Spinoncaea tenuis Boettger-Schnack , 2003

Cho, Kyuhee, Park, Chailinn & Boettger-Schnack, Ruth, 2021, Taxonomy of three species of the genus Spinoncaea (Copepoda, Oncaeidae) in the North Pacific Ocean with focus on morphological variability, ZooKeys 1043, pp. 147-191 : 147

Spinoncaea tenuis Boettger-Schnack, 2003 Figs 8 , 9 , 10 , 11

Spinoncaea tenuis Böttger-Schnack, 2003: 215-225, figs 12-16 (Red Sea, Mediterranean, Arabian Sea, Pacific Ocean).

Material examined.

(1) Northeastern Pacific (a) 10°30'N, 131°20'W (EP-1), 21 August 2009: One female (habitus of S. tenuis female in Fig. 8A, B ) and one male (habitus of S. tenuis male in Fig. 11A ) undissected on H-S slide, respectively. Five females and two males dissected on several slides, respectively. Three females dissected on H-S slide, respectively. Six dissected females (NIBRIV0000882784-882789) and one dissected male (NIBRIV0000882790) and one undissected female (NIBRIV0000882782) and one undissected male (NIBRIV0000882783) on respective H-S slide were deposited in the NIBR. (b) 9°52'1.38"N, 131°45'38.28"W (EP-2), 19 March 2019. Two undissected females and two undissected males in alcohol vial (NIBRIV0000882791) were deposited in the NIBR. (2) Northwestern Pacific, 13°20'3.42"N, 144°20'2.7"E (WP-2), 4 April 2016. One undissected male in alcohol vial (NIBRIV0000882792) was deposited in the NIBR. (3) Korea Strait, 33°44'50.50"N, 128°15'39.02"E (KS), 7 October 2008: One female (NIBRIV0000882793) and one male (NIBRIV0000882794) dissected on H-S slide, respectively. All dissected specimens and one undissected female (in alcohol, NIBRIV0000882795) were deposited in the NIBR .

Description.

Female (Figs 8 - 10 , Tables 3 , 4 ). Body length in lateral view (telescoping of somites not considered) (Fig. 8B ) 320-355 µm in Pacific specimens (Table 3 ), somewhat larger than in the Red Sea (280-300 µm, Böttger-Schnack 2003: 215).

Prosome 1.7 × length of urosome, excluding caudal rami, 1.5 × urosome length including caudal rami in specimens figured (Fig. 8B ), calculated by not correcting for the telescoping of somites. Variation of prosome to urosome length (including CR) 1.3-1.7 in Pacific specimens (Table 3 ), single value from Korea Strait smallest. The respective values provided for Red Sea specimens (1.5 incl. CR; Böttger-Schnack 2003: fig. 12A, calculated by not correcting for telescoping of somites) are within the range of values from the Pacific.

P5-bearing somite with paired midventral spinous processes variable in number (two or three processes) and one pair of ventrolateral lobate processes (arrowed in Fig. 8D, E ). Variation in number of midventral spinous processes was not mentioned for Red Sea specimens and ventrolateral lobes were not described, but are vaguely discernible from Böttger-Schnack (2003: fig. 12I).

Genital double-somite (Fig. 8C, D, E ) 2.1 × as long as maximum width in specimen figured (measured in dorsal aspect) and ~ 2.1 × as long as postgenital somites combined; variation in length to width ratio 1.8-2.3 in Pacific specimens (Table 3 ), respective values from Red Sea fall within this range. Largest width measured at 2/5 the distance between anterior and posterior margin, similar to Red Sea specimens, where it is "about halfway". Ventral surface with few rows of minute spinules in some specimens (Fig. 8E ), difficult to discern; this ornamentation was not mentioned for Red Sea specimens. Paired genital apertures located dorsally at about same position as in Red Sea specimens, armature difficult to discern. Weakly pronounced undulate hyaline frill on posterior margin of genital double-somite and postgenital somites and pore pattern as figured (Fig. 8D, E ).

Anal somite (Fig. 8C ) length to width ratio ranging between 1.1-1.3 (Table 3 ), ornamentation as figured (Fig. 8C, D, E ).

Caudal ramus (Fig. 8A, C ) length to width ratio 1.8-2.5 measured along inner margin and 2.3-3.0 measured along outer margin (Table 3 ). Caudal seta III ornamented with few minute spinules along medial margin (Fig. 8C ), not observed in Red Sea specimens. Length ratio between seta IV and III 1.4-2.3 (Table 3 ), seta IV unipinnate, not bipinnate as in Red Sea specimens ( Böttger-Schnack 2003: fig. 12C).

Antennule 6-segmented (Fig. 9A ). Armature formula and ornamentation as for S. ivlevi .

Antenna 3-segmented, armature and ornamentation as figured (Fig. 9B ). Distal endopod segment with length to width ratio 3.3-4.1 in Pacific specimens (Table 3 ), seta II longer than seta I (as illustrated for Red Sea specimens, Böttger-Schnack 2003: fig. 13A, but erroneously described as being "shorter than seta I" in text on p. 217).

Labrum with ornamentation as figured (Fig. 9G, H ). including difference to S. ivlevi in (1) size of three marginal teeth along distal (ventral) margin on each lobe (arrowed in Fig. 9G ) being somewhat smaller than in S. ivlevi , and (2) presence of two paired rows of long setules on anterior surface (Fig. 9G ), not only a single paired row as in S. ivlevi .

Mandible with armature and ornamentation as figured. (Fig. 9C ), small element D on gnathobase absent, as typical for S. tenuis (cf. Böttger-Schnack 2003: 218, fig. 13D).

Maxillule (Fig. 9D ) similar to S. ivlevi , except for middle element on inner lobe naked.

Maxilla (Fig. 9E ) with additional ornamentation on surface of syncoxa (arrowed in Fig. 9E ), not reported earlier for Red Sea specimens.

Maxilliped as figured (Fig. 9F ), surface of syncoxa ornamented with few spinules (arrowed in Fig. 9F ), which was not recorded for Red Sea specimens.

Swimming legs 1-4 (Fig. 10A-D ), with armature as in S. ivlevi (Table 2 ). Intercoxal sclerites of P1 ornamented with paired long, fine setules (but only one paired setule shown in Fig. 10A ), which are difficult to discern. Ornamentation on inner portion of basis in P1-P3 as figured (Fig. 10A-C ).

Exopods with variability of proportional spine lengths given in Table 4 , respective values from the Red Sea fall within this range ( Böttger-Schnack 2003: fig. 14A-D), except for the proportional lengths of outer spines on P3, which are larger in Pacific specimens than in the Red Sea specimens. Distal spine slightly longer than (P1) or almost equal in length (P2-P4) to distal exopod segment, similar to Red Sea specimens ( Böttger-Schnack 2003: fig. 14A-D).

Endopods. Length ranges of outer subdistal spine and outer distal spine relative to distal spine on P2-P4enp-3 as given in Table 4 generally similar to Red Sea specimens ( Böttger-Schnack 2003: fig. 14A-D).

P5 (Fig. 8C, D, F ) exopod 1.7 × longer than wide, armature and ornamentation as figured.

P6 (Fig. 8C ) as figured, possibly armed with a short spinule, which is difficult to discern.

Male (Fig. 11 , Tables 3 , 4 ). Body length 292-325 µm (Table 3 ). Sexual dimorphism in antennule, maxilliped, P6, and in genital segmentation, slight modification in setal length of P5. Pore pattern on prosome not discerned.

P5-bearing somite with paired row midventral spinous processes variable in number as in female and one pair of ventrolateral lobate processes (Fig. 11C ).

Caudal rami (Fig. 11A, B, C, G ) with length to width ratio 1.9-2.4 measured along inner margin and 2.2-2.8 measured along outer margin (Table 3 ), ornamentation as figured (Fig. 11D ). Caudal setae with proportional lengths as in female, variation in length ratios as given in Table 3 .

Antennule (Fig. 11E ) 4-segmented, armature as for S. ivlevi .

Antenna (not figured) with variation in length to width ratio of distal endopod segment similar to female (Table 3 ).

Maxilliped (Fig. 11D ) 3-segmented, armature and ornamentation as figured.

Swimming legs 1-4 with armature and ornamentation as in female. Variability in proportional spine lengths on rami given in Table 4 , values of equatorial Pacific fall within the range of females, but proportional lengths of exopodal spines on P2 and P4 from Korea Strait larger than those of females.

P5 (Fig. 11F ) with exopodal seta and outer basal seta somewhat shorter than in female.

P6 (Fig. 11C ) with ornamentation pattern as figured.

Remarks.

Böttger-Schnack (2003) reported two variants of female S. tenuis which differed in geographical distribution. The typical form appeared in the entire Red Sea and in the northern Arabian Sea, while the elongate form was found in the Mediterranean Sea and in the NW Pacific (Kuroshio Extension); specimens from the NE Pacific (Monterey), on the other hand, showed intermediate values between both forms. In the present study, females from the NE equatorial Pacific also displayed intermediate values in morphological characters between the two forms of S. tenuis , which are as follows: (1) the length to width ratio of the genital double-somite has a wide range (1.8-2.3), including values of both form types; (2) the position of the genital apertures is at 2/5 of distance from the anterior margin as in the elongate form (from the Adriatic Sea); (3) the basal seta on P4 is more similar to the typical Red Sea form, reaching as far as the middle of the distal exopod segment, whereas this seta is much longer in the elongate form (from the Adriatic Sea), reaching beyond the tip of the distal spine on the exopod segment (cf. Böttger-Schnack 2003: fig. 16C); (4) the outer basal seta on P5 reaching as far as 4/5 the distance from the anterior margin of the genital double-somite in our Pacific specimens, but extending almost beyond the posterior margin of the genital double-somite in the elongate form (from the Adriatic Sea), (5) the length to width ratio of the caudal ramus measured along inner or outer margin in our specimens (1.8-2.5 or 2.3-2.9 ×, respectively) is larger than in the typical form from the Red Sea (1.9-2.0 or 2.1-2.3 ×) at least for ratio of the outer margin, and the range of values corresponds approximately to those of the elongate form from the Adriatic Sea (1.8-2.4 or 2.2-2.6 ×) and the NE Pacific (off Monterey, California) (2.1 or 2.4-2.7 ×) ( Böttger-Schnack 2003: table 8).

In terms of ornamentation details, which are described for the typical form only, our Pacific specimens differed from the typical S. tenuis mainly by some details such as: (1) (1a) on the syncoxa of the maxilla and (1b) on the proximal element of the maxilliped; (2) short spinule(s) on the inner margin of bases on P2 and P3; (3) ornamentation with few minute spinules along the medial margin of CR seta III; and (4) variable number of midventral spinous processes on the P5-bearing somite.

Unlike the females, males of S. tenuis could not clearly be classified into form types in Böttger-Schnack’s account. When compared to the typical form from the Red Sea, specimens from the equatorial Pacific are similar in morphology, except for some minor differences including (1) the length to width ratio of the genital somite, which is longer than in our specimens (1.8-2.0 ×) than in the Red Sea specimens (1.7 ×), (2) the caudal rami (inner 1.9-2.2 ×, outer 2.2-2.6 ×) were slightly longer than in the Red Sea specimens (inner 1.9 ×, outer 2.3 ×), and (3) the length ratio of caudal setae VII and IV, respectively, with seta VII being 1.6-1.9 × longer than seta IV in the Pacific specimens, whereas seta VII is only 1.4 × the length of seta IV in the Red Sea specimens. Also, the number of paired midventral spinous processes on the P5-bearing somite differs, showing only two processes in the Pacific, as compared to three processes in the Red Sea specimens. However, as the male specimen from the Korea Strait also showed three paired processes (not figured), and differences among individuals of S. tenuis females (two or three processes) were apparent, this ornamentation seems to be due to individual variation, and cannot be regarded as a regional difference.

According to Böttger-Schnack (2003), some slight morphological differences occurred between males of S. tenuis from the Red Sea and the Adriatic Sea (e.g., the proportional lengths of the genital somite and the caudal rami), but the determination of an elongate male appeared to be ambiguous. In our case, the above mentioned two characters are intermediate between the typical form (from the Red Sea) and the elongate form (from the Adriatic Sea) and the range of these values could be perceived as a variation among individuals (cf. Table 3 ). However, the single male of S. tenuis from the Korea Strait (not figured) seemed to be similar to the elongate form from the Adriatic Sea, as it differed from specimens from the equatorial Pacific specimens in the following characters (Table 3 ): (1) smaller body length: 292 μm; (2) the genital somite being slightly longer than in the equatorial Pacific, with a length to width ratio of 2.0:1; (3) the length to width ratio of the caudal rami being greater/higher (inner 2.4 ×, outer 2.8 ×) than in the equatorial Pacific (Table 3 ); (4) the anal somite slightly longer than in the equatorial Pacific, 1.2 × longer than wide; and (5) the outer basal seta on P5 reaching the posterior margin of the genital somite. In summary, the observed variation of features for S. tenuis in the Pacific indicates that the previously described form types of this species are not clearly separated; however, distinct form types may occur due to regionally reduced ranges of variation in the morphological details described here.

The female of S. tenuis can easily be confused with the elongate form of female S. ivlevi from the Pacific Ocean, due to the shape of the genital double-somite. However, as Böttger-Schnack (2003) mentioned the distinction between S. tenuis and S. ivlevi elongate form from the equatorial Pacific are: (1) the number of elements on the mandible (four in S. tenuis , but five in S. ivlevi elongate form) and (2) the number of spinules patches on the anterior surface of the labrum (two pairs in S. tenuis , but one pair in S. ivlevi elongate form, generally). Further morphometric differences between females of the two species may be found in (3) the proportional lengths of caudal setae III: II, which is smaller in S. tenuis (1.0-1.5 ×) as compared to S. ivlevi elongate form (1.6-2.0 ×) and (4) the length ratio between the distal spine and distal exopod segment on P2-P4, with the distal spine being almost equal in length to the distal segment in S. tenuis , whereas the spine is shorter than the segment in S. ivlevi elongate form (esp. on P4) (Table 4 ). Further minor differences between the two species are found in the patterns of the ornamentation on the ventral surface of the genital double-somite, as the elongate form of S. ivlevi (Fig. 7B ) has a larger number of spinular rows than S. tenuis (Fig. 8E ) and the ornamentation on the inner margin of caudal ramus, which is absent in S. tenuis , but is present in S. ivlevi elongate form.

Figures

Figure 7. Spinoncaea ivlevi (Shmeleva, 1966), female, elongate form, general (northwestern equatorial Pacific) A urosome, dorsal, caudal seta V on both sides missing; elongate form, aberrant (northeastern equatorial Pacific) B P 4 - bearing somite and urosome, ventral, showing ornamentation on P 4 and P 5 - bearing somites and genital double-somite, ventrolateral lobes arrowed, caudal seta V on left side missing C P 5 - bearing somite and genital double-somite, lateral, showing 2 long setules on ventral side of double-somite, midventral spinous process and ventrolateral lobe arrowed. Spinoncaea ivlevi (Shmeleva, 1966), female, robust form, variation, (northwestern equatorial Pacific) D setular patch on tip of cephalosome, dorsal E setular patch on cephalosome, lateral. Scale bars in μm.

Figure 8. Spinoncaea tenuis Boettger-Schnack, 2003, female (northeastern equatorial Pacific) A habitus, dorsal (outer basal seta and exopodal seta of P 5 on right side damaged, caudal seta V on left side missing) B habitus, lateral C urosome, dorsal (outer basal seta and exopodal seta of P 5 on right side missing, caudal seta V on left side missing) D urosome, lateral, midventral spinous processes and ventrolateral lobe arrowed E urosome, ventral, midventral spinous processes and ventrolateral lobes arrowed (caudal setae IV and VI on left side omitted and seta V missing). Scale bars in μm.

Figure 9. Spinoncaea tenuis Boettger-Schnack, 2003, female (northeastern equatorial Pacific) A antennule B antenna C mandible D maxillule E maxilla, arrows indicating spinules F maxilliped, anterior G labrum, anterior, arrows indicating three marginal teeth H labrum, posterior. Scale bars in μm.

Figure 10. Spinoncaea tenuis Boettger-Schnack, 2003, female (northeastern equatorial Pacific) A P 1, anterior B P 2, anterior C P 3, posterior D P 4, anterior, intercoxal sclerite not figured, seta on basis figured separately. Scale bars in μm.

Figure 11. Spinoncaea tenuis Boettger-Schnack, 2003, male (northeastern equatorial Pacific) A habitus, dorsal (outer basal seta on left side of P 5 - bearing somite missing) B urosome, dorsal (P 5 and the outer seta of P 5 - bearing somite on left side missing, caudal seta V on both sides missing) C urosome, ventral, ventrolateral lobes on P 5 - bearing somite arrowed (P 5 and the outer seta of P 5 - bearing somite on left side missing, caudal seta V on both sides missing) D maxilliped, anterior E antennule F P 5 exopod and outer basal seta, lateral G Anal somite and caudal ramus of another specimen, ventral. seta IV on left side and seta V on right side omitted. Scale bars in μm.