Calanopia media Gurney, 1927

El-Sherbiny, Mohsen M. & Al-Harbi, Mamdouh A., 2020, New morphological and molecular data on the little-known pontellid Calanopia media Gurney, 1927 (Crustacea, Copepoda, Calanoida) from the Red Sea, with notes on its diel vertical distribution, ZooKeys 922, pp. 13-33 : 13

Calanopia media Gurney, 1927 Figs 1 , 2 , 3 , 4 , 5 , 6

Material examined.

36 females (body length: 1.17-1.32 mm, mean ± SD: 1.25 ± 0.051 mm) and 25 males (body length: 1.10-1.26 mm, mean ± SD: 1.14 ± 0.048 mm); whole specimens in 70% ethanol were deposited in the Natural History Museum, London [Registration number: NHMUK 2018. 1538-1547]. All specimens were collected at Obhur Creek, central Red Sea (21°42'32.23"N, 39°5'41.56"E) on 21 January 2016 by M.M. El-Sherbiny.

DNA-barcode.

The mitochondrial gene cytochrome oxidase subunit (mtCOI) sequences were submitted to GenBank (GenBank Accession numbers for C. elliptica : MN796254; C. media : MN445608-MN445611; C. minor : MN796251-MN796253; C. thompsoni : MN796255-MN796256).

Description.

Female. Prosome (Fig. 1A, B ) elliptical, without lateral hooks; cephalosome and first pedigerous somite completely separated; fourth and fifth pedigerous somites completely fused, with dorsal suture visible; posterior corners of prosome symmetrical, sharply pointed, extending nearly one-third of way along genital compound somite. Rostrum with broad base and pair of rounded lobes, each terminating in a tapering point (Figs 1C , 2A ). Urosome (Figs 1D, E , 2B, C ) with 2 free somites: genital compound somite symmetrical in dorsal view, with 2 unequal ventral spinules on right side, ventral surface with smooth, evenly rounded operculum located posterior to mid-length (Figs 1E , 2C ); second urosomite symmetrical and slightly shorter than genital compound somite; caudal rami asymmetrical; right ramus broader and expanded anteromedially, slightly shorter than left ramus, each ramus carrying 5 plumose setae (II-VI) along distal margin and a reduced seta (seta VII) located on dorsal surface near medial distal angle.

Antennules (Fig. 1F, G ) 18-segmented, slightly exceeding end of genital compound urosomite. Fusion pattern and armature elements as follows: ancestral segment I (segment 1) = 2 setae + aesthetasc (ae), II-VI (2) = 8 + ae, VII (3) = 2 + ae, VIII-X (4) = 7 (2 spiniform) + 2ae, XI (5) = 2 + ae, XII-XIII (6) = 4 (2 spiniform) + 2ae, XIV (7) = 1 +ae, XV (8) = 1 + ae, XVI (9) = 2 + ae, XVII-XVIII (10) = 4 + 2ae, XIX (11) = 2 + ae, XX (12) = 2 + ae, XXI (13) = 2 + ae, XXII (14) = 1, XXIII (15) = 1, XXIV (16) = 1 + 1, XXV (17) = 1 + ae+ 1, XXVI-XXVIII (18) = 5 + ae.

Antenna (Fig. 1H ) biramous; coxa with plumose seta distomedially; basis carrying 2 subequal plumose setae at distomedial angle; exopod 5-segmented, second segment longest with setal formula of 0, 2, 2, 1, 3. Endopod 2-segmented, first endopodal segment with 2 subequal lateral setae distally and furnished with fine setules distolaterally; second endopodal segment armed with 8 and 6 setae on proximal and distal lobes, respectively, laterodistal border with row of posterior spinules.

Mandible (Fig. 3A ). Gnathobase with eight teeth on cutting edge, third and fourth ventralmost teeth bicuspidate; patches of dagger-like spinules arranged at base of third to sixth ventralmost teeth; mandibular palp basis with 4 setae; endopod 2-segmented, first and second segments carrying 3 and 6 setae, respectively; exopod 5-segmented, first to fourth segments each with one seta and fifth segment with 3 setae.

Maxillule (Mx1) (Fig. 3B ). Praecoxal endite well developed and extended distally with 9 marginal and 4 posterior setae; coxal exite bearing 9 setae along distal margin; coxal endite with 3 setae; basal exite with long seta, proximal and distal endites with 3 and 2 setae, respectively. Exopod 1-segmented, with a total of 9 terminal setae. Endopod fused to basis, bearing 4 medial and 5 terminal setae.

Maxilla (Mx2) (Fig. 3C ). Praecoxal endite of syncoxa with 4 setae; proximal and distal coxal endites bearing 3 setae each; proximal and distal basal endites with 3 and 3 setae, respectively; endopod 3-segmented, with setal formula of 1, 1, 4.

Maxilliped (Mxp) (Fig. 3D ). Praecoxa and coxa completely fused, syncoxa with three endites carrying 2, 3, 2 setae on proximal, middle and distal endites, respectively; basal endite with 2 distal setae; endopod 4-segmented, first endopodal segment long, with 2 setae distally; other three endopodal segments shorter, bearing 1, 1 and 3 setae, respectively.

Legs 1-4 as in other members of the genus, with 3-segmented exopods and 2-segmented endopods as well as lateral spines with serrated hyaline margins (Fig. 4A-D ): coxa of legs 1 to 3 bearing one medial seta and a patch of fine setules; coxa of leg 4 without medial seta. Seta and spine formula as follows (spines, Roman numerals; setae, Arabic numerals):

Leg 5 (Fig. 4E ) asymmetrical but with same number of spines and processes; coxa and intercoxal sclerite completely fused; right basis broader and slightly shorter than left basis, each with one posterior plumose seta; exopod 2-segmented; first exopodal segment of right leg shorter than that of left leg; with 1 lateral fused process and 1 strong spine distally; second exopodal segment of right leg slightly longer than that of left leg, extending into tapering process fused to its segment, with 2 lateral articulated spines (proximal one smaller).

Male. Prosome (Fig. 5A ) 2.1 times as long as urosome; cephalosome and first pedigerous somite completely separated; fourth and fifth pedigerous somites completely fused (Figs 5A , 6B ); rostrum as in female (Fig. 5 B, C ); posterior corners of prosome slightly asymmetrical (right one slightly longer than left), with a sharp triangular process directed posteriorly and with a distinct ventral knob or process on its right medial margin, which cannot be seen in dorsal view (Figs 5D , 6B ). Urosome composed of 5 free somites, genital somite with genital aperture located ventrolaterally on posterior left side margin; second urosomite longer than other somites; anal somite shorter than preceding somite; caudal rami symmetrical, 2.2 times longer than wide, each ramus with 6 setae (II-VI) and seta VII small, inserted in ventrodistal medial margin.

Antennule (Figs 5E, F , 6C ) geniculate on right side, left one similar to that of female (except for second segment, which carries longer posterior setae): right one indistinctly 17-segmented, segments 3-4 incompletely fused ventrally, segments 5-6 and 7-8 completely fused dorsally, segment 13 with long denticles on proximal 1/4 and short denticles that extend to distal fourth part, segment 14 tooth ridge possessing triangle denticles proximally, which extend back to distally-directed spure-like process, armature as follows: ancestral segment I (segment 1) = 2 setae + aesthetasc (ae), II-V (2) = 8 + 2ae, VI (3) = 2 + ae VII (4) = 2 + ae, VIII-IX (5) = 4 (2 spiniform) + 2ae, X-XI (6) = 4 (1 spiniform) + ae, XII (7) = 1 + ae, XIII (8) = 1 + ae, XIV (9) = 2+ ae, XV (10) = naked, XVI-XVII (11) = 3 + 2 ae, XVIII-XIX (12) = 1+ process] + ae, XX (13) = 1+ ae, XXI-XXIII (14) = 2 + process +ae, XXIV (15) = 1 + 1, XXV (16) = 1+ ae + 1, XXVI-XXVIII (17) = 5 + ae.

Antenna, mouthparts and legs 1-4 as in female. Leg 5 (Figs 5G , 6D ) uniramous, asymmetrical; coxae and intercoxal sclerite completely fused. Left leg basis carrying 1 plumose seta posteriorly near two-thirds of its length; exopod 2-segmented, first segment shorter than basis (0.46 times) with small laterodistal spine; second segment nearly 1.35 times as long as first one, bearing 3 articulated spines (2 stout apically and one small laterally), medial hirsute margin with one distal fused spine. Right leg 5 (Figs 5G , 6D ), longer than left; basis slightly longer than coxa, carrying one posterior plumose seta; right exopod 2-segmented, first segment with small thumb-like process located at approximately one-third of segment length, with small seta near base of thumb, lateral margin concave with bilobed flap-like process; second exopodal segment approximately 0.7 as long as first exopod segment, curved at about mid-length and bluntly rounded distally, bearing 2 setae in depression (one proximal and one central) and 2 unequal outer setae at mid-length (proximal one longer than distal).

Variations.

On the ventral surface of the female genital compound somite of some specimens, a small fold in the cuticle may be found on the right or left side. Also, the degree of anteromedial expansion of the female right caudal ramus varies among specimens. The anteromedial expansion of the female right caudal ramus was present in most of the specimens collected from the study area (about 90% of the population), and sometimes the degree of this expansion varied greatly among specimens. In some specimens, the right caudal ramus had a concave or straight medial margin. Moreover, the ventral knob on the right side of the male prosome posterior corners varies in size.

Remarks.

We compared our specimens with the paratypes deposited at the Natural History Museum, London (BMNH 1926.2.16.69-88), and concluded that our specimens are C. media . Both our specimens and the paratypes shared most of the diagnostic features of the species, such as: the shape of the fifth pediger, the presence of 2 ventral spinules on the right side of the female genital compound somite, and the structure of both female and male leg 5. However, the asymmetry of female leg 5 (right leg basis broader and slightly shorter than left, first exopodal segment of right leg shorter than that of left leg and second exopodal segment of right leg slightly longer than on left leg) and the presence of a ventral knob on the right side of the prosome were probably overlooked in the original description by Gurney (1927). Nevertheless, our specimens differ in the asymmetry of the caudal rami, of which the right ramus is broader and expanded anteromedially, and slightly shorter than the left one.

Distribution.

Calanopia media was originally described from the Suez Gulf and the southern part of the Suez Canal ( Gurney 1927) during the CAMBRIDGE Expedition. Subsequently, Pesta (1941) collected this species during the POLA Expedition in the southern Red Sea (15°26'12"N, 40°05'24"E). In 1956, Rose recorded this species from the Vietnamese waters. Later, it was recorded from the Levantine Basin by Berdugo (1968) and Lakkis (1984) and considered to be a Lessepsian migrant species. In the present study, C. media was found in considerable abundance (106 ind. m-3) in samples collected at sunset (6:00 pm; UTC+3), with the highest densities at midnight (150 ind. m-3). Copepodid stages were relatively low, constituting 9 and 5% of the population at sunset and midnight, respectively. It was completely absent from near the surface in morning and midday samples. The sex ratio (males/females) of C. media varied between 0.46 and 0.54 at 6:00 pm and 12:00 am (UTC+3), respectively.

Molecular diversity.

A 624-bp region of the mtCOI was obtained for four female individuals of C. media , which varied in the degree of anteromedial expansion of the female right caudal ramus in specimens collected from Obhur Creek, central Red Sea. Results showed that the four analyzed specimens have nearly identical mtCOI sequences, with a distance ranging between 0.013 and 0.016 based on the pairwise distance method and Kimura 2 parameter model. The intraspecific variation in the mtCOI sequences of the other Red Sea species, C. minor and C. thompsoni , were 0.000 and 0.002, respectively.

Moreover, in the current analysis, sequences were obtained for three other Calanopia species collected from the study area ( C. elliptica , C. minor and C. thompsoni ) and sequences of one species ( C. thompsoni ) were obtained from NCBI. The mtCOI sequences of Calanopia species (i.e., C. elliptica , C. media , C. minor and C. thompsoni ) from the Red Sea differ between 21.3% and 29.4% (Table 1 and Fig. 7 ). A Neighbor Joining phylogenetic analysis using the Kimura 2 parameter model showed that C. media was clearly distinct from its congeneric species (Fig. 7 ). Concerning C. thompsoni , the only sequenced mtCOI in NCBI, it is clear that the average distance between Red Sea specimens and Indian Ocean specimens (KP068656-KP068659) was 0.035 (0.030-0.042), whereas for the China seas’ specimens it was 0.201 (0.193-0.211).

Figures

Figure 1. Calanopia media female from the Red Sea A habitus, dorsal view B habitus, lateral view C rostrum, lateral view (rudimentary rostral notch indicated by arrow) D abdomen, ventral view E abdomen, dorsal view F-G antennule H antenna. Scale bars in mm.

Figure 2. SEM micrographs of Calanopia media female from the Red Sea A rostrum, ventral view B abdomen, dorsal view C abdomen, ventral view D leg 5, posterior view.

Figure 3. Calanopia media female from the Red Sea A mandible B maxillule C maxilla D maxilliped. Scale bars in mm.

Figure 4. Calanopia media female from the Red Sea A leg 1, anterior view B leg 2, anterior view C leg 3, anterior view D leg 4, posterior view E leg 5, posterior view. Scale bars in mm.

Figure 5. Calanopia media male from the Red Sea A habitus, dorsal view B rostrum, lateral view C enlarged rostral filaments (rudimentary rostral notch indicated by arrow) D abdomen, ventral view (knob indicated by arrow) E right antennule F enlarged segments XVIII-XXIII G leg 5, posterior view. Scale bars in mm.

Figure 6. SEM micrograph of Calanopia media male from the Red Sea A rostrum, ventral view B prosomal end with abdomen, ventral view (medial notch indicated by arrow) C enlarged segments XX-XXIII D male leg 5, posterior view.

Figure 7. Neighbor Joining phylogenetic tree based on the mtCOI genes of Calanopia elliptica, C. media, C. minor and C. thompsoni from the Red Sea (indicated by *). Calanopia thompsoni sequences (indicated by +) from GenBank were used for comparative analysis.