Gaussia princeps (Scott, 1894)

Suárez-Morales, Eduardo, 2007, The mesopelagic copepod Gaussia princeps (Scott) (Calanoida: Metridinidae) from the Western Caribbean with notes on integumental pore patterns, Zootaxa 1621, pp. 33-44 : 34-42

Gaussia princeps (Scott, 1894)

( Figs 1–4 )

Material examined. One adult male, coll. 31 March, 2006 from sta. 0 54 (18°18'33.84" N; 87°25'06.96"W), oceanic waters, at a depth of 25–50 m, Mexican Caribbean Sea. Specimen partly dissected, ethanol-preserved, vial and slides deposited in the collection of Zooplankton of ECOSUR under catalogue number ECO-CHZ- 0 0 3536.

Description of adult male. Body length 10.1 mm, including caudal rami. Length of prosome: 7.0 mm. Body with typical calanoid shape, relatively slender, prosome slightly narrower anteriorly and widest at first prosome somite (width: 2.8 mm) ( Fig. 1 A–B). Prosome anteriorly an acute process; depression anterior to process visible in lateral view ( Fig. 1 C). Rostrum wide, with 2 long filaments each armed with short setules along both margins ( Fig. 3 G). Prosome apparently 5-segmented; first segment a complex of 5 cephalic somites and thoracic somites 1 and 2; 3–6 articulating. Fifth thoracic somite symmetrical, slightly produced posteriorly to form subtriangular processes ( Fig. 1 D).

Urosome representing 30% of total body length, 5-segmented. Relative lengths of urosomites from anterior to posterior somites: 21.4: 21.5: 17.1: 15.6: 24.4 = 100. Anal somite with wing-like lateral processes reaching proximal 1/3 of caudal rami ( Fig. 1 E); dorsal rounded process visible in lateral view ( Fig. 1 F). Caudal rami relatively short, robust, symmetrical, about 1.3 times longer than wide ( Fig. 1 E); inner and outer margins lightly setose from proximal 1/3. Each ramus with five setae: 1 lateral, 1 subterminal and 3 terminal; all ramal setae biserially plumose.

Left antennule ( Fig. 2 A–C) 23-segmented, reaching beyond caudal rami by 2–3 segments. Armature per segments as follows (numerals = segment, numerals in parentheses = number of setae, ae = aesthetasc): 1(6+3ae), 2(2+ae), 3, 4(2+ae), 5(2+ae+ small spine), 6(2+ae), 7(6+3ae), 8–12(2+ae), 13(1+ae), 14(2+ae),15 (2+ae), 16(1), 17(2+ae), 18(1), 19(1), 20(2), 21(1+ae), 22(1), 23(5+ae).

Right antennule 20-segmented, geniculated between segments 16 and 17 ( Figs. 2 D–E). Armature as follows: 1(7+4ae), 2(2+ae), 3–7(2+ae), 8, 9(1+ae), 10, 11(2+ae), 12(2+ae), 13(3+ae+short spiniform seta), 14(2), 15–17(2+spine), 18(4+ae), 19(1), 20 (5+ae). Segment 11 with additional modified, globose seta ( Fig. 2 F); segment 17 with spiniform, acute distal ventral process reaching beyond distal end of segment ( Fig. 2 G). Antenna ( Fig. 3 A) coxa with1 long seta (not illustrated), basis with 2 subequal setae on outer distal margin. Endopod 2-segmented, proximal segment long, with 1 seta inserted at 2/3 length, second with 9 medial and 7 distal setae and subterminal patch of spinules. Exopod 8-segmented, segments 2–4 partially fused, segments armed with 1 seta on segments 1, 3–6, and 2 on segment 7; 4 setal elements on distal segment.

Mandible ( Fig. 3 B–C): Gnathobase with 8 wide-based bicuspidal or tricuspidal teeth distinctly separated from large distal tooth by a wide distemma. Bases of teeth 3–8 with clusters of spinules. Proximal seta with spinulose inner margin. Basis of mandibular palp with 4 medial subequal setae. Endopod 2-segmented; proximal segment with 4 setae; distal segment with 10 terminal setae. Exopod 5-segmented, with 1, 1, 1, 1, 2 setae.

Maxillule not illustrated, as in Soh et al. (1998): with praecoxal (= protopodal segment proximal to coxa) arthrite bearing 16 spiniform setae [12 ventral, 3 posterior, 1 on anterior]. Coxal epipodite with 9 setae, coxal endite with 5 setae. Basis with proximal and distal endites bearing 4 and 5 setae, respectively, basal exite with single seta. Endopod reduced, 1 articulating segment with 6 medial and 11 distal setal elements. Exopod oblong, with 11 subequal setae.

Maxilla ( Fig. 3 D) precoxa and coxa partially fused, with syncoxa and basis fused; praecoxal lobe with 6 setae, coxal lobe with 3 setae, proximal and distal lobes of basis with 3 setae each. Proximal segment of endopod large, with 4 setae, antepenultimate and penultimate segments small, with 2 setae each, distal segment small, with 2 long, curved setae. All setae biserially setulated.

Maxilliped ( Fig. 3 E) precoxa and coxa incompletely segmented, 4 endites with 1, 2, 4, and 4 setae, respectively; distal and subdistal endites each with patch of short spinules at base and tuft of hair-like setules between them. Basis and endopod partially fused; basis with 3 setae, dense longitudinal row of spinules along medial surface; neither proximal nor distal basal lobe developed. Endopod 6-segmented, distal segment small, partially inserted on penultimate one; endopod armed as follows: 2, 4, 4, 3, 3, 4. Gland openings on all endopodal segments of the maxilliped, except distalmost.

First swimming leg ( Fig. 3 F) with 3-segmented exopod and endopod; coxa with long plumose seta on internal margin reaching beyond distal margin of basipodal segment. Basipod with long, recurved seta on inner margin reaching middle of third endopodal segment. Outer margin of second and third exopodal segments with combined rows of denticles and short setules. Outer spines of same segments with terminal lash. Proximal and second segments of endopod with outer margin covered with minute spinules of different sizes; third segment with small patch of spinules on anterior surface. Second, third and fourth legs with 3-segmented exopods and endopods ( Fig. 4 A, C–D). Second leg with endopod bearing characteristic bifid process on distal inner margin; proximalmost hook shorter than distal one, both strongly chitinized. Same segment with slen- der, acute accessory process on medial-proximal position; process directed posteriorly ( Fig. 4 B). For armature formula of legs 1–4 see table 1.

Fifth legs modified, uniramous ( Fig. 4 E–H). Basis with short, slender seta inserted distally on outer margin. Endopod absent. Left leg exopod 2-segmented, proximal segment cylindrical, naked; second segment with 2 proximal processes, 1 on anterior surface, 1 on posterior surface of segment; former represented by long, triangular protuberance, naked; second process wide-based, distally covered with hair-like setules. Distal segment with 3 distal sensilla. Right leg with intersegmental sutures suggesting 3 segments, but second and third putative segments partially fused. Outer margin of terminal segment with proximal seta and small subdistal seta. Same segment with medial rounded protuberance and 2 distal sensilla.

Remarks. The Caribbean specimen was identified as G. princeps by having a combination of the following characters ( Soh et al. 1998): 1) cephalosome produced to form a stout process; 2) rostral area indented in lateral view; 3) proximally directed medial process of second exopodal segment of male left fifth leg relatively short, not reaching the distal margin of the basipodite; 4) in addition to hook-like process on lateral margin, spiniform process on proximal surface of first exopodal segment of second swimming leg present.

Gaussia princeps was originally described from a single male specimen 12.0 mm long, collected in the Gulf of Guinea, eastern tropical Atlantic (Scott 1894). The known range of the body length of the males from different geographical areas is between 9 and 12 mm (Scott 1894; Vervoort 1965; Saraswathy 1973b; Soh et al. 1998). The size of the Caribbean specimen (10.4 mm) falls within the overall average size range of the species. According to data presented by Vervoort (1965), males from the Indo-Pacific tend to be smaller than those from the eastern Atlantic. However, Saraswathy (1973b) suggested males from the Pacific are the largest (10.1–11.1 mm), and those from the Atlantic are the smallest (9.1–10.4 mm). According to these figures, the Caribbean specimen (10.2 mm) would be at the upper end of the range of the Atlantic males.

There are some morphological differences between the males from the Indian Ocean (and the holotype male), as described by Soh et al. (1998) with respect to the Caribbean male: 1) the shape and size of the accessory spiniform process of the first endopod of the second leg; this structure is relatively short and robust in the former group of specimens and it is slenderer, relatively longer in the Caribbean male; 2) the spiniform process on the right antennule segment 17 is more acute and relatively longer in the Caribbean specimen, it stretches well beyond the end of the distal margin of the segment, whereas it barely reaches this margin in the Indian Ocean specimens. This process is also short in G. s e w e l l i ( Saraswathy & Bradford 1980; fig. 1A), but it is well developed, also stretching beyond the same segment in G. intermedia ( Defaye 1998; fig. 2D); 3) the globose setal element with cone-shaped distal protuberance inserted on segment 11 is slightly wider, shorter in the Indian Ocean specimens and the paired aesthetasc is short, slightly longer than the modified seta; in the Caribbean male, the globose seta is more slender and the accompanying aesthetasc is longer. Soh et al. (1998) also noticed a difference in the shape of this seta between the holotype male from the Eastern Atlantic and specimens from the Indian Ocean. In the Caribbean specimen, this structure is, in fact, more similar to that depicted by Saraswathy and Bradford (1980, fig. 2C) for G. s e w e l l i, with a long accompanying aesthetasc. Soh et al. (1998) stated that the shape of the bulbous seta on segment 11 of the right male antennule has some range of variation within the species; these differences were also reported by Saraswaty (1973a). Apparently, this seta has a considerable range of intra- and interspecific variability and its morphology is not consistent within one species of Gaussia .

Integumental pore patterns. The pore pattern has been used in other marine calanoid copepods such as Eucalanus Dana ( Fleminger 1973) and even in another genus of the family Metridinidae ( Pleuromamma Giesbrecht ) as a consistent character to separate species ( Park & Mauchline 1994; Park 1995). The number and general distribution of the pores have been found to express important differences between congeners in Gaussia . In G. princeps and G. s e w e l l i, these integumental structures are distributed on all the appendages (Barnes & Case 1972; Soh et al. 1998). In the other two known species, G. asymmetrica and in G. intermedia , pores are present on the antennules and the swimming legs only (see Björnberg & Campaner 1990; Defaye 1998). The presence of pores on all the appendages, including the mouthparts, was observed in the Caribbean specimen, thus agreeing with the general pattern of G. princeps .

Soh et al. (1998) described and compared the pore patterns in groups of female specimens of G. princeps from Japanese waters and the Indian Ocean, without finding intraspecific differences. They did not comment on the pore pattern related to male sexual characters such as the right geniculate antennule and the fifth leg.

The Caribbean specimens showed a pore pattern that differs in number with that described from specimens from the Indian Ocean and Japan ( Soh et al. 1998) on different appendages, as follows: (number of pores of Indian and Japanese waters vs number in Caribbean specimens between parentheses; appendages/ segments with identical patterns ommitted:

Left antennule. First segment 9(7); third segment 3(2); fifth and sixth segments 2(1); seventh segment 6(3); eleventh segment 1(0).

Right antenule. First segment 12(9); second and third segments 3(2); fourth segment 2(1); sixth and eighth segments 2(1); tenth segment 1(0).

Mandible. Basis 3(1).

Differences on swimming legs 1–4 in Table 4, on fifth legs, see Table 3 .

All the other integumental pores depicted by Soh et al. (1998) were found in the Caribbean specimens. However, in some cases, the position was not the same, i.e. in the maxilliped, all five endopodal segments have a pore each, but in the Indian specimens two of these are in medial position, whereas all five pores are distal or subdistal in the Caribbean specimens.

As noticed by Soh et al. (1998), the number of gland openings is the largest among the species of Gaussia . This is true when considering the mouthparts, the male and female swimming legs 1–4, and the female antennules; however, this is not as clear in reference to the male fifth leg (see Table 3 ), or the right male antennule (see Table 4). Pores are present on the second endopodal segment of legs 2–4 and on the third endopodal segment of legs 3 and 4 of G. princeps only (see Table 3 ). The Indian Ocean specimens ( Soh et al. 1998) have the highest number of pores on the swimming legs 1–4. Overall, the Caribbean specimen has consistently fewer pores on all these appendages, but in each case, the figure is still higher than the other two species for which pores of swimming legs have been examined ( G. asymmetrica , G. intermedia ) (see Table 4). The Caribbean specimen has a single pore on the first endopodal segment of leg 2, a position not reported in any other specimens of G. princeps ; the only segment in which the Caribbean specimen has more pores than those found by Soh et al. (1998) is the third exopodal segment of the fourth swimming leg (7 vs 6), the remaining pattern agrees with their observations.

According to this comparative analysis, the species with the highest number of pores on the male right antennule is G. sewelli (see Table 2 ; Saraswathy & Bradford 1980); Gaussia princeps has the highest pore number on the fifth leg, followed closely by G. sewelli , but patterns remain undescribed for males of G. intermedia and G. asymmetrica . These patterns should be compared in order to state the overall range of interspecific variation involving secondary sexual characters.

GpC GpI GaSA GiNP

P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4 P1 P2 P3 P4 Cx – 2 1 1 – 1 1 1 – 1 1 1 – 1 – – Bp 1 2 1 – 2 2 1 – – 1 1 1 – 1 – – Enp1 – 1 – – – – – – – – – – – – – – Enp2 – 1 1 – 1 1 1 1 – – – – – – – – Enp3 1 – 1 1 1 1 2 2 1 – – – – 1 – – Exp1 – 2 3 3 – 3 3 3 – 1 2 1 – 1 3 3 Exp2 – 3 3 3 – 4 4 4 – 4 4 4 – 4 3 3 Exp3 1 7 10 8 2 7 9 9 1 7 9 9 2 3 7 9 Total 3 18 14 19 6 19 21 20 2 14 17 16 2 11 13 17

Figures

FIGURE 1. Gaussia princeps (Scott, 1894) from the western Caribbean. Adult male. A. habitus, lateral view; B. habitus, dorsal view; C. cephalic area showing frontal process and depression, lateral view; D. fifth pedigerous and first urosomites, dorsal view; E. anal somite and caudal rami, dorsal view; F. same, lateral. Scale bars: A, B = 3 mm, C – F = 0.7 mm.

FIGURE 2. Gaussia princeps (Scott, 1894) from the western Caribbean. Adult male. A. left antennule segments 1 – 10; B. same, segments 11 – 16; C. same, segments 17 – 23. D. right antennule, segments 1 – 10; E. same, segments 11 – 20; F. detail of modified seta and aesthetasc; G. detail of segment 17 and distal acute process. Position of integumental pores indicated by solid circles. Scale bars: A – E = 0.7 mm, F = 0.05 mm, G = 0.35 mm.

FIGURE 3. Gaussia princeps (Scott, 1894) from the western Caribbean. Adult male. A. antenna; B. mandibular palp; C. mandibular blade showing detail of teeth and blade ornamentation; D. maxilla; E. maxilliped; F. first swimming leg. G. rostral area, ventral view. Some setae cut short. Position of integumental pores indicated by solid circles. A, B – G = 0.7 mm, C = 0.2 mm.

FIGURE 4. Gaussia princeps (Scott, 1894) from the western Caribbean. Adult male. A. second swimming leg; B. detail of hook-like and spiniform processes on first endopodal segment of second swimming leg; C. third swimming leg; D. fourth swimming leg; E. fifth swimming leg, anterior view; F. basipod and exopod of left fifth leg, lateral view; G. detail of processes of second exopodal segment of left fifth leg; H. detail of distal exopodal segment of right fifth leg, showing ornamentation. Position of integumental pores indicated by solid circles. Scale bars A, C – E, H = 0.7 mm, B, F, G = 0.3 mm.

Tables

TABLE 3. Comparison of pore number of male fifth leg of Gaussia princeps from the Caribbean (this survey) (GpC), the Indian Ocean (Soh et al. 1998) (GpI), and G. sewelli (Saraswathy & Bradford 1980) (GsI) from the Indian Ocean. Enp = endopod; Exp = exopod; Cx = coxa; Bp = basipodite; R = right; L = left.

	GpC	GpI	GsI
Cx	3	4	2
RBp	3	3	3
LBp	3	4	4
RExp1	1	3	4
LExp1	2	3	3
RExp2	4	4	5
LExp2	5	6	5
Total pores	21	27	26

TABLE 2. Comparison of pore number of male right antennule of Gaussia princeps from the Caribbean (this survey) (GpC), the Indian Ocean (Soh et al. 1998) (GpI), and G. sewelli (Saraswathy & Bradford 1980) (GsI) from the Indian Ocean. Antennule segments grouped in three sectors: first segment, segments 2 – 5, and 6 – 12.

GpC	GpI	GsI
First segment 9	11	11
Segments 2–5 7	10	13
Segments 6–12 6	9	10
Total pores 22	29	34