Spinoncaea ivlevi (Shmeleva, 1966)

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

publication ID	https://dx.doi.org/10.3897/zookeys.1043.64438
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scientific name	Spinoncaea ivlevi (Shmeleva, 1966)
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Treatment

Spinoncaea ivlevi (Shmeleva, 1966) Figs 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 5 , 6 View Figure 6 , 7 View Figure 7 , 16 View Figure 16

Oncaea ivlevi Shmeleva, 1966: 932-933, figs 1.1-1.11 (Adriatic).

Oncaea ivlevi : Shmeleva 1969: 5-8, 27, figs 3a-i, 4a-h (Adriatic, tropical Atlantic).

Oncaea ivlevi : Malt 1982: 186-187, 193, figs 3a-k, 4a-d (temperate Atlantic).

Spinoncaea ivlevi : Böttger-Schnack 2003: 193-207, figs 2-7 (Red Sea, Mediterranean Sea, Indian and Pacific oceans).

Material examined.

1. Robust form. (1) Northwestern Pacific (a) 13°23'46.44"N, 143°55'0.60"E (WP-1), 27 March 2016: Five females and four males dissected on several slides, respectively. Four dissected females (NIBRIV0000882743-882746) and four dissected males (NIBRIV0000882747-882750) were deposited in the NIBR; (b) GoogleMaps 13°20'3.42"N, 144°20'2.7"E (WP-2), 4 April 2016: Six females dissected on several slides, respectively. Four dissected females (NIBRIV0000882751-882754), one undissected female (NIBRIV0000882755) and one undissected male (NIBRIV0000882756) mounted on H-S slide, respectively, and five undissected females and three undissected males in alcohol vial (NIBRIV0000882757) were deposited in the NIBR. (2) Northeastern Pacific GoogleMaps , 10°30'N, 131°20'W (EP-1), 21 August 2009: Six females (NIBRIV0000882758-882763) and four males (NIBRIV0000882764-882767) dissected on several slides, respectively. All dissected specimens, one undissected female (NIBRIV0000882768) and one undissected male (NIBRIV0000882769) on respective H-S slide, and five undissected females and two undissected males in alcohol vial (NIBRIV0000882770) were deposited in the NIBR. (3) Korea Strait GoogleMaps , 33°44'50.50"N, 128°15'39.02"E (KS), 7 October 2008: Three females (NIBRIV0000882771-882773) and one male dissected (NIBRIV0000882774) on H-S slide, respectively. All dissected specimens and two undissected females and two undissected males in alcohol vial (NIBRIV0000882775) were deposited in the NIBR GoogleMaps .

2. Elongate form. (1) Northwestern Pacific , 13°23'46.44"N, 143°55'0.60"E (WP-1), 27 March 2016: One female (NIBRIV0000882776) dissected on two slides. This specimen was deposited in NIBR. (2) Northeastern Pacific, 10°30'N, 131°20'W (EP-1), 21 August 2009: Three females (NIBRIV0000882777-882779) dissected on one slide or three slides, respectively. Two females (aberrant) (NIBRIV0000882780) dissected on H-S slide. The morphometric data provided in Tables 3 View Table 3 and 4 View Table 4 included only four specimens (three normal females and one aberrant female). All dissected specimens except for one specimen of aberrant female and one undissected aberrant female (in alcohol, NIBRIV0000882781) were deposited in the NIBR GoogleMaps .

Description.

Female (robust form, Figs 2 View Figure 2 - 4 View Figure 4 , 6 View Figure 6 , 7D, E View Figure 7 , 16A-D View Figure 16 , Tables 3 View Table 3 , 4 View Table 4 ). Body length (in lateral view, telescoping of somites not considered) range 318-373 µm in Pacific specimens (Table 3 View Table 3 ), showing a wider size range than in the Red Sea (330-340 µm, Böttger-Schnack 2003: 193).

Prosome 1.9 × length of urosome, excluding caudal rami, 1.6 × urosome length, including caudal rami (Fig. 2B View Figure 2 ), calculated by not correcting for the telescoping of somites. Variation of prosome to urosome length (including CR) ratio 1.5-1.7 in Pacific specimens (Table 3 View Table 3 ). The respective values provided for Red Sea specimens are not comparable as they were based on length data corrected for the telescoping of somites. When calculating the body proportions of the female from Böttger-Schnack’s fig. 2A by not correcting for the telescoping of somites, the respective ratio of prosome to urosome length (incl. CR) would account to 1.5, which is within the range of values for Pacific specimens.

P5-bearing somite with paired row of midventral spinous processes (Fig. 2E View Figure 2 ), variable in number, generally two or three processes, difference per body side may appear as in Fig. 2E View Figure 2 : four (right) and two (left). No such variation was mentioned for the Red Sea specimens.

Posterior margin of genital double-somite and postgenital somites with undulate hyaline frill (Fig. 2C, E View Figure 2 ), as typical for Spinoncaea species, shown in detail in Fig. 16D View Figure 16 .

Genital double-somite (Figs 2C, D, E View Figure 2 , 16D View Figure 16 ) 2.0 × as long as maximum width in specimen figured (measured in dorsal aspect) and ~ 1.5 × as long as postgenital somites combined; variation in length to width ratio 1.6-2.0 in Pacific specimens (Table 3 View Table 3 ), surface ornamentation and pore pattern as figured (Figs 2E View Figure 2 , 16D View Figure 16 ).

Anal somite approximately as wide as long, with insignificant variation in length to width ratio (Table 3 View Table 3 ), ornamentation as figured (Fig. 2C, E View Figure 2 ).

Caudal ramus (Fig. 2A, C, G View Figure 2 ) with length to width ratio 1.9-2.2 measured along inner margin and 2.4-2.9 measured along outer margin (Table 3 View Table 3 ). Caudal seta II with a single long spinule (as in male, e.g., Fig. 16E View Figure 16 ), which is difficult to discern, and which was not reported for Red Sea specimens, and seta IV with ornamentation being unipinnate, while it is bipinnate in Red Sea specimens. Variation in length ratios among setae II, III, and IV as given in Table 3 View Table 3 , denoting a smaller ratio for seta III:II (1.3-1.9) than in the Red Sea (2.2; Böttger-Schnack 2003: fig. 2F).

Antennule 6-segmented (Fig. 3A View Figure 3 ) with armature formula: 1-[3], 2-[8], 3-[5], 4-[2+ae], 5-[2 (ae not discernible)], 6-[5+(1+ae)], typical for Spinoncaea species.

Antenna 3-segmented, armature as for Red Sea specimens, including the absence of seta IV on the lateral armature of the distal endopod segment (Fig. 3B View Figure 3 , setae I-III indicated). Distal endopod segment reflexed (Fig. 3B View Figure 3 ), 3.0-3.9 × longer than wide (Table 3 View Table 3 ), somewhat longer than reported for Red Sea specimens (ca 3:1; discussed under “Remarks”). Ornamentation of elements differing slightly from Red Sea specimens in (1) the coxobasis with a long seta at inner distal corner is ornamented with long spinules unilaterally along entire length, including a single very long spinule at distal part, but only a short row of small spinules at anterior half (Fig. 3B View Figure 3 ), while in specimens from the Red Sea this seta is ornamented with strong spinules bilaterally and lacking a single long spinule ( Böttger-Schnack 2003: fig. 3A), and on (2) the proximal endopod segment is lacking single strong spine on expanded outer margin in specimen figured (Fig. 3B View Figure 3 ), but is present in specimen from Korea Strait (Fig. 6A View Figure 6 ), as specified for Red Sea specimens.

Labrum (Figs 3G, H View Figure 3 , 16A View Figure 16 ) showing variable ornamentation on anterior surface, paired row of long setules in specimen figured (Fig. 3G View Figure 3 , indicated by white arrow in Fig. 16A View Figure 16 ) as specified for Red Sea specimens, additional row of setules indicated in specimen from Korea Strait (Fig. 6C View Figure 6 ).

Mandible (Fig. 3C View Figure 3 ) gnathobase with five elements, with dorsal element D shortest and inserting near base of seta E, as typical for S. ivlevi (cf. Böttger-Schnack 2003: 191) difficult to discern in some specimens from the Pacific.

Maxillule (Figs 3D View Figure 3 , 16B View Figure 16 ) with six elements [innermost element on outer lobe absent, as typical for Spinoncaea species]; ornamentation of middle and innermost element on inner lobe as well as of element next to innermost on outer lobe (Fig. 16B View Figure 16 ) slightly modified as compared to Red Sea specimens.

Maxilla (Fig. 3E View Figure 3 ) with additional ornamentation on syncoxa showing rows of short spinules along outer margin and long spinules along inner margin (arrowed in Fig. 3E View Figure 3 ), not reported earlier for Red Sea specimens.

Maxilliped (Fig. 3F View Figure 3 , syncoxa missing) with basis ornamented with fringe of short spatulated spinules between distal seta and articulation with endopod, as illustrated for Red Sea specimens ( Böttger-Schnack 2003: fig. 3G, but erroneously described as “… between proximal seta and articulation with endopod;.." in text on p 200).

Swimming legs 1-4 (Fig. 4A-D View Figure 4 ) with armature formula shown in Table 2 View Table 2 . Intercoxal sclerite of P1 ornamented with paired long, fine setules (Figs 4a View Figure 4 , 16C View Figure 16 ), which were not discernible in some specimens. Outer seta on basis of P1 slightly shorter than in Red Sea specimens and naked. Anterior surrounding of bases of spines on exopodal and endopodal segments (= small spinules) not discerned in Pacific specimens.

Exopods with general characteristics as for Red Sea specimens, including a reduced length of spine on middle segment (= exp-2) of P2 and P3 (Fig. 4B, C View Figure 4 ) and of proximal spine on distal segment (= exp-3) of P2 (Fig. 4B View Figure 4 ); variability of proportional spine lengths, however, indicates that extent of size reduction of spine on exp-2 differs between legs: most obvious on P2, less obvious on P3 and insignificant on P4 (Table 4 View Table 4 ). Distal spine on P1 slightly longer, on P2-P4 shorter than distal exopod segment, variability of respective length ratios (Table 4 View Table 4 ) indicating that the respective size difference is less obvious in P2 as compared to P3 and P4.

Endopods with length ranges of outer subdistal spine and outer distal spine relative to distal spine given in Table 4 View Table 4 generally similar to Red Sea specimens ( Böttger-Schnack 2003: fig. 4A-D).

P5 (Fig. 2C, D, F View Figure 2 ) with length to width ratio of exopod segment 1.6, as for Red Sea specimens.

P6 (Fig. 2C View Figure 2 ) represented by operculum closing off each genital aperture; possibly armed with a short spinule, which is difficult to discern in Pacific specimens.

Female (elongate form, Fig. 7A View Figure 7 - C, Tables 3 View Table 3 , 4 View Table 4 ). Body length range 305-345 μm, based on five specimens from tropical northeastern and northwestern Pacific, not significantly different from robust form (Table 3 View Table 3 ).

Prosome 1.3-1.4 × length of the urosome (incl. CR), smaller than in the robust form (1.5-1.7, Table 3 View Table 3 ).

Genital double-somite with shape slightly different from robust form, degree of tapering being stronger (Fig. 7A View Figure 7 ) than in robust form (Fig. 2C View Figure 2 ). Length to width ratio of the genital double-somite (1.9-2.2) slightly larger than in robust form (1.6-2.0), but values overlap (Table 3 View Table 3 ).

Anal somite with length to width ratio larger in elongate form (1.2-1.4) than in robust form (1.0-1.1) (Table 3 View Table 3 ); longer than CR (measured along outer margin) while in the robust form the anal somite is shorter than the CR (cf. Fig. 2A, C, E View Figure 2 ).

Caudal ramus with ranges in length to width ratio overlapping between the two female form variants (Table 3 View Table 3 ).

Antennule (not figured) 6-segmented. Armature formula as for S. ivlevi robust form.

Antenna (not figured) 3-segmented, armature as for S. ivlevi robust form. Distal endopod segment with variation of length to width (Table 3 View Table 3 ).

Mandible, maxillule, maxilliped (not figured) similar to those of the robust form.

Swimming legs variable in proportional lengths of endopodal and exopodal spines on P2-P4 as given in Table 4 View Table 4 , showing similar ranges of variation among both forms of the species (cf. Table 4 View Table 4 ).

Male (Figs 5 View Figure 5 , 16E View Figure 16 , Tables 3 View Table 3 , 4 View Table 4 ). Body length range 298-331 µm in Pacific specimens (Table 3 View Table 3 ). Sexual dimorphism in antennule, maxilliped, P6, and in genital segmentation, slight modification in setal length of P5.

P5-bearing somite with paired row of midventral spinous processes (Fig. 5D View Figure 5 ), variable in number, generally two or three processes.

Caudal rami (Fig. 5A, D, F View Figure 5 ) with length to width ratio 1.7-2.0 measured along inner margin and 2.3-2.7 measured along outer margin (Table 3 View Table 3 ). Caudal setae with variations in proportional lengths of caudal setae III:II and setae IV:III as given in Table 3 View Table 3 , similar to female. CR seta II ornamented with single long spinule in some specimens (Fig. 16E View Figure 16 ), not noted for specimens from Red Sea.

Dorsal surface of genital somite ornamented with pattern of minute denticles or spinules (Fig. 5D View Figure 5 ), which are less distinct than in Red Sea specimens ( Böttger-Schnack 2003: fig. 5D), ventral surface with spinule pattern on anterior part (Fig. 5F View Figure 5 ) not observed in Red Sea specimens ( Böttger-Schnack 2003: fig. 5E). Surface of genital flaps covered with several rows of strong denticles or spinules (Fig. 5E, F View Figure 5 ), few denticles also observed on inner distal part (Fig. 5D View Figure 5 ) not observed in Red Sea specimens ( Böttger-Schnack 2003: fig. 5D).

Antennule (Fig. 5G View Figure 5 ) 4-segmented, armature formula: 1-[3], 2-[8], 3-[4], 4-[9+2ae+(1+ae)], aesthetascs very small and slender, segment 4 with small middle aesthetasc close to seta present, which is not discernible in the female. Ornamentation as figured.

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

Maxilliped (Fig. 5B, C View Figure 5 ) 3-segmented, comprising syncoxa, basis and 1-segmented endopod, armature and ornamentation as figured. Basis with only one long seta within longitudinal cleft, corresponding to distal seta in female, proximal seta absent (Fig. 5C View Figure 5 ). Endopod represented by long curved claw, tip of claw without hyaline apex.

Swimming legs 1-4 with armature and ornamentation as in female. Variability in length ratios of outer spine on exp-1 relative to outer spines on exp-2 and exp-3 of P2-P4, and length ratios of outer subdistal spine and outer distal spine relative to distal spine on enp-3 of P2-P4 given in Table 4 View Table 4 , not significantly different from female.

P5 (Fig. 5E, F View Figure 5 ) exopod with general shape and armature as in female; exopodal seta and outer basal seta somewhat shorter than in female.

P6 (Fig. 5F View Figure 5 ) represented by posterolateral flap closing off genital aperture on either side, ornamented as described above, posterolateral corners well discernible in dorsal aspect (Fig. 5A, D View Figure 5 ).

Remarks.

Böttger-Schnack (2003) provided a comprehensive redescription of S. ivlevi from the Red Sea and various other regions and included a detailed discussion of Shmeleva’s descriptions of the species in 1966 (original account) and in 1969 and of that record by Malt (1982). Therefore, these papers are not further discussed in the present paper and the data presented by Böttger-Schnack (2003) were mainly used as a reference for comparison with the Pacific specimens. However, one detail of Shmeleva’s original illustration is noteworthy, as the shape of the distal endopod segment on the antenna is much more slender in both sexes ( Shmeleva 1966: fig. 1.4; 1969: figs 3d, 4c) than figured in Böttger-Schnack’s account for the robust form of the female (2003: fig. 3A). In specimens of both female form variants from the Pacific the distal endopod segment of the antenna appears to be relatively longer and more slender than figured for the Red Sea specimens, showing a range of variation in length to width ratio of 3.0-3.9 in Pacific (cf. Table 3 View Table 3 ), while this ratio is described as "about three times longer than wide" in the Red Sea ( Böttger-Schnack 2003: 198). As the figure of the specimen from the Korea Strait (Fig. 6A View Figure 6 ) also shows a somewhat stronger reflexed orientation of the distal segment compared to the specimen from the equatorial Pacific (Fig. 3B View Figure 3 ), the length to width ratio may be underestimated. But the respective figure (Fig. 6A View Figure 6 ) does not give clear evidence about its actual length to width ratio, because the strongly reflexed orientation of the distal antennary segment makes it difficult to measure it from this figure.

Some other differences between our study and Böttger-Schnack’s redescription were detected in the presence of few long fine setules on the intercoxal sclerite of P1 in both sexes (Figs 4a View Figure 4 , 16C View Figure 16 ), and the distinct ornamentation of the ventral anterior surface of the genital somite in the male (Fig. 5F View Figure 5 ). The first character mentioned has so far been found only in one other Spinoncaea species, S. tenuis (cf. Böttger-Schnack 2003: fig. 14A), and is recorded for S. ivlevi in the present account for the first time, but seemed to be variable, being present in most but not all specimens examined from the three locations in the Pacific (e.g., eight of eleven females and three of four males in the northwestern Pacific).

Additional or different ornamentation details found in the Pacific specimens of S. ivlevi , not mentioned and/or not figured by Böttger-Schnack (2003) included mainly details on the surface of elements such as on the maxilla (syncoxa with additional spinule pattern, Fig. 3E View Figure 3 ), or small details on setae, such as on the inner seta on the coxobasis of the antenna (Figs 3B View Figure 3 , 6A View Figure 6 ) and on the middle element on the outer lobe of the maxillule (Figs 3D View Figure 3 , 16B View Figure 16 ). These delicate ornamentation details can be discerned much better under a scanning electronic microscope as used in the present study than under a light microscope.

Despite the ornamentation differences between the redescription ( Böttger-Schnack 2003) and the present account, specimens from the equatorial and temperate Pacific Ocean were regarded as conspecific with S. ivlevi because our specimens showed basic morphological characters of S. ivlevi , such as:

the mandible showing the full set of 5 elements,

the length to width ratio of the caudal ramus,

the proportional lengths of caudal setae,

the shape of caudal seta IV, which is setiform and not dilated as in S. tenuis,

the shape and ornamentation of the female genital double-somite, and

the paired row of long setules on the anterior surface of the labrum.

In addition, the results of the molecular genetic analysis, which are presented, also supports this opinion, and is briefly discussed below.

Similar to the report from the Red Sea ( Böttger-Schnack 2003), females of S. ivlevi exhibited two form variants in the equatorial northeastern and northwestern Pacific. Taking into consideration the variability of morphological characters of the two variants as examined in the present account (Tables 3 View Table 3 , 4 View Table 4 ), the following differences between the two female forms reported by Böttger-Schnack (2003: 204) could be confirmed for specimens from the Pacific: (1) the length to width ratio of the anal somite, which is larger in the elongate form (1.2-1.4) than in robust form (1.0-1.1), (2) the length ratio of prosome to urosome which is smaller in the elongate form (1.3-1.4) than in the robust form (1.5-1.7), and (3) the shape of the genital double-somite, which shows a stronger degree of tapering in the elongate form (Fig. 7A View Figure 7 ) as compared to the robust form (Fig. 2C View Figure 2 ). On the other hand, the difference between the two forms in the length to width ratio of the genital double-somite indicated in Böttger-Schnack´s study (2003: 204) was not confirmed, because the respective values in the Pacific specimens overlapped. (Table 3 View Table 3 ). Also, the variability of the length to width ratio of the caudal ramus is similar for both variants, and the range of values of proportional spine lengths of endopodal and exopodal spines on P2-P4 overlap between the two forms, including the values of these spines calculated from the robust form in Böttger-Schnack (2003: fig. 4B-D). The P5-bearing somite of the elongate form from the equatorial Pacific exhibits one pair of weakly developed ventrolateral lobes (Fig. 7B, C View Figure 7 ), which is not mentioned in the descriptive text of Böttger-Schnack (2003), but was shown in her fig. 6b. In the robust form, these lobes were not observed in specimens from the two locations in the equatorial Pacific areas (cf. Fig. 2E View Figure 2 ), but were weakly pronounced in specimens from the Korea Strait (Fig. 4D View Figure 4 , arrowed).

In the Pacific, individual variation between specimens was found e.g., in the number of midventral spinous processes on the P5-bearing somite, either two or three in both sexes, and some individuals also had different numbers between left and right side (Fig. 2E View Figure 2 ). It is common that there is no fringe of long setules on outer margin of proximal endopod segment of P4 in S. ivlevi , but in some individuals this fringe was present (not figured). Furthermore, individual variation in ornamentation appeared (1) in the caudal seta II in some individuals, ornamented with a single long spinule in both sexes, (2) in the ornamentation on the dorsal anterior surface of the genital double-somite of females (cf. Fig. 2C View Figure 2 ), not observed in all specimens. One of the robust females from the Korea Strait (Fig. 6A-D View Figure 6 ) showed intraspecific variation in the outer distal part of the first endopod segment of the antenna with broad and more numerous spinules (arrowed in Fig. 6a View Figure 6 ), in additional setules on the anterior surface of the labrum (Fig. 6C View Figure 6 ), in one of the spinules on the inner margin of the syncoxa of the maxilla being relatively long (arrowed in Fig. 6B View Figure 6 ), and in the weak development of the ventrolateral lobes on the P5-bearing somite (Fig. 6D View Figure 6 ).

A number of morphological aberrations found in some specimens of S. ivlevi were summarized in Table 5 View Table 5 . In the northwestern Pacific Ocean, three out of eleven robust female form variants and one out of four males showed abnormalities. Two aberrant specimens were ornamented with a patch of long setules on the anterior part of the cephalosome (Fig. 7D, E View Figure 7 ) and the other one robust female was ornamented with a very long setule on the dorsal anterior surface of the genital double-somite. In the northeastern Pacific Ocean, three out of six elongate females showed a pair of extremely long setules on both sides of the P4-bearing somite in ventral view (e.g., Fig. 7B View Figure 7 ), and one of them had also two extremely long setules on the ventral anterior surface of the genital double-somite (Fig. 7B View Figure 7 ). In the Korea strait, one robust female showed an atypical spine count on the right leg of P2, with only two outer spines on P2exp-3 [typical for the spine count on P2exp-3 of S. humesi ] and with an inner setal count of four setae instead of five setae, while the armature on the right leg was normal. One male from the Korea strait showed imperfect and/or flawed segmentation of endopod segments on the antenna, and the distal part of abnormal distal segment has aberrant four setae.