Dodecas decacentrum Stebbing, 1910b

Takeuchi, Ichiro & Lowry, James K., 2015, A taxonomic study on the Phtisicidae (Crustacea: Amphipoda) of New South Wales, Australia, Journal of Natural History 50, pp. 603-648 : 605-610

Dodecas decacentrum Stebbing, 1910b

( Figures 1 – 3 )

Dodecas decacentrum Stebbing, 1910b: 629 – 631 , 652, pl. 60. – Briggs, 1914: 77 – 78. – Guiler, 1952: 31. – Guiler, 1954: 541. – McCain and Steinberg: 1970: 49. – Lowry and Stoddart, 2003: 35.

Type material

Syntypes. 232 individuals, AM P2546 , 34°03.5 ʹ S, 151°12.5 ʹ E, 2 – 3 km off Port Hacking, New South Wales, 69 m depth, sandy, 10 March 1898, Stn 35; 1 male, AM P2547 , 34°05 ʹ S, 151°15 ʹ E, 3 – 4 km off Botany Bay , New South Wales, 91 – 95 m depth, mud, 11 March 1898, Stn 37; 1 male, AM P3383 , 34°05 ʹ S, 151°15 ʹ E, 3 – 4 km off Botany Bay , New South Wales, 91 – 95 m depth, mud, 11 March 1898, Stn 37; 220 individuals, AM P46931, 34°05 ʹ S, 151°15 ʹ E, 3 – 4 km off Botany Bay , New South Wales 91 – 95 m depth, mud, 11 March 1898, Stn 37; 36 individuals (17 males, 18 mature females and 1 juvenile), AM P46917, 34°10 ʹ S, 151°11 ʹ E, 5.5 – 6.5 km off Wattamolla , New South Wales, 99 – 108 m depth, mud, 2 March 1898, Stn 57; 1 female, AM P46941, 34°05 ʹ S, 151°15 ʹ E, 3 – 4 km off Botany Bay , New South Wales, 91 – 95 m depth, mud, 11 March 1898, Stn 37; 53 individuals (24 males, 25 mature and 4 immature females), AM P46942, 34°05 ʹ S, 151°15 ʹ E, 3 – 4 km off Botany Bay , New South Wales, 91 – 95 m depth, mud, 11 March 1898, Stn 37; 1 male, AM P46943, 34°05 ʹ S, 151°15 ʹ E, 3 – 4 km off Botany Bay , New South Wales, 91 – 95 m depth, mud, 11 March 1898, Stn 37 .

Additional material examined

AM P46929, 5 males, 43°15 ʹ S, 147°16 ʹ E, D ’ Entrecasteaux Channel , Tasmania, 3.5 – 20 m depth, on the hydrozoan Sertularia operculata .

Type locality

Botany Bay and off Wattamolla, New South Wales, Australia, 69 – 108 m depth .

Description

Based on syntype, male, body length 18.98 mm, AM P2547 and male, 13.35 mm, AM P3383, for pereopods 6 and 7. Head and pereonite 1 combined length 2.71 mm; pereonite 2, 1.91 mm; pereonite 3, 2.49 mm; pereonite 4, 3.42 mm; pereonite 5, 4.58 mm; pereonite 6, 3.33 mm; pereonite 7, 0.53 mm.

Head and pereonites slender. Head smooth, rounded dorsally; eye large, distinctive; head/ pereonite 1 concave along dorsal margin; pereonite 1 with paired dorsodistal projections.

Antenna 1 slender; 0.67 × body length; peduncle articles 2 and 3 subequal in length; peduncular article 3 straight; flagellum 0.17 × peduncular length, with more than five articles; proximal article result of fusion of 3 – 4 articles. Antenna 2 slender; 0.33 × antenna 1 length; peduncle with several feeble setae; flagellum 0.33 × peduncular length, with seven articles.

Upper lip notched, forming rounded quadrilateral projections. Mandible right side incisor with six teeth; lacinia mobilis a reverse trapezoid plate followed by one smaller plate; accessory setal row with 10 setae; palp article 2 with one lateral seta; palp article 3 setal formula 1 – 16 – 1; left side incisor with five teeth; left side lacinia mobilis with five teeth, followed by one trapezoid plate; left side accessory setal row with one bundled seta and 10 setae; palp article 2 with five lateral setae; palp article 3 setal formula 1 – 17 – 1. Lower lip finely setose on inner and outer lobes. Maxilla 1 outer plate with six stout apical setal-teeth; palp distal margin with six triangular projections, each with one slender or robust seta, with a row of slender setae. Maxilla 2 inner plate triangular with nine apical robust setae; outer plate elongate with 12 apical setae. Maxilliped inner plate (basal endite) oval, with three stout serriform setae on expanded inner half of distal margin, with one or two setae on outer half; outer plate (ischial endite) oval, 0.5 × length of inner plate (basal endite), inner margin smooth, with four setae on inner margin; palp article 2 setose on inner margin; palp article 3 with moderately dense distal setae; palp article 4 weakly falcate.

Pereon. Pereonite 2 with anterolateral triangular projection, with midlateral projection, with paired anteriorly curved mid-dorsal projections, with paired anteriorly curved dorsodistal projections. Pereonite 3 with paired mid-dorsal projections. Pereonite 4 with paired small mid-dorsal projections. Pereonite 5 longest.

Gnathopod 1 propodus triangular, with four rows of submarginal setae near dorsal margin; palm begins about 1/4 along posterior margin, smooth, with two robust setae near corner of palm; dactylus curved. Gnathopod 2 situated near middle of pereonite 2; gill length about 4.5 × width, 1/2 × pereonite length; coxa vestigial; basis 2.2 × length of pereonite 2; carpus 0.55 × basis length; propodus elongate, subrectangular, large, length 2.6 × width, anterodistal margin straight; palm proximal projection with three robust (one large and two small) setae, palm margin straight, smooth, with narrow well-developed distal shelf with two small triangular projections distally.

Pereopod 3 very slender; gill length about 1/2 × corresponding pereonite, curved anteriorly; basis to carpus straight and cylindrical; basis longer than other pereopod articles, 2/5 × the pereopod length; propodus without distal palm and robust setae; dactylus spatulate. Gill 4 length about 1/4 × corresponding pereonite. Pereopod 5 broad; articulation between articles 1 and 2 oblique; dactylus medium length, falcate. Pereopod 6 basis longer than propodus; carpus with four robust setae and one slender seta on anterior margin; propodus with two pairs of robust setae on proximal margin and one robust and one slender seta; dactylus curved, not setose. Pereopod 7 basis shorter than propodus; merus subequal in length to basis; carpus longer than basis with five robust setae; propodus and dactylus similar to those of pereopod 6.

Pleon. Pleopod one pair, tiny. Uropod 1; peduncle elongate, about 6 – 8 × width; ramus 0.2 × peduncular length. Uropod 2; peduncle about 5 – 6 × width, shorter than uropod 1; ramus 0.5 × peduncular length.

Female (sexually dimorphic characters). Based on female syntype, body length 16.67 mm, AM P 46941. Antenna 1 about 0.5 × body length; flagellum 0.25 × peduncular length. Length of head and pereonite 1 combined length 1.47 mm, and each length of pereonites 2–7, 1.91 mm, 1.82 mm, 2.98 mm, 4.58 mm, 3.24 mm and 0.67 mm, respectively. Gnathopod 2 situated near anterior end of pereonite 2; palm margin of propodus convex, with one small triangular projection distally. Gill 3 length about as long as corresponding pereonite, distal part curved posteriorly. Gill 4 length about 1/3 × corresponding pereonite.

Remarks

Of the seven species of Dodecas , two species, D. decacentrum Stebbing, 1910b and D. hexacentrum Mayer, 1903 , have been reported along the coasts of New South Wales and redescribed in the present study. Guiler (1954) synonymized D. decacentrum with D. hexacentrum based on the Tasmanian specimens.

Dodecas decacentrum is, however, easily distinguished from D. hexacentrum , which coexists along the New South Wales coasts by the following diagnosis: (1) in D. decacentrum pereonite 2 possesses a pair of dorsodistal projections in addition to mid-dorsal projections on pereonite 2, while in D. hexacentrum the pereonite 2 lacks the dorsodistal ones; (2) in D. decacentrum pereonites 3 – 4 possess a pair of mid-dorsal projections, while in D. hexacentrum pereonites 3 – 4 lack these projections; (3) gills on pereonites 2 and 3 of male D. decacentrum are half the length of the corresponding pereonites, whereas those of male D. hexacentrum are longer than the corresponding pereonites; (4) in D. decacentrum the carpus of pereopods 6 and 7 are elongated with four or five spines along the anterior margin, while in D. hexacentrum the carpus of pereopods 6 and 7 lacks anterior spines.

The other five species have been reported so far from Tasmania and sub-Antarctic and Antarctic regions. Especially, D. grandimanus Guiler, 1954 and D. tasmaniensis Guiler, 1954 were established based on specimens which were collected from off Mountain Creek, Tasmania by Guiler (1954). Guerra-García and Takeuchi (2004) showed the lateral view of a male and a mature female of D. tasmaniensis Guiler, 1954 from Tasmania. The male, 9.41 mm in body length, is a little smaller than a mature female, 9.47 mm in body length, in both Guerra-García and Takeuchi (2004). The figure of the male indicates the presence of paired posterodorsal projections on pereonite 1, midlateral projection, paired anteriorly curved mid-dorsal projections, and anteriorly curved dorsodistal projection on pereonite 2 and paired mid-dorsal projections on pereonite 3 (see Guerra-García and Takeuchi 2004, fig. 3A). These characteristics show the close relationship of D. tasmaniensis to D. decacentrum ( Guerra-García and Takeuchi 2004) . However, the male specimen of D. tasmaniensis might not be a fully grown male, which fits precisely the species diagnosis of the present study, since in D. decacentrum and D. hexacentrum males grow larger than females (see Figures 1 and 4 ) similar to Caprella (see Takeuchi and Hirano 1991; Nakajima and Takeuchi 2008). Moreover, the lateral figure of a mature female of D. tasmaniensis lacks the posterodorsal projections on pereonite 1 and paired anteriorly curved mid-dorsal projections on pereonite 2 (see Guerra-García and Takeuchi 2004, fig. 3B), while an immature female, 5.22 mm in body length, collected from King George Sound, Western Australia possessed paired anteriorly curved middorsal projections and anteriorly curved dorsodistal projection on pereonite 2, paired mid-dorsal projections and shallow dorsodistal projection on pereonite 3, a middorsal projection on pereonite 4 and a shallow mid-dorsal projection on pereonite 5 (see Guerra-García 2004a, fig. 7). These differences indicate that it is necessary to find fully adult males to clarify the species diagnosis of “ D. tasmaniensis ” collected from off Mountain Creek, Tasmania.

Figures

Figure 1. Dodecas decacentrum Stebbing, 1910b from 3–4 km off Botany Bay, New South Wales. Male, syntype, 18.98 mm, AM P2547; female, syntype, 16.67 mm, AM P46941. Bar indicates 2.0 mm.

Figure 2. Dodecas decacentrum Stebbing, 1910b from 3–4 km off Botany Bay, New South Wales. Male, syntype, 18.98 mm, AM P2547, A2, G1, G2, P3, P5, ABD (LT); male, syntype,13.35 mm, AM P3383,P6, P7; female, syntype, 16.67 mm, AM P46941, G2, ABD (LT). Bars in ABD (LT) (F) and ABD (LT) (M) indicate 0.20 mm,those in G1, P3, P5,P6 and P7 indicate 0.50 mm and those in G2 (M) and G2 (F) indicate 1.0 mm.

Figure 3. Dodecas decacentrum Stebbing, 1910b from 3–4 km off Botany Bay, New South Wales. Male, syntype, 18.98 mm, AM P2547. Bars indicate 0.10 mm.

Figure 4. Dodecas hexacentrum Mayer, 1903 from Watsons Bay, Port Jackson, New South Wales. Male, syntype, 14.01 mm, AM G2555; B, female, syntype ‘b’, 10.90 mm, AM P46932. Bar indicates 1.0 mm.