral margin, and symmetrical with 6 plumose setae ( II–VII). I absent, V longest, and VII inserted anterodorsally.

Antennule (Fig. 2C) reaching beyond posterior end of second urosomite, symmetrical, 17-segmented; segments II–VIII completely fused; segments II, IV, VII, VIII, XI, XV, and XVII with aesthetasc (ae). Fusion pattern and setal elements as follows (Roman numerals represent ancestral segments): I = 1, II–VIII = 7 + 2ae, IX = 1+ (1 spiniform), X–XI = 2 + (1 spiniform) + ae, XII = 1, XIII–XV = 3 + ae, XVI= 1 + ae, XVII–XVIII = 2 + (1 process) + ae, XIX = 1 + (1 process), XX = 1, XXI= 1 + (1 process) + ae, XXII = 1, XXIII = 1, XXIV = 2, XXV = 2 + ae, XXVI = 2, XXVII–XXVIII = 4 + ae.

Antenna (Fig. 2D) coxa with single seta; basis fused to elongated first endopodal segment forming allobasis with eight setae on outer medial margin, and single lateral seta and transverse row of small spinules terminally; second segment with eight outer setae and fine setules along inner margin; free terminal segment short with six setae. Exopod 3-segmented, setation formula 1, 4, 3.

Mandible (Fig. 3A) gnathobase having two sharp cuspid teeth, one blunt tooth, and three small sharp teeth bordered by small spinules at the proximal end; basis with fine setules on medial outer marginal, single seta distally, and patch of small spinules on surface at midlength; first endopodal segment short with two short setae, second segment with seven setae; exopod 4-segmented, first to fourth with setation formula 1, 1, 2, 2; first segment with row of small spinules.

Maxillule (Fig. 2E) with precoxal arthrite bearing nine strong spines; coxal endite with three terminal setae; coxal epipodite with one short and eight long setae; basal exite with one terminal seta and one proximal seta; exopod 1-segmented fused with basis and bearing two long medial setae, five setae terminally, with fine spinules along outer marginal; endopod absent.

Maxilla (Fig. 3B) with syncoxal endite bearing 5, 3, 3, 2 setae; basis with one long seta; endopod with four long, one medium, and one short setae.

Maxilliped (Fig. 2F) highly reduced; syncoxa with setation formula of two long, one medium and one short seta; basis with one short strong, one long setae and row of setules along inner margin; endopod 2-segmented, with four inner spines and terminal spiniform element.

Legs 1 to 4 (Fig. 3 C–F) biramous, each with 2-segmented endopod and 3-segmented exopod; coxa unarmed; second endopodal segments of leg 1 and 4 and third exopodal segment of leg 1 with row of small spinules anteriorly. Seta and spine formula as shown in Table 1.

Leg 5 (Fig. 3G) symmetrical, coxae and intercoxal sclerite completely fused; basis longer than wide, outer margin with single lateral seta, slightly longer than terminal seta of exopod; exopod with knob-like projection basally, distal half spinulose.

Male. Body (Fig. 4A, B) similar to that of female; cephalosome anterior bluntly triangular in dorsal view; rostrum (Fig. 4E) with paired filaments (Figs 4E, 5E). Posterior prosome symmetrical with pair of short acute processes dorsolaterally and longer ventrolateral acute processes, with pair of small prominences between two dorsolateral processes. Posterior margin of prosome naked. Urosome composed of five somites, symmetrical in dorsal view; genital somite (= first urosomite) as long as wide, bearing 2 dorsolateral rows of small spinules; second urosomite with two pairs of strong posterior dorsolateral, processes (Figs 4A, B, 5 E), outer shorter than inner, and furnished with three rows of minute spinules ventrolaterally (Figs 4B, 5F); third urosomite with pair of strong acute processes dorsally (Figs 4 A, 5C, D); fourth urosomite 4.5 times shorter than wide and furnished with pair of small medium-sized acute processes dorsally; anal somite with setules along outer margins. Caudal rami symmetrical, approximately 1.5 times as long as wide, having lateral setules along inner margin (Fig. 5D) and 6 plumose ( II–VII) setae as in female.

Left antennule (Fig. 4C) incompletely 21-segmented; segments 2, 3 and 21 incompletely fused; armature elements and fusion pattern as follows (Roman numerals represent ancestral segments): I = 1, II–V = 3 + ae, XI–IX = 4 + ae, X= 1 + (1 spiniform), XI= 2 + ae, XII = 0, XIII = 0, XIV= 1 + (1 spiniform) + ae, XV = 1, XVI = 1 + ae, XVII = 1, XVIII = 1 + ae, XIX = 1, XX= 1, XXI= 1 + ae, XXII = 1, XXIII = 1, XXIV = 2, XXV = 2 + ae, XXVI = 2, XXVII–XXVIII = 4 + ae.

Right antennule (Fig. 4D) geniculate, incompletely 17-segmented, not reaching beyond posterior end of fifth pedigerous somite; segments 2 and 3 incompletely fused; segments 2, 3, 4, 7, 9, 15, 17 each with aesthetasc (ae); segments 4, 7 and 12 each with spiniform element. Armature elements and fusion pattern as follows (Roman numerals represent ancestral segments): I = 1, II–VI = 3 + (1 minute) + ae, VII–VIII = 2 + ae, IX–XI = 2 (2 spiniforms) + ae, XII = 0, XIII = 1 minute, XIV = 1 (1 spiniform) + ae, XV = 1, XVI = 1 + ae, XVII = 1, XVIII = 1, XIX = 1 + (1 spiniform), XX = 1 + longitudinal row teeth, XXI–XXIII = 3 + longitudinal row teeth, XXIV–XXV = 3+ ae, XXVI = 2, XXVII–XXVIII = 4 + ae.

Leg 5 (Fig. 3H) uniramous, coxae unarmed and completely fused with intercoxal sclerite; each side of basis with outer plumose seta subterminally, left basis approximately 2.5 times as long as wide, with concave inner margin. Right exopod 3-segmented, first segment approximately 3 times as long as wide with single seta subterminally, second segment with small subterminal spine at mid-length of inner irregularly triangular knob, third segment curved inward with small terminal spine and small inner spine midway. Left exopod 2-segmented, first segment about 2.5 times as long as wide, second segment with long inner seta and small terminal spine.

SEM observation of Acartia (Odontacartia) edentata sp. n.

The absence of paired dorsal processes on the female genital double-somite and the thick rostrum were confirmed with scanning electron microscopy (Fig. 5 A, D). Paired genital slits are located at midlength and moderately separated (Fig. 5B, C). A row of setules is located along the anterior margin of each genital slit (Fig. 5C).

SEM observations of the male urosomite clearly showed fine ornamentation on the posterior border of first urosomite (Fig. 5E) and second segment furnished with three rows of minute spinules ventrolaterally (Fig. 5F). No fine setules were observed along the inner posterior margin of prosome, as described in the above descriptions of the type specimens.

Remarks.

The subgenus Acartia (Odontacartia) is composed of two species groups, the centrura and erythraea species groups ( Steuer 1923). Acartia (O.) lilljeborgi is regarded as an intermediate type between the centrura and erythraea species groups ( Steuer 1923; Ueda 1986). The centrura species group is defined as follows: in the female, the genital double-somite with a pair of large processes, the first antennulary segment without a large spine, relatively long caudal rami, the exopod of leg 5 with a knob situated or extending to midlength; in the male, the third and fourth urosomites have a dorsal pair of large acute processes, the first exopodal segment of left leg 5 without an outer spine.

Among the centrura species, the female of A. (O.) edentata sp. n. is unique in lacking paired posterior dorsolateral processes on the genital double-somite unlike those of the closely related A. (O.) pacifica (Table 2). Such an absence can also be found in females of A. (O.) bowmani from India ( Abraham 1976), but the morphology of posterior prosome and fifth legs of both sexes of A. (O.) bowmani differ from that of A. (O.) edentata sp. n.: (1) posterior prosomal border of female and male rounded with one pair of medium spines and one pair of small spines dorsally, (2) posterior margin of antennule with small spines on segments 4, 5, 10, 11 and 13 in female, (3) exopod in female fifth leg bulbous basally, (4) first segment of male urosomite bilobed and with fine setules on lateral margins, (5) second segment of right exopod of male fifth leg with quadrilateral shape of inner lobe, and (6) second segment of left exopod of male fifth leg with short seta and short segment. Irrespective of the presence or absence of the dorsal processes on the female genital double-somite, females of the new species and A. (O.) pacifica share the following features: (1) moderately long caudal rami (ca 2.7 times as long as wide), (2) the presence of a basal knob on the exopod of leg 5. Males of these species are also characterized together as follows: (1) the fine setules along posterior margin of first urosomite, (2) the presence of ventrolateral rows of minute spinules on the second urosomite laterally, and (3) dorsal processes on the third urosomite twice as long as those on the fourth urosomite.

Since " A. (O.) pacifica " s.l. morphologically and genetically consists of several cryptic species ( Ueda and Bucklin 2006; Srinui et al. unpublished), we genetically compared specimens obtained from Japan (the Seto Inland Sea) and South Korea near the type localities to the new species (see "Molecular diversity" below). In conclusion, our Japanese and Korean specimens of A. (O.) pacifica clearly coincided with A. (O.) pacifica s.s. as morphologically/genetically redefined by Ueda and Bucklin (2006). Therefore, a comparison is made between the new species from the Philippines and A. (O.) pacifica s.s. obtained from Japan and South Korea in the present study. In addition to the absence of dorsal processes on the genital double-somite, females of the new species are distinguished from those of A. (O.) pacifica s.s. by: (1) segment 5 (VII) of right antennule with 1 seta (absent in A. (O.) pacifica s.s.), (2) dorsal processes on the second urosomite nearly reaching the posterior border of the anal somite (at most half the length of anal somite in A. (O.) pacifica s.s.), (3) length ratio of lateral seta of the basis to terminal seta of leg 5 is relatively short, about 1.3 (ca 2 in A. (O.) pacifica s.s.), (4) mandibular processes on gnathobase 1 blunt and 2 cuspidate (1 blunt and 6 cuspidate in A. (O.) pacifica s.s.) (Table 2). Males of the new species are differentiated from those of A. (O.) pacifica s.s. by: (1) Dorsal and lateral spines on the second somite are of medium-sized (longer in A. (O.) pacifica s.s.), (2) dorsal processes of the third urosomite are long enough to reach beyond those of the fourth urosomite (not reaching in A. (O.) pacifica s.s.), (3) terminal exopod segment of left leg 5 with an inner seta inserted midway (subterminally in A. (O.) pacifica s.s.), and (4) medial projection of second exopodal segment of right leg 5 with an inner irregularly triangular knob (rounded triangular in A. (O.) pacifica s.s. (Table 2).

Ueda and Bucklin (2006) described both left and right antenules of male A. (O.) ohtsukai , and we can compare setation in the new species as follows: Right: (3) VII–VIII– 2 + ae (3 and 1 ae in A. (O.) ohtsukai ), (4) IX–XI– 2 (2 spiniform) + ae (plus 2 spinules in A. (O.) ohtsukai ), (6) XII– 1 minute (minute absent in A. (O.) ohtsukai ), (7) XIV– 1 (1 spiniform) + ae (plus 1 spinule in A. (O.) ohtsukai ), (11) XVIII– 1 (plus 1 ae in A. (O.) ohtsukai ), (12) XIX– 1 (spiniform) (plus longitudinal row teeth in A. (O.) ohtsukai ), (14) XXI–XXIII– 3 + longitudinal row teeth (longitudinal row teeth absent in A. (O.) ohtsukai ), (15) XXIV–XXV– 3 + ae (plus 1 spinule in A. (O.) ohtsukai ). The segmentation and setation of the right antennule of A. (O.) ohtsukai are alternately interpreted as follows: 16-segmented with those of (1) I– 1, (2) II–VI– 4 + ae, (3) VII–VIII– 3 + ae, (4) IX–XI– 4 (2 spiniforms) + ae, (5) XII–unarmed, (6) XIII–unarmed, (7) XIV– 2 (1 spiniform) + ae, (8) XV– 1, (9) XVI– 1 + ae, (10) XVII– 1, (11) XVIII– 1 + ae, (12) XIX– 1 + process, (13) XX– 1 (14) XXI–XIII– 4, (15) XXIV–XXV– 4 + ae, (16) XXVI– 2 (17) XXVII–XXVIII– 4 + ae. The segmentation and setation of the left antennule are similar in the A. (O.) edentata sp. n. and A. (O.) ohtsukai .

The taxonomy of the Indo-West Pacific A. (O.) pacifica should be revised, because the presence of several cryptic species has already been suggested by our study and oth ers. The above-mentioned sexual dimorphic features are species-specific, and should be carefully compared among A. (O.) pacifica s.l. to resolve the issue (see Discussion).

Etymology.

The new species of Acartia was named edentata (Latin, meaning toothless) with reference to the absence of tooth-like processes on the posterodorsal border of the genital double-somite in females.

Genetic diversity.

We obtained sequence data from mitochondrial 16S and COI genes for 14 individual specimens at three sites. A 282 bp fragment of the 16S gene was analyzed for five adult female specimens from the Philippines ( A. (O.) edentata sp. n.), and a 162 bp 16S fragment was analyzed for A. (O.) pacifica specimens from Ariake Bay, the Seto Inland Sea, Japan and South Korea. A 636 bp fragment of the mitochondrial COI gene was analyzed in the new species and in specimens from Japan and Korea. GenBank sequences for A. (O.) pacifica (accession number DQ071175 for 16S and DQ071177 for COI) and two out group species of subgenus A. (Odontacartia) , A. (O.) ohtsukai (accession number DQ071174 for 16S and DQ071176 for COI), A. (O.) erythraea (accession number DQ320504 for 16S) and A. (O.) tsuensis (accession number KC287427 for COI), were also used for comparison. The intraspecific variation in the 16S sequences from the five A. (O.) edentata sp. n. individuals was 0%, whereas A. (O.) edentata sp. n. sequences differ from those of A. (O.) pacifica from Japanese and Korean waters, A. (O.) pacifica based on GenBank, A. (O.) ohtsukai , and A. (O.) erythraea by 20-21%, 20-21%, 28%, and 31%, respectively. The COI sequences from A. (O.) edentata sp. n. individuals differ by only 0.02-0.08%; A. (O.) pacifica (from Japan, and Korea), A. (O.) pacifica (GenBank), A. (O.) ohtsukai , and A. (O.) tsuensis sequences differ from A. (O.) edentata sp. n. COI sequences by 16-18%, 16-17%, 16-17%, 22%, and 24%, respectively (Fig. 6; Table 3).

Ecology.

Temperature and salinity appear to be important factors determining the distribution and abundance of copepods. In the Indo-West Pacific, A. (O.) pacifica occurs in the tropical and subtropical zones of the Pacific and Indian oceans. In the East China Sea (subtropical zone), A. (O.) pacifica was abundant in August (salinity 15.0) in the Changjiang (Yangtze River) Estuary, China ( Gao et al. 2008), while in Korean waters, A. (O.) pacifica is strictly stenohaline, occurring waters of more than 32 in salinity ( Moon et al. 2008). Kang (2011) also observed the A. (O.) pacifica and A. (O.) erythraea in Korean waters with temperature ranges of 18.0-27.2 °C and 14.6-26.4 °C and salinity ranges of 21.0-32.9 and 21.0-33.7, respectively. In Japanese waters A. (O.) ohtsukai was found in the estuary of the Rokkaku River, Ariake Bay in surface waters, where water temperature was 29.0 °C and salinity was 5.0, while A. (O.) pacifica was found in waters of 26.0 °C and 33.0 in the Seto Inland Sea, Japan ( Ueda and Bucklin 2006). Furthermore, A. (O.) pacifica was dominant in Moreton Bay, Queensland waters with temperature ranges above 22.0 °C and salinities ranging from 34.0 to 36.5 ( Greenwood 1981).

In the tropical zone, A. (O.) edentata sp. n. specimens were collected in the Philippines during the rainy season (August 2013), when water temperature and salinity were 30.2 °C and 33.5, respectively. In contrast, A. (A.) tsuensis represents the dominant species in brackish pond water from Panay Island in central Philippines during the dry season (November - April), with salinity ranging from 14.0 to 40.0 ( Golez et al. 2002).

In Bintulu, Sarawak, Malaysia, Johan et al. (2013) compared to A. (O.) pacifica in coastal waters with temperatures of 28.8-29.0 °C and high salinities (24.0-32.0). In the Indian waters, Wellershaus (1969) recorded the occurrence of female specimens of A. (O.) pacifica in waters with salinity ranging from 10.0 to 30.0, including in the Andaman Sea, Thailand (Surin Islands National Park, Phang Nga Province), A. (O.) pacifica were abundant in waters with temperatures of 29.7-31.0 °C and salinities ranging from 29.9 to 35.8 ( Treeramaethee et al. 2013). However, we concluded that three species of Acartia appear to occupy water bodies differing in temperature and salinity of the tropical and subtropical zone of the Pacific and Indian oceans (Fig. 7).