Journal of Marine BiologyVolume 2011 (2011), Article ID 842821, 18 pagesdoi:10.1155/2011/842821

Research Article

Sexually Dimorphic Morphological Characters in Five North Atlantic Deepwater Skates (Chondrichthyes: Rajiformes)

Alexei M. Orlov¹ and Charles F. Cotton²

¹Russian Federal Research Institute of Fisheries and Oceanography (VNIRO), 17 V. Krasnoselskaya, Moscow 107140, Russia²Department of Fisheries Science, Virginia Institute of Marine Science, P.O. Box 1346, Gloucester Point, VA 23062, USA

Received 16 March 2011; Accepted 9 May 2011

Academic Editor: Jakov Dulčić

Copyright © 2011 Alexei M. Orlov and Charles F. Cotton. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Skates exhibit a variety of manifestations of sexual dimorphism. However, this phenomenon has been poorly documented in deepwater species. New data on the sexual dimorphism of five species of deepwater skates from the North Atlantic Ocean are presented: Amblyraja jenseni, Bathyraja pallida, Bathyraja richardsoni, Rajella bigelowi, and R. kukujevi. These skates exhibit sexual dimorphism most frequently in interorbital width, disc length and width, length of the third gill slit, horizontal diameter of the orbit, length of the fifth gill slit, space between the first and fifth gill slits, length of the second dorsal fin base, mouth width, length and width of each lobe of the nasal curtain, distance from the snout to the center of anus, distance from the snout tip to the point of maximum disc width, length of the lateral fold, length of the orbit + spiracle (measured together), tail height at the pelvic fin tips, and the number of median thorns.

1. Introduction

Skates (Rajoidei) comprise an important component of shelf and continental slope benthic communities of the world’s oceans. They are major consumers of cephalopods, decapods, and fishes, often utilizing the same food resources as the apex predators, such as sea birds, marine mammals, and sharks [1]. Skates, like all chondrichthyan fishes, exhibit sexual dimorphism in the structure of the pelvic fins, which in males are modified to act as copulatory organs, or claspers [2]. Also, mature male skates have alar thorns on the edges of the disc, which help to grasp the female during copulation [3–5]. In many species sexual differences have been reported in length, body mass, size and age of sexual maturity, other external morphological features, and diet [3, 5–16]. Mature females and males of several skate species exhibit differences in the form and length of the teeth [3, 15, 17–23]. In one species, differences were observed in the shape and size of the shoulder and pelvic girdle [24]. Sexual differences in size and shape of the olfactory and electrical organs have also been found in some skates [3, 25–27]. Even with this variety of manifestations of sexual dimorphism in skates, the phenomenon has been poorly documented in deepwater species. This study presents new data on sexually dimorphic morphological characters of five deepwater skate species from the North Atlantic Ocean: Jensen's skate, Amblyraja jenseni [28]; pale ray, Bathyraja pallida [29]; Richardson's ray, Bathyraja richardsoni [30]; Bigelow's ray, Rajella bigelowi [31]; Mid-Atlantic skate, R. kukujevi [32].

2. Materials and Methods

Our comparative analysis of skate morphology was based on the measurement of 45–47 morphometric and 2–15 meristic characters, according to protocols used in recent years [33–35]. For some meristic characters (preorbital thorns, postorbital thorns, orbital thorns, interspiracular thorns, and scapular thorns) we observed differences in thorn counts on the left and right side of each specimen. Therefore, we present maximum, minimum, and mean values for one side for each character. The material examined by the authors during 2005-2006 consisted of the majority of specimens of five deepwater skate species available from ichthyological collections worldwide (MCZ-Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, USA; ZMUB-Zoological Museum, University of Bergen, Bergen, Norway; USNM-National Museum of Natural History, Smithsonian Institute, Washington DC, USA; VIMS-Virginia Institute of Marine Science, Gloucester Point, Virginia, USA; NMSZ-National Museum of Scotland, Edinburgh, Scotland; BMNH-British Museum of Natural History, London, Great Britain; NMHN-Museum National d’Histoire Naturelle, Paris, France; ZISP-Zoological Institute, Sankt-Peterburg, Russia). All measurements were conducted in accordance with conventional procedures [3, 36–38]. The following juvenile, adolescent, and mature specimens were examined (the numbers of females and males, resp., of each species are indicated in parenthesis).

Amblyraja jenseni   (7/6)—MCZ 37899, 38354, 55011, 138020-1, 138020-2, 155628; ZMUB 19529, 19462, 19463; VIMS 11757; USNM 35592, 23483, 33457. The USNM specimens were not examined in this study but the morphological data were taken from Bigelow and Schroeder [28, 36, 39].

Bathyraja pallida    (3/7)—ZMUB 19465, VIMS 11577, VIMS 11758, NMSZ 2000.130.462, NMSZ 2000.130.80, BMNH 1967.2.13.2, BMNH 1985.11.14.1, BMNH 1985.11.14.2, BMNH 1985.11.14.3, BMNH 1985.11.14.4.

Bathyraja richardsoni  (14/13)—BMNH 1999.2.2.1, BMNH 1999.2.2.2, BMNH 1999.2.2.3, BMNH 1999.2.2.4, BMNH 1999.10.1.1, BMNH 1999.10.1.2, NMHN 1999-1156, NMSZ 2000.130.260, ZMUB 19522-1, ZMUB 19522-2, ZMUB 19522-3, ZMUB 19514-1, ZMUB 19514-2, ZMUB 19514-3, ZMUB 19514-4, ZMUB 19476, ZMUB 17600, ZMUB 19448, ZMUB 19464, ZMUB 19364, ZMUB 19446, ZMUB 19528, ZMUB 19535, VIMS 11756.

Rajella bigelowi  (8/10)—NMHN 1988-361, NMHN 1987-482, NMHN 1999-1162; ZMUB 18406, ZMUB 18405; MCZ 55316(A), MCZ 55316(B), MCZ 55316(C), MCZ 58444, MCZ 55314, MCZ 158964, MCZ 57327, VIMS 5562 (), VIMS 8444, VIMS 3347, VIMS 5424.

Rajella kukujevi  (2/6)—ZISP 46195, ZMUB 15709, NMHN 1998-0727, NMHN 1999-1165, NMHN 1996-1157, NMHN 1996-1153, NMHN 1996-235, NMHN 1996-1154.

Furthermore, for our comparative analyses of B. richardsoni and A. jenseni, we also included published data [28, 30, 36, 39, 40]. We used conventional abbreviations to denote museum collections [41, 42].

Because of the small number of deepwater skates available in ichthyological collections we had an opportunity to examine only 13 specimens of A. jenseni, 10 specimens of B. pallida, 27 specimens of B. richardsoni, 12 specimens of R. bigelowi, and eight specimens of R. kukujevi. Since sample size was too small, the statistical significance of the compared material was not evaluated using standard statistical tests. Instead, we evaluated the significance of variation in morphological characters between males and females by comparing the mean () and standard error () for each set of measurements. If the standard errors of the means did not overlap () between males and females, such differences were considered significant.

3. Results and Discussion

Amblyraja jenseni was described in the middle of the past century [28] and since then, several records of this skate have been reported from the North Atlantic [43–47]. Until recently, morphometric and meristic data were limited to three specimens [28, 36, 39]. Although a recent study [35] considerably expanded the knowledge of morphological features of A. jenseni, information about sexual dimorphism of this species, until now, was absent.

Our results show that the greatest differences in male and female A. jenseni are found in the disc length, horizontal diameter of the orbit, height of the tail at the pelvic fin tips, length of the third gill slit, space between the fifth gill slits, distances from the center of the anus to the first and second dorsal fins (Table 1).

Table 1: Morphometrics and meristics of male and female Jensen’s ray Amblyraja jenseni from different areas of the North Atlantic (significant differences are given in bold, number of specimens examined are shown in parentheses, m = standard error).

Females (7)
Males (6)

Character
Absolute
% TL
Absolute
% TL

Min
Max
Mean
Min
Max
Mean ±m
Min
Max
Mean
Min
Max
Mean ±m

TL, mm
369.0
1065.0
686.6

223.0
1113.0
635.0

Disc width
264.0
795.0
510.4
69.5
81.3
74.2 ± 1.51
156.5
837.0
466.4
70.2
75.5
73.3 ± 0.91

    length
207.0
593.0
390.5
55.7
60.0
57.1 ± 0.60
119.8
632.0
351.5
51.9
56.8
55.1 ± 0.79

Snout length, preorbital
61.2
157.0
103.3
14.0
16.6
15.2 ± 0.31
32.1
174.0
89.6
10.3
15.8
14.2 0.84

    preoral
54.9
135.0
92.9
11.6
16.3
13.6 ± 0.71
37.1
149.0
88.7
12.6
16.9
14.5 ± 0.65

    prenasal
38.7
80.3
65.0
7.5
10.6
9.7 ± 0.72
29.9
115.0
66.8
7.4
11.1
9.7 ± 0.81

Orbit, horizontal diameter
10.6
40.0
27.0
2.5
5.8
4.2 ± 0.52
9.2
22.0
16.1
1.8
4.7
3.1 ± 0.52

Interorbital width
27.8
68.1
46.1
6.0
7.8
6.8 ± 0.26
13.8
77.2
47.4
6.2
11.1
7.5 ± 0.75

Spiracle length
11.2
34.4
19.6
2.3
3.4
3.0 ± 0.16
5.6
37.3
20.4
2.5
3.8
3.1 ± 0.18

Interspiracular width
40.8
110.0
75.8
10.3
12.0
11.1 ± 0.19
23.6
115.0
74.4
10.2
15.8
11.7 ± 0.85

Orbit + spiracle length
20.9
56.3
42.2
5.3
6.9
6.2 ± 0.33
16.2
60.0
39.9
4.8
7.2
5.8 ± 0.41

D1, height
6.0
42.0
17.3
1.5
4.9
2.3 ± 0.46
3.8
25.6
13.2
1.7
2.3
2.0 ± 0.10

    base length
17.8
47.0
30.7
4.3
5.4
4.7 ± 0.16
11.2
48.9
28.4
3.9
5.0
4.6 ± 0.16

D2, height
5.6
28.0
14.4
1.3
3.3
2.0 ± 0.25
3.3
30.4
13.8
1.5
2.7
2.0 ± 0.18

    base length
19.7
44.6
32.3
4.2
5.8
5.0 ± 0.27
8.8
53.0
29.8
3.3
5.2
4.5 ± 0.29

Interdorsal space
0.0
29.0
6.3
0.0
3.4
0.9 ± 0.44
0.0
3.1
1.3
0.0
1.1
0.4 ± 0.20

C, base length
13.1
28.3
20.1
2.4
3.6
3.0 ± 0.28
10.9
25.0
18.0
2.2
4.0
2.7 ± 0.34

Tail, postdorsal length
14.1
45.0
23.9
2.4
5.3
3.5 ± 0.49
8.7
30.0
18.1
2.2
5.5
3.3 ± 0.51

    height at V-tips
8.9
25.3
17.0
2.4
2.7
2.5 ± 0.06
6.8
23.0
15.9
2.0
2.5
2.3 ± 0.10

    width at V-tips
11.4
30.3
21.1
2.7
3.6
3.1 ± 0.18
8.1
31.7
21.5
2.8
3.6
3.1 ± 0.14

    height at D1 origin
4.2
9.8
7.0
0.9
1.2
1.0 ± 0.06
3.0
11.0
7.2
0.8
1.3
1.0 ± 0.07

    width at D1 origin
4.9
13.5
8.5
0.9
1.6
1.3 ± 0.12
4.2
14.3
9.1
1.1
1.6
1.3 ± 0.08

    lateral fold length
143.0
337.0
243.0
31.6
38.8
36.4 ± 1.62
107.0
345.0
235.6
30.5
39.6
34.2 ± 1.81

Head length, ventrally
105.3
281.0
205.3
26.4
33.3
30.4 ± 1.32
77.9
309.0
212.4
27.8
34.5
30.2 ± 1.22

Mouth width
45.5
157.0
93.3
11.9
15.9
13.3 ± 0.58
27.9
150.0
84.7
12.3
14.5
13.0 ± 0.35

Internasal width
46.6
126.0
84.1
11.8
13.0
12.4 ± 0.22
28.5
133.0
76.5
11.1
13.7
12.3 ± 0.36

Nasal curtain, length
20.5
56.4
34.6
3.7
5.6
5.1 ± 0.35
18.0
65.3
36.3
3.4
6.7
5.4 ± 0.56

    width each lobe
10.0
23.6
13.8
1.5
2.8
2.1 ± 0.24
6.8
29.0
18.5
1.9
4.2
2.6 ± 0.41

    space between lobes
30.4
107.0
65.7
8.2
10.1
9.4 ± 0.34
21.8
115.0
70.5
7.8
12.1
9.7 ± 0.85

Gill slit length, first
5.2
14.2
8.7
1.3
1.4
1.3 ± 0.02
2.9
18.5
9.6
1.3
1.7
1.4 ± 0.09

    third
5.4
16.8
9.9
1.3
1.6
1.5 ± 0.05
3.6
22.5
12.1
1.6
2.0
1.7 ± 0.07

    fifth
4.4
14.9
7.3
0.9
1.4
1.1 ± 0.08
2.2
16.9
8.2
0.9
1.5
1.2 ± 0.12

Space btwn first gill slits
68.7
186.0
137.5
17.5
22.1
20.2 ± 0.62
42.1
225.0
135.1
16.3
24.8
20.4 ± 1.43

    fifth gill slits
52.3
139.0
104.9
13.1
18.1
15.6 ± 0.74
31.2
170.0
87.2
10.5
16.4
13.4 ± 1.10

V-length, anterior lobe
55.7
123.0
81.0
11.5
15.1
12.5 ± 0.54
23.9
143.0
78.2
10.7
15.2
12.4 ± 0.63

    posterior lobe
74.6
206.0
131.5
18.1
20.2
19.3 ± 0.36
54.0
229.0
138.3
16.7
20.6
19.0 ± 0.81

Clasper, postanus length
na
na
na
na
na
na
20.5
240.0
116.9
7.6
23.2
13.5 ± 3.51

Snout tip to center of anus
196.0
625.0
367.8
52.8
58.7
55.2 ± 1.02
114.8
678.0
359.1
51.1
60.9
54.8 ± 1.65

    to max. disc width
142.0
358.0
259.2
33.6
42.6
38.6 ± 1.44
105.0
400.0
255.8
31.4
39.7
36.5 ± 1.63

Center of anus to D1
121.0
356.0
223.8
30.4
34.8
32.5 ± 0.52
74.9
349.0
180.8
17.1
33.6
29.3 ± 2.52

    to D2
139.0
398.0
264.3
35.4
39.7
37.2 ± 0.59
104.0
390.0
246.4
30.2
38.5
34.9 ± 1.39

    to tail tip
175.0
470.0
306.7
42.7
47.4
45.0 ± 0.81
108.2
475.0
270.5
38.1
48.5
43.7 ± 1.47

Snout angle
85
91
88.0 ± 3.00

89
103
94.0 ± 4.51

No. preorbital thorns
1
2
1.6 ± 0.24

1
3
2.0 ± 0.32

No. postorbital thorns
1
2
1.8 ± 0.20

1
2
1.8 ± 0.20

No. interspiracular thorns
1
2
1.6 ± 0.24

1
2
1.4 ± 0.24

No. scapular thorns
3
5
4.0 ± 0.45

3
6
4.0 ± 0.63

No. nuchal thorns
1
2
1.6 ± 0.24

0
2
1.6 ± 0.40

No. interscapular thorns
1
1
1.0 ± 0.00

1
3
1.4 ± 0.40

No. median thorns
23
28
25.0 ± 0.95

20
31
24.4 ± 1.91

No. middorsal thorns
0
1
0.2 ± 0.20

0
1
0.2 ± 0.20

Tooth rows upper jaw
52
63
58.5 ± 2.53

59
64
61.3 ± 1.11

    lower jaw
54
61
57.3 ± 1.89

52
64
57.8 ± 2.53

Bathyraja pallida was described in the second half of the past century [29]. Although there have been eight subsequent specimens collected [34, 45, 48–50], morphometric and meristic data are limited to two specimens, and these accompanied the original description of the species [29]. A recent publication [35] contributed to the existing morphological data available for this species; however, that study did not include information about sexual dimorphism. Furthermore, these data were based on the examination of only three specimens.

Maximum differences between male and female B. pallida were found in the following characters (Table 2): interorbital width, interspiracular width, height of the first dorsal fin, lengths of the first and second dorsal fin bases, tail height at the pelvic fin tips, mouth width, nasal curtain length and width, width of each lobe of the nasal curtain, length of first gill slit, space between the first gill slits and the fifth gill slits, distance from the snout tip to the center of the anus, distance from the center of the anus to the tail tip, and the number of teeth in the lower jaw.

Table 2: Morphometrics and meristics of male and female pale ray Bathyraja pallida from different areas of the North Atlantic (significant differences are given in bold, number of specimens examined are shown in parentheses, m = standard error, na = not available).

Females (3)
Males (7)

Character
Absolute
% TL
Absolute
% TL

Min
Max
Mean
Min
Max
Mean m
Min
Max
Mean
Min
Max
Mean ±m

TL, mm
1285.0
1565.0
1437.0

311.0
1490.0
948.3

Disc, width
845.0
1015.7
932.2
64.0
65.8
64.9 ± 0.50
197.0
1050.0
623.0
60.5
70.5
65.5 ± 1.19

    length
672.0
859.0
791.0
52.3
58.8
55.0 ± 1.95
174.0
851.0
531.7
54.2
57.1
56.1 ± 0.48

Snout length, preorbital
246.0
261.4
253.7
16.7
16.8
16.8 ± 0.06
55.4
215.0
132.8
14.0
19.2
16.1 ± 0.89

    preoral
228.5
228.5
228.5
14.6
14.6
14.6 ± na
54.3
285.0
143.1
13.2
19.2
16.8 ± 1.13

    prenasal
na
na
na
na
na
na
45.8
184.0
111.8
12.1
15.7
13.50.66

Orbit, horizontal diameter
36.0
68.4
56.4
2.3
5.1
4.0 ± 0.86
13.5
63.9
34.3
1.8
4.7
3.9 ± 0.49

Interorbital width
81.4
94.0
87.1
5.2
7.3
6.1 ± 0.62
18.6
80.4
47.7
4.2
6.0
5.3 ± 0.21

Spiracle length
38.5
39.9
39.2
2.7
3.0
2.9 ± 0.13
8.3
50.5
26.6
2.4
3.4
2.8 ± 0.13

Interspiracular width
125.2
161.0
138.7
8.0
12.5
9.8 ± 1.39
25.3
127.0
73.0
6.6
8.8
7.9 ± 0.27

Orbit + spiracle length
83.6
91.5
87.6
5.7
7.1
6.4 ± 0.70
17.1
85.7
49.9
4.3
6.1
5.5 ± 0.29

D1, height
23.9
26.6
25.3
1.6
1.7
1.7 ± 0.03
6.4
33.5
17.7
1.5
2.2
1.9 0.12

    base length
45.9
51.6
48.8
3.1
3.3
3.2 ± 0.08
12.2
56.5
34.2
3.4
4.3
3.8 ± 0.15

D2, height
25.7
32.9
29.3
1.8
2.1
1.9 ± 0.17
6.7
32.4
19.3
1.8
2.4
2.0 ± 0.07

    base length
42.6
53.2
47.9
2.9
3.4
3.2 ± 0.24
12.1
53.9
33.1
3.2
4.6
3.6 ± 0.24

Interdorsal space
9.9
23.5
16.7
0.7
1.5
1.1 ± 0.41
6.4
27.8
14.2
0.9
2.4
1.7 ± 0.27

C, base length
21.9
na
na
1.5
na
1.5 ± na
12.2
34.5
18.8
1.0
3.9
2.20.59

Tail, postdorsal length
34.4
na
na
2.2
na
2.2 ± na
17.0
32.1
24.6
2.2
2.7
2.4 ± 0.26

    height at V-tips
32.9
36.8
34.9
2.5
2.6
2.5 ± 0.02
6.8
36.3
22.2
1.9
2.5
2.3 ± 0.05

    width at V-tips
38.2
43.9
41.1
2.6
3.4
3.0 ± 0.40
8.5
52.0
28.7
1.6
3.6
2.9 ± 0.15

    height at D1 origin
16.1
na
na
1.1
na
1.1 ± na
3.4
17.1
10.3
0.9
1.2
1.1 ± 0.04

    width at D1 origin
22.4
na
na
1.5
na
1.5 ± na
3.2
25.3
13.0
0.9
1.7
1.2 ± 0.13

    lateral fold length
455.0
na
na
31.1
na
31.1 ± na
129.0
544.0
360.0
25.2
41.5
35.1 ± 3.15

Head length, ventrally
351.0
455.0
403.0
27.3
31.1
29.2 ± 1.90
92.5
429.0
276.7
28.2
30.7
29.5 ± 0.34

Mouth width
140.0
170.6
150.9
9.7
10.9
10.5 ± 0.39
23.1
142.0
87.4
7.4
9.5
8.8 ± 0.44

Internasal width
134.6
147.0
140.0
8.6
10.8
9.8 ± 0.64
26.6
157.0
86.6
8.2
10.5
9.0 ± 0.37

Nasal curtain, length
44.2
45.6
44.9
3.0
3.5
3.3 ± 0.26
14.1
70.6
37.4
3.5
4.7
4.0 ± 0.24

    width
145.0
149.0
147.0
10.2
11.3
10.7 ± 0.55
28.9
155.0
87.9
9.2
10.8
9.7 ± 0.37

    width each lobe
24.4
33.7
29.1
1.9
2.3
2.1 ± 0.20
5.0
28.2
17.0
0.8
2.5
1.8 ± 0.12

    space between lobes
na
na
na
na
na
na
10.0
86.3
48.2
1.6
5.8
3.7 ± 0.26

Gill slit length, first
21.8
25.1
24.0
1.6
1.7
1.7 ± 0.04
4.2
29.0
14.8
1.3
1.9
1.5 ± 0.10

    third
23.7
30.4
27.4
1.8
2.1
1.9 ± 0.09
5.0
37.2
18.1
1.4
2.5
1.8 ± 0.16

    fifth
17.1
23.5
20.3
1.3
1.6
1.4 ± 0.10
3.6
23.9
12.8
0.9
1.6
1.3 ± 0.07

Space between first gill slits
246.0
270.7
256.9
17.3
19.1
17.9 ± 0.60
48.6
241.0
141.8
12.6
16.7
15.0 ± 0.63

    fifth gill slits
186.0
190.9
188.0
12.2
14.6
13.2 ± 0.71
34.2
166.0
99.3
8.7
11.5
10.7 ± 0.45

V-length, anterior lobe
87.0
167.0
127.0
6.8
11.4
9.1 ± 2.33
43.9
177.0
106.8
9.9
15.4
12.2 ± 0.98

    posterior lobe
174.0
255.0
214.5
13.5
17.4
15.5 ± 1.95
46.3
269.0
156.6
14.5
18.1
16.1 ± 0.51

Clasper, postanus length
na
na
na
na
na
na
24.5
402.0
212.3
7.9
27.6
17.3 ± 4.20

Snout tip to center of anus
702.0
890.5
812.5
54.6
57.8
56.4 ± 0.94
151.1
855.0
504.6
48.6
57.4
52.2 1.51

    to max. disc width
435.0
539.0
487.0
33.9
36.9
35.4 ± 1.51
110.3
545.0
331.8
31.8
37.5
35.4 ± 0.88

Center of anus to D1
505.0
519.6
512.3
33.2
34.5
33.9 ± 0.67
109.0
529.0
332.5
32.6
36.9
35.2 ± 0.69

    to D2
599.0
na
na
41.0
na
41.0 ± na
129.0
602.0
378.0
36.9
42.7
40.4 ± 0.90

    to tail tip
621.0
694.9
657.9
42.5
44.4
43.4 ± 0.96
156.0
674.0
490.2
44.3
50.2
46.7 ± 1.09

No median thorns
16
na
16.0 ± na

16
19
17.6 ± 0.60

No middorsal thorns
1
na
1.0 ± na

0
2
0.8 ± 0.31

Tooth rows upper jaw
25
31
28.0 ± 3.00

24
30
26.2 ± 0.95

    lower jaw
25
35
30.0 ± 5.00

22
26
24.0 ± 0.73

The morphological and meristic characters of Bathyraja richardsoni have been examined more thoroughly than most other species in our study. Information on the morphology of 16 mature individuals of this species has been published in several early papers [30, 40, 51, 52]. However, these studies omitted many of the morphometric and meristic characters used in our study. More recently [34], a similar measurement scheme on four advanced embryos of this species was used, and the same measurement scheme was used on several specimens caught on the Mid-Atlantic ridge during the MAR-ECO expedition (http://www.mar-eco.no/), as well as material from the BMNH and MNHN collections [35]. Nevertheless, none of these prior studies offer information about sexual dimorphism of B. richardsoni.

Our results show B. richardsoni to be sexually dimorphic in the following characters (Table 3): disc width, disc length, horizontal diameter of the orbit, length of the orbit + spiracle (measured together), tail width at the pelvic fin tips, length of the lateral fold, mouth width, lengths of the third and fifth gill slits, distance from the snout tip to the center of the anus, distance from the snout tip to the line of maximum disk width, and the number of median thorns.

Table 3: Morphometrics and meristics of male and female Richardson’s ray Bathyraja richardsoni from different areas of the North Atlantic (significant differences are given in bold, number of specimens examined are shown in parentheses, m = standard error).

Females (14)
Males (13)

Character
Absolute
% TL
Absolute
% TL

Min
Max
Mean
Min
Max
Mean ±m
Min
Max
Mean
Min
Max
Mean ± m

TL, mm
240.0
1530.0
924.0

182.0
1133.0
570.2

Disc, width
159.1
1042.0
626.6
64.4
72.3
68.1 ± 0.56
93.9
784.0
382.5
51.6
70.3
65.7 ± 1.45

    length
121.0
922.0
525.6
50.4
60.3
56.2 ± 0.65
76.1
637.9
314.5
41.8
59.9
53.1 ± 1.59

Snout length, preorbital
21.6
211.0
118.3
9.0
15.4
12.9 ± 0.53
14.6
145.0
74.5
7.5
15.3
12.2 ± 0.78

    preoral
24.5
206.6
120.5
10.2
14.5
13.2 ± 0.38
14.9
147.3
76.1
8.2
16.3
12.7 ± 0.72

    prenasal
15.4
139.0
76.3
6.4
11.5
9.4 ± 0.56
12.0
102.0
52.0
5.9
11.9
9.2 ± 0.62

Orbit, horizontal diameter
11.3
31.1
20.4
1.7
5.0
2.6 ± 0.30
8.7
28.7
17.4
1.9
5.6
3.7 ± 0.40

Interorbital width
16.8
161.5
70.5
5.2
11.1
7.5 ± 0.45
9.6
117.8
41.7
5.3
10.4
6.9 ± 0.34

Spiracle length
5.3
53.3
23.2
2.0
3.5
2.6 ± 0.15
4.0
20.0
11.9
1.8
3.4
2.4 ± 0.15

Interspiracular width
28.8
180.4
104.7
9.0
12.8
11.3 ± 0.25
23.5
122.4
63.9
10.2
14.1
11.3 ± 0.27

Orbit + spiracle length
15.4
70.0
42.1
4.4
6.8
5.2 ± 0.21
11.5
52.4
28.5
4.8
7.5
5.7 ± 0.27

D1, height
5.8
20.5
13.8
1.1
2.4
1.6 ± 0.10
4.2
16.6
9.1
1.3
2.8
1.8 ± 0.13

    base length
10.1
50.9
30.8
2.8
4.6
3.4 ± 0.12
9.5
38.3
19.8
2.8
5.4
3.7 ± 0.21

D2, height
5.0
20.4
12.7
1.0
2.1
1.6 ± 0.10
4.4
20.4
9.8
1.4
2.5
1.8 ± 0.11

    base length
10.3
45.0
28.4
2.9
4.3
3.5 ± 0.12
9.1
36.0
19.6
2.8
5.0
3.8 ± 0.23

Interdorsal space
0.0
14.5
5.2
0.0
1.0
0.6 ± 0.08
0.0
7.9
3.6
0.0
1.0
0.6 ± 0.08

C, base length
11.4
44.2
27.1
2.8
5.5
3.8 ± 0.27
11.8
32.8
19.7
2.9
7.0
4.4 ± 0.43

Tail, postdorsal length
0.0
52.4
29.1
0.0
7.9
4.0 ± 0.65
0.0
44.2
21.9
0.0
8.1
4.4 ± 0.68

    height at V-tips
8.6
47.7
25.1
2.5
3.6
2.9 ± 0.10
7.8
38.8
15.7
2.4
4.3
3.1 ± 0.16

    width at V-tips
7.6
64.0
32.3
2.6
4.8
3.7 ± 0.18
10.2
45.1
21.6
3.5
5.6
4.3 ± 0.17

    height at D1 origin
3.8
18.8
10.0
1.0
1.7
1.2 ± 0.06
3.3
13.4
6.3
0.9
2.0
1.3 ± 0.08

    width at D1 origin
3.3
20.3
11.1
1.1
2.3
1.3 ± 0.10
4.0
15.1
7.1
1.0
2.7
1.5 ± 0.16

    lateral fold length
111.1
481.0
274.0
30.9
46.3
36.5 ± 1.58
98.8
384.0
196.8
33.9
54.3
40.1 ± 1.97

Head length, ventrally
53.3
413.0
221.3
22.2
28.3
26.0 ± 0.53
44.4
264.0
133.6
20.0
28.7
25.1 ± 0.78

Mouth width
22.1
197.0
103.0
9.2
12.9
10.8 ± 0.29
14.4
130.3
58.8
7.9
11.5
9.9 ± 0.32

Internasal width
26.4
174.6
99.6
8.0
12.8
11.0 ± 0.31
16.0
124.6
62.6
8.8
14.2
10.9 ± 0.37

Nasal curtain, length
11.8
70.6
37.3
3.0
5.5
4.6 ± 0.17
6.6
37.6
24.6
3.4
12.0
5.0 ± 0.65

    width each lobe
5.0
77.7
22.0
1.6
12.4
2.9 ± 0.87
2.2
20.7
10.3
1.2
2.4
2.0 ± 0.1

    space between lobes
13.8
117.7
61.0
2.2
8.5
7.0 ± 0.48
12.7
84.4
42.2
5.8
8.3
7.2 ± 0.28

Gill slit length, first
2.6
26.2
14.9
1.1
1.9
1.5 ± 0.06
2.0
20.4
8.7
0.9
1.8
1.4 ± 0.08

    third
3.1
30.2
17.3
1.3
2.2
1.8 ± 0.08
2.2
23.8
9.5
1.0
2.1
1.6 ± 0.08

    fifth
2.2
25.5
14.0
0.8
1.9
1.5 ± 0.09
1.3
19.3
8.1
0.7
1.7
1.3 ± 0.10

Space between first gill slits
46.6
308.5
171.7
13.6
21.4
18.5 ± 0.59
27.5
235.7
102.8
14.8
20.8
17.9 ± 0.56

    fifth gill slits
36.5
227.2
129.9
10.3
16.6
14.2 ± 0.51
20.9
174.5
80.2
11.1
15.9
13.9 ± 0.40

V-length, anterior lobe
19.2
149.0
77.9
6.2
10.1
8.7 ± 0.38
11.5
118.3
55.2
6.1
11.7
9.2 ± 0.59

    posterior lobe
33.4
238.0
132.5
12.9
18.7
15.6 ± 0.56
20.7
147.0
79.5
11.0
18.6
14.4 ± 0.77

Clasper, postanus length
na
na
na
na
na
na
12.2
127.7
48.6
6.4
12.0
8.6 ± 0.47

Snout tip to center of anus
113.8
1010.0
539.9
46.1
66.0
56.0 ± 1.56
75.0
691.1
299.1
32.5
61.0
49.6 ± 2.05

    to max. disc width
91.0
570.0
293.9
31.0
39.0
35.0 ± 0.74
32.4
386.0
170.2
9.0
36.4
31.1 ± 2.20

Center of anus to D1
90.5
566.0
298.1
25.1
37.7
31.5 ± 0.96
68.4
362.6
171.9
25.7
38.2
31.5 ± 1.22

    to D2
103.0
471.0
276.4
29.3
42.9
35.3 ± 1.22
80.4
358.0
176.3
29.1
44.2
35.6 ± 1.54

    to tail tip
129.1
601.0
358.4
37.4
53.8
43.1 ± 1.42
109.6
471.3
236.7
36.2
60.2
44.3 ± 2.15

Number of median thorns
12
21
16.5 ± 0.74

13
18
15.2 ± 0.46

Snout angle
70
120
90.8 ± 3.69

70
133
96.4 ± 5.41

The morphology of Rajella bigelowi has been documented better than all the other species in our study, with published data on 30 specimens [31, 33]. In a recent study, additional data on the morphology and meristics of R. bigelowi from the North Atlantic was provided [35], based on specimens from the MCZ and MHNH and two neonates collected from the Mid-Atlantic Ridge (ZMUB). However, there have been no investigations concerning the sexual dimorphism of this species. Our analysis of 18 specimens of R. bigelowi showed that males and females of this species differ significantly in the following characters (Table 4): interorbital width, length of the orbit + spiracle (measured together), head length, length of the nasal curtain, width of each nasal curtain lobe, length of the third and fifth gill slits, and the number of preorbital thorns. Some of the thorn counts for this species varied on the left and right sides of the specimen (i.e., orbital thorns, scapular thorns, etc.). Therefore, the maximum, minimum, and mean values for these characters are shown for one side of the animal. Apparent sexual dimorphism of meristic characters could be due to our small sample size or low variation among the specimens observed.

Table 4: Morphometrics and meristics of male and female Bigelow’s ray Rajella bigelowi from different areas of the North Atlantic (significant differences are given in bold, number of specimens examined are shown in parentheses, m = standard error, na = not available).

Females (4)
Males (8)

Character
Absolute
% TL
Absolute
% TL

Min
Max
Mean
Min
Max
Mean ±m
Min
Max
Mean
Min
Max
Mean ±m

TL, mm
102.0
453.0
283.8

101.0
436.0
231.0

Disc width
57.0
234.0
144.3
47.9
55.9
51.3 ± 0.87
54.0
215.0
116.1
47.8
54.3
50.6 ± 0.70

    length
43.0
209.0
128.9
40.6
47.1
44.8 ± 0.82
46.0
196.0
102.8
41.3
49.0
44.5 ± 0.72

Snout length, preorbital
10.5
55.3
31.5
9.4
13.4
11.0 ± 0.44
8.0
41.0
23.1
7.9
12.7
10.2 ± 0.45

    preoral
10.5
56.4
33.5
10.3
14.4
11.7 ± 0.48
11.0
44.5
26.6
10.2
14.8
11.8 ± 0.44

    prenasal
8.5
46.0
28.5
8.3
12.7
10.0 ± 0.46
8.0
37.0
21.8
7.9
11.8
9.5 ± 0.35

Orbit, horizontal diameter
4.2
19.0
11.8
3.8
4.7
4.2 ± 0.12
4.0
17.0
9.4
3.1
5.3
4.1 ± 0.19

Interorbital width
5.0
16.5
10.5
3.4
4.9
3.8 ± 0.17
5.0
16.0
9.3
3.2
5.3
4.3 0.21

Spiracle length
2.2
12.6
6.7
2.0
2.8
2.3 ± 0.10
2.0
11.5
5.7
2.0
3.0
2.4 ± 0.10

Interspiracular width
7.2
29.9
19.5
6.6
7.1
6.9 ± 0.06
8.0
30.0
14.9
6.4
8.0
7.1 ± 0.19

Orbit + spiracle length
5.0
21.5
13.1
4.1
5.2
4.7 ± 0.11
5.5
21.5
11.4
4.1
9.1
5.3 ± 0.45

D1, height
1.4
12.0
6.5
1.4
2.6
2.1 ± 0.18
2.0
12.0
5.4
1.7
3.2
2.2 ± 0.15

    base length
7.7
23.8
15.3
4.1
7.5
5.5 ± 0.36
5.0
24.0
12.4
4.1
6.1
5.1 ± 0.19

D2, height
1.6
12.1
6.3
1.4
2.7
2.1 ± 0.19
2.0
13.0
5.4
1.7
3.0
2.2 ± 0.15

    base length
6.2
21.0
13.9
3.5
6.2
5.0 ± 0.34
5.5
22.0
12.4
4.7
5.9
5.4 ± 0.15

Interdorsal space
0.0
5.0
1.1
0.0
2.8
0.6 ± 0.37
0.0
3.0
0.7
0.0
1.9
0.50.20

C, base length
3.9
12.0
8.9
1.9
5.9
3.4 ± 0.57
3.0
14.0
6.7
1.6
3.6
2.9 ± 0.21

Tail, postdorsal length
4.0
12.0
8.6
1.9
5.9
3.4 ± 0.50
3.0
14.0
7.0
2.1
3.9
3.1 ± 0.18

    height at V-tips
3.2
12.5
8.2
2.6
3.1
2.9 ± 0.07
3.5
11.5
6.4
2.3
3.5
2.8 ± 0.10

    width at V-tips
4.5
18.7
11.4
3.2
4.6
4.0 ± 0.15
4.0
19.5
9.3
3.1
4.5
4.0 ± 0.14

    height at D1 origin
1.4
4.8
3.0
1.0
1.4
1.1 ± 0.04
1.5
5.0
2.7
1.0
1.8
1.2 ± 0.09

    width at D1 origin
1.8
8.0
4.4
1.1
1.8
1.5 ± 0.12
1.8
8.3
3.6
1.1
2.0
1.6 ± 0.09

    lateral fold length
21.3
122.0
66.1
12.1
29.0
20.5 ± 2.59
17.0
106.0
55.7
14.0
27.0
20.6 ± 1.72

Head length, ventrally
21.5
109.2
67.0
21.1
25.6
23.4 ± 0.47
22.0
91.0
50.7
20.4
25.6
22.3 ± 0.53

Mouth width
8.3
28.3
17.0
4.9
8.1
6.3 ± 0.32
7.0
35.0
15.4
5.9
8.0
6.7 ± 0.22

Internasal width
7.2
33.2
19.0
5.8
7.3
6.7 ± 0.17
8.0
29.0
14.5
5.5
7.9
6.9 ± 0.23

Nasal curtain, length
3.1
15.3
8.7
2.6
3.7
3.0 ± 0.16
2.0
13.0
6.0
2.0
3.3
2.5 ± 0.14

    width each lobe
1.6
7.7
4.6
1.4
1.9
1.7 ± 0.06
1.0
6.5
3.2
0.8
2.0
1.4 ± 0.11

    space between lobes
5.2
13.0
9.8
2.9
5.7
4.1 ± 0.49
4.8
16.0
9.6
3.0
5.9
4.6 ± 0.44

Gill slit length, first
1.0
8.8
4.2
1.0
1.9
1.4 ± 0.11
1.5
5.0
2.8
0.9
1.6
1.3 ± 0.07

    third
0.7
9.3
4.4
0.7
2.1
1.5 ± 0.14
1.0
6.0
3.0
0.9
1.6
1.2 ± 0.08

    fifth
1.0
7.1
3.3
1.0
1.6
1.1 ± 0.07
1.0
4.8
2.3
0.7
1.6
0.9 ± 0.09

Space between first gill slits
13.8
62.9
37.6
12.4
14.0
13.2 ± 0.21
14.0
58.0
30.5
12.3
14.2
13.3 0.19

    fifth gill slits
9.7
38.6
24.3
7.7
9.5
8.6 ± 0.21
9.0
35.0
18.4
6.8
9.1
8.2 ± 0.23

V-length, anterior lobe
11.0
62.3
36.2
10.7
13.8
12.5 ± 0.42
13.0
56.0
29.7
11.5
13.8
12.8 ± 0.25

    posterior lobe
14.5
81.2
43.0
12.8
17.9
14.7 ± 0.55
13.0
61.0
32.6
11.9
16.0
13.8 ± 0.42

Clasper, postanus length
na
na
na
na
na
na
7.0
45.0
17.7
5.4
10.3
7.2 ± 0.43

Snout tip to center of anus
38.0
195.0
116.1
37.3
43.4
40.3 ± 0.85
39.0
180.0
91.3
36.0
41.3
39.4 ± 0.49

    to max. disc width
31.5
125.0
73.1
12.5
32.8
27.2 ± 2.23
23.3
130.0
56.5
10.2
30.1
24.5 ± 2.30

Center of anus to D1
40.0
199.0
128.9
39.2
48.5
45.2 ± 0.99
47.0
190.0
106.5
43.0
48.9
46.1 ± 0.61

    to D2
46.0
222.0
144.5
45.1
53.6
50.9 ± 1.00
53.0
215.0
119.5
49.3
54.5
51.9 ± 0.43

    to tail tip
62.0
254.0
166.2
56.1
63.2
59.2 ± 0.82
59.0
249.0
137.4
57.1
61.7
59.5 ± 0.50

Snout angle
106.0
111.0
108.5 ± 2.50

107.0
110.0
108.3 ± 0.88

No. preorbital thorns
2
6
3.1 ± 0.52

1
5
2.1 ± 0.39

No. postorbital thorns
2
4
2.4 ± 0.26

1
4
2.1 ± 0.26

No. interspiracular thorns
0
4
1.3 ± 0.53

0
3
0.8 ± 0.42

No. scapular thorns
2
8
4.6 ± 0.75

2
10
3.6 ± 0.79

No. nuchal thorns
2
4
2.6 ± 0.32

2
4
2.4 ± 0.22

No. interscapular thorns
1
1
1.0 ± na

1
1
1.0 ± na

No. median thorns
25
33
29.4 ± 0.96

26
33
28.6 ± 0.69

No. middorsal thorns
0
1
0.1 ± 0.13

0
1
0.1 ± 0.10

Tooth rows upper jaw
22
38
30.7 ± 2.35

22
40
32.3 ± 2.63

    lower jaw
24
36
29.2 ± 2.15

22
38
30.3 ± 2.60

Morphometric and meristic data from Rajella kukujevi are published in a single paper, which is the original description of the species [32]. Unfortunately the description of this species included only a small number of characters. Since then, only three specimens have been collected [50, 53], but these subsequent records lack any reporting of morphometric or meristic data. Therefore, the morphology of this species has been scantly reported and no data on sexual dimorphism are available.

The males and females we examined differ in the following characters (Table 5): disc width, interorbital width, length of the second dorsal fin base, lateral fold length, space between the first gill slits, distance between the snout and the point of maximum disc width, number of interspiracular and median thorns, number of lateral spines on the right and left sides of the tail, and the number of trunk vertebrae. Like R. bigelowi, the apparent sexual dimorphism of meristic characters could be due to our small sample size (only two females examined) or low variation among the specimens observed. Conversely, other characters might be sexually dimorphic but not detected by our analysis, owing to very high variation in some characters (i.e., median thorns and lateral spines). Also, like R. bigelowi, the maximum, minimum, and mean values for some of the thorn counts were given for one side of the animal.

Table 5: Morphometrics and meristics of male and female Mid-Atlantic skate Rajella kukujevi from different areas of the North Atlantic (significant differences are given in bold, number of specimens examined are shown in parentheses, m = standard error, na = not available).

Females (2)
Males (6)

Character
Absolute
% TL
Absolute
% TL

Min
Max
Mean
Min
Max
Mean ± m
Min
Max
Mean
Min
Max
Mean ± m

TL, mm
602.0
604.0
603.0

597.0
757.0
716.2

Disc, width
335.0
340.0
337.5
55.6
56.3
56.0 ± 0.32
337.0
442.0
412.2
56.0
58.5
57.5 ± 0.44

    length
301.0
308.0
304.5
50.0
51.0
50.5 ± 0.50
299.0
394.0
366.5
50.1
52.3
51.1 ± 0.35

Snout length, preorbital
83.3
86.6
85.0
13.8
14.3
14.1 ± 0.25
83.8
111.0
99.4
13.2
15.0
13.9 ± 0.26

    preoral
85.7
90.7
88.2
14.2
15.0
14.6 ± 0.39
94.2
111.5
103.9
13.2
16.0
14.6 ± 0.48

    prenasal
74.5
81.2
77.9
12.4
13.4
12.9 ± 0.53
80.1
96.9
89.1
11.8
13.4
12.5 ± 0.27

Orbit, horizontal diameter
21.8
27.5
24.7
3.6
4.6
4.1 ± 0.47
24.0
37.2
29.0
3.2
4.9
4.1 ± 0.26

Interorbital width
24.2
25.4
24.8
4.0
4.2
4.1 ± 0.09
23.9
32.1
27.7
3.7
4.2
3.9 ± 0.09

Spiracle length
15.7
17.8
16.8
2.6
2.9
2.8 ± 0.17
14.0
21.7
19.1
1.8
3.1
2.7 ± 0.19

Interspiracular width
40.8
42.5
41.7
6.8
7.0
6.9 ± 0.13
42.4
56.8
51.0
6.7
7.5
7.1 ± 0.14

Orbit + spiracle length
28.6
30.9
29.8
4.8
5.1
4.9 ± 0.18
33.3
42.9
36.3
4.5
5.7
5.1 ± 0.19

D1, height
14.7
18.3
16.5
2.4
3.0
2.7 ± 0.30
14.0
27.6
22.2
2.3
3.7
3.1 ± 0.19

    base length
29.0
31.6
30.3
4.8
5.2
5.0 ± 0.22
31.0
38.0
34.6
4.1
5.9
4.9 ± 0.24

D2, height
17.1
18.2
17.7
2.8
3.0
2.9 ± 0.10
17.9
28.6
22.0
2.7
3.8
3.1 ± 0.16

    base length
33.3
36.2
34.8
5.5
6.0
5.8 ± 0.25
27.3
42.3
36.1
4.6
5.6
5.0 ± 0.15

Interdorsal space
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0

C, base length
8.9
10.8
9.9
1.5
1.8
1.6 ± 0.16
8.2
12.0
10.2
1.2
1.6
1.4 ± 0.06

Tail, postdorsal length
8.9
10.8
9.9
1.5
1.8
1.6 ± 0.16
8.2
12.0
10.2
1.2
1.6
1.4 ± 0.06

    height at V-tips
17.2
17.4
17.3
2.8
2.9
2.9 ± 0.02
15.9
23.0
20.2
2.4
3.0
2.8 ± 0.10

    width at V-tips
25.2
26.0
25.6
4.2
4.3
4.2 ± 0.06
20.7
38.9
29.1
3.0
5.1
4.0 ± 0.34

    height at D1 origin
6.7
7.2
7.0
1.1
1.2
1.2 ± 0.04
6.2
11.0
8.0
1.0
1.5
1.1 ± 0.07

    width at D1 origin
11.3
11.4
11.4
1.9
1.9
1.9 ± 0.01
9.2
16.4
13.3
1.5
2.2
1.8 ± 0.12

    lateral fold length
124.0
128.0
126.0
20.6
21.2
20.9 ± 0.30
78.6
170.0
133.1
13.2
22.5
18.4 ± 1.47

Head length, ventrally
151.4
162.0
156.7
25.1
26.8
26.0 ± 0.84
156.6
203.5
182.9
22.8
26.9
25.6 ± 0.63

Mouth width
45.0
45.8
45.4
7.5
7.6
7.5 ± 0.05
45.5
57.8
54.4
7.3
7.8
7.6 ± 0.06

Internasal width
58.0
64.2
61.1
9.6
10.6
10.1 ± 0.50
58.0
79.0
67.6
7.7
10.5
9.5 ± 0.44

Nasal curtain, length
25.6
26.9
26.3
4.2
4.5
4.4 ± 0.12
22.9
35.9
31.1
3.8
4.8
4.3 ± 0.13

    width
48.4
50.6
49.5
8.0
8.4
8.2 ± 0.17
42.5
67.5
57.4
7.1
9.0
8.2 ± 0.40

    width each lobe
11.2
12.9
12.1
1.9
2.1
2.0 ± 0.14
8.6
16.0
13.9
1.4
2.1
1.9 ± 0.11

    space between lobes
na
na
na
na
na
na
33.0
33.7
33.4
4.4
4.5
4.5 ± 0.09

Gill slit length, first
8.5
9.8
9.2
1.4
1.6
1.5 ± 0.11
8.9
12.8
10.8
1.3
1.7
1.5 ± 0.06

    third
8.8
12.2
10.5
1.5
2.0
1.7 ± 0.28
9.0
13.1
11.7
1.2
1.8
1.6 ± 0.10

    fifth
7.0
9.6
8.3
1.2
1.6
1.4 ± 0.21
7.0
10.5
8.5
1.0
1.4
1.2 ± 0.06

Space between first gill slits
82.0
84.9
83.5
13.6
14.1
13.8 ± 0.22
80.5
103.5
92.9
11.4
13.7
13.0 ± 0.35

    fifth gill slits
51.4
53.8
52.6
8.5
8.9
8.7 ± 0.18
62.9
68.7
64.9
8.3
10.6
9.1 ± 0.33

V-length, anterior lobe
79.4
83.2
81.3
13.1
13.8
13.5 ± 0.34
85.0
109.1
98.4
11.2
14.9
13.8 ± 0.56

    posterior lobe
103.0
105.2
104.1
17.1
17.5
17.3 ± 0.21
101.2
149.0
129.6
15.2
19.8
18.1 0.71

Clasper, postanus length
na
na
na
na
na
na
49.9
168.0
118.4
8.4
22.2
16.2 ± 2.43

Snout tip to center of anus
275.0
283.0
279.0
45.7
46.9
46.3 ± 0.59
276.0
367.0
335.7
44.9
48.7
46.9 ± 0.53

    to max. disc width
87.4
90.0
88.7
14.5
15.0
14.7 ± 0.24
78.2
244.0
150.7
11.3
32.2
20.9 ± 3.29

Center of anus to D1
237.0
248.0
242.5
39.2
41.2
40.2 ± 0.98
243.0
317.0
289.5
39.0
41.9
40.4 0.38

    to D2
269.0
279.0
274.0
44.5
46.3
45.4 ± 0.90
273.0
351.0
322.5
43.4
46.4
45.0 ± 0.43

    to tail tip
311.0
321.0
316.0
51.5
53.3
52.4 ± 0.92
315.0
403.0
364.5
47.2
53.2
51.0 0.88

Snout angle
87.0
91.0
89.0 ± 2.00

79.0
96.0
87.0 ± 2.44

No. of preorbital thorns
2
6
3.5 ± 1.00

3
6
4.1 ± 0.54

No. of postorbital thorns
2
4
2.8 ± 0.75

0
5
3.0 ± 0.79

No. of orbital thorns
5
8
6.3 ± 0.25

2
10
6.6 ± 1.21

No. of interspiracular thorns
1
2
1.8 ± 0.25

0
2
1.2 ± 0.17

No. of nuchal thorns
4
8
6.0 ± 2.00

2
5
3.8 ± 0.54

No. of scapular thorns
2
4
3.0 ± 1.00

0
5
2.4 ± 0.62

No. of interscapular thorns
0
2
1.0 ± 1.00

0
4
2.0 ± 0.63

Total no. of median thorns
21
29
25.0 ± 4.00

0
25
14.0 ± 3.79

Lateral spines right
57
58
57.5 ± 0.50

55
79
66.3 ± 3.55

Lateral spines left
39
56
47.5 ± 8.50

57
89
68.8 ± 5.09

No. trunk vertebrae
25
25
25.0 ± 0.00

25
28
26.2 ± 0.58

No. caudal vertebrae
105
113
109.0 ± 4.00

99
118
106.4 ± 3.44

Tooth rows upper jaw
40
na
40.0 ± na

35
43
38.7 ± 1.17

Tooth rows lower jaw
38
40
39.0 ± 1.00

31
44
39.7 ± 1.91

Studies of sexual dimorphism in batoids are limited. However, these previous studies found sexual dimorphism in various morphological characters. For example, males and females of the South American apron ray, Discopyge tschudii, and Patagonian skate, Bathyraja macloviana, differ in disc length [15, 26]. Male and female zipper sand skates, Psammobatis extenta, from Argentinean waters exhibit sexual dimorphism in the prenasal snout length, distance between the anus and tail tip, and distance between the snout tip and anus [14]. The thorny skate, Raja clavata, from the Adriatic Sea exhibits sexual dimorphism in the disc length and maximum eye diameter [6]. Also, many North Pacific skate species from Japanese waters have shown sexual dimorphism in snout lengths, lengths of the pelvic fins, and tail lengths [3].

Our study shows that North Atlantic deepwater skates from the genera Amblyraja, Bathyraja, and Rajella exhibit sexual dimorphism most frequently in the following characters: interorbital width and length of the third gill slit (three of five species), disc length and width, horizontal diameter of the orbit, length of the fifth gill slit, space between first gill slits, space between fifth gill slits, length of the second dorsal fin base, mouth width, length and width of each lobe of the nasal curtain, distance from the snout tip to the center of the anus, distance from the snout to the point of maximum disc width, length of lateral fold, length of the orbit + spiracle (measured together), tail height at the pelvic fin tips, and the number of median thorns (two of five species).

Sexual dimorphism in disc length and the distance from the snout tip to the anus has been noted for other skate species [6, 14, 15, 26]. This is probably due to the larger size attained by females, compared to males of the same age [8–11, 16]. The same reason may cause differences between males and females in the length of the lateral fold, distance from the snout tip to the point of maximum disc width, and the number of median thorns.

Our study shows that females of some species have a relatively wider disc compared to males and this is likely responsible for the observed differences in interorbital width, length of gill slits, length of the orbit + spiracle (measured together), and the space between the gill slits. Differences in mouth width in some species may be caused by sexual dimorphism of the shape and number of teeth in adult skates, as well as differences in diet [3, 9–11, 15, 17–23]. The same cause might be responsible for differences between males and females in size of the nasal curtain.

We also observed differences between males and females in the tail height at the tips of pelvic fins. Since the tail is used in courtship and copulation, it is reasonable to assume that these differences can be attributed to reproductive behaviour.

Forster [37] noted the existence of minor sexual differences in colour patterns of Bathyraja richardsoni that may be related to limited observations (single male and single female examined) and is likely attributable to individual variation. Our study did not reveal significant differences in colouration between males and females in any of the five species examined. However, ontogenetic changes in colour pattern were observed in four of the five species (A. jenseni, B. pallida, B. richardsoni, R. bigelowi), while R. kukujevi demonstrated considerable individual variation in colouration.

Acknowledgments

This paper would not have been possible without the input of our many colleagues. Matthias Stehmann (ICHTHYS, Hamburg, Germany) helped with the ID of skates. James Orr (Alaska Fisheries Science Center, Seattle, USA) and Hajime Ishihara, (W & I Associates Corporation, Fujisawa, Japan) explained some specific measurements. Romain Causse (Museum National d’Histoire Naturelle, Paris, France) made X-ray images of Rajella kukuevi and R. bigelowi from museum specimens. Ingvar Byrkjedal, (Zoological Museum, University of Bergen, Bergen, Norway), Bernard Séret, (Museum National d’Histoire Naturelle, Paris, France), Karsten Hartel and Andy Williston, (Museum of Comparative Zoology, Harvard, USA), Patrick Campbell and Roberto Miguez, (British Museum of Natural History, London, Great Britain), Geoff Swinney, (National Museums of Scotland, Edinburgh, Scotland), and Monty Priede, (Oceanlab, University of Aberdeen, Newburgh, Scotland) made possible the examination of museum material and provided the facilities and equipment for these analyses. Boris Shieko (Zoological Institute Russian Academy of Science, St. Petersburg, Russia) made measurements of the R. kukujevi holotype. Natalia Popova (VNIRO) made the merisitic calculations from X-ray images. The authors are grateful to these colleagues for their valuable contributions to this paper.

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