Alexandrella australis ( Chilton, 1912 )

Berge, Jørgen & Vader, Wim, 2005, The amphipod genus Alexandrella (Amphipoda, Stilipedidae): taxonomic status, allometric growth and description of two new species, Journal of Natural History 39 (17), pp. 1327-1346 : 1330-1334

Alexandrella australis ( Chilton, 1912)

( Figure 1 )

Acanthonotozoma australis Chilton 1912, p 487 .

Alexandrella australis: Holman and Watling 1983 a, p 33 –37.

Alexandrella australis: Barnard and Karaman 1991, p 703 .

Bathypanoplea australis (in part): Lowry and Bullock 1976, p 11.

Material examined

Two females (35 and 40 mm), 62 ° 52–559S, 59 ° 27–159W, 884–935 m, collected on the RV Eltanin, 7 January 1963, cruise 6, station 432, Blake Trawl, USNM account no. 1011851 .

Distribution

Known from the Weddell Sea (type locality Cape Norvegia), Scotia Sea and Drake Passage, 884–2609 m .

Remarks

This species was thoroughly figured and described by Holman and Watling (1983), and the two examined specimens agree very well with their description. Hence no redescription is deemed necessary. However, the juveniles taken from the brood pouch of one of the females (40 mm, see details above) were examined, and their morphology needs to be further described and figured. First of all, the body was totally smooth, except for the small tooth on urosomite 1. In contrast (see Holman and Watling 1983, p 34, Figure 1 ), the adult female has large dorsal projections on pereonite 7 and pleonites 1–3. Furthermore, the telson was relatively shorter, compared to the breadth, with a deeper cleft. The mouthparts, especially the maxilliped and the two maxillae, were in general far less setose than for the adults, as is usually found for immature/juvenile amphipod specimens, and the maxilliped palp was relatively longer than in adults. The labrum of the unhatched juveniles was strongly asymmetrical with a large right lobe, hence very similar to the character state found for all its adult congeners, while the adults of A. australis , compared to all other Alexandrella spp. , are characterized by a short and only weakly asymmetrical labrum. The mandibles of the juvenile specimens showed some character states of even greater interest: at the present developmental stage of the hatchlings, the mandibles are short and compact, with a weakly toothed incisor. The lacinia mobilis was present on both mandibles; the left lacinia mobilis smaller than that of the adult, and the right conspicuously broader and more powerful than in adults (see Figure 1 ). Through the cuticula of the mandibles, the next moult could clearly be seen (see Figure 1 ), in which the incisor was strongly toothed and narrow. Thus it is evident that the mandible of A. australis goes through several developmental stages before the specimens are hatched. The incisor seen through the cuticula closely resembles that of Astyra , being strongly toothed and narrow (versus broad).

Alexandrella dentata Chevreux, 1912

( Figures 2 , 3 )

Alexandrella dentata Chevreux 1912, p 213 .

Alexandrella dentata: Chevreux 1913, p 134 .

Alexandrella dentata: Lowry and Bullock 1976, p 145 (in part).

Alexandrella dentata: Holman and Watling 1983, p 37 –39.

Alexandrella dentata: Barnard and Karaman 1991, p 703 .

Alexandrella mixta: Barnard 1958, p 127 .

Alexandrella mixta: Holman and Watling 1983 , 39–41.

Alexandrella mixta: Barnard and Karaman 1991, p 703 .

Parandaniexis mixtus: Nicholls 1938, p 42 , Figure 22.

Pseudandaniexis mixtus Lowry and Bullock 1976, p 136 .

Parandaniexis mixtus: Lowry 1982, p 320 .

Material examined

Three females, 72 ° 499540S, 19 ° 269300W, 470–473 m, collected on the RV Polarstern , 25 February 1998, station 264, Agassiz Trawl ( IRSNB); two females, 71 ° 129000S, 13 ° 149000W, 402–412 m, collected on the RV Polarstern , 18 February 1989, station 284, Agassiz Trawl ( IRSNB); one female, 36 ° 319S, 178 ° 389W, 4520 m depth, collected on the RV Galathea , 24 February 1952, station 663 ( Zoological Museum of Copenhagen , Denmark); five females, 71 ° 099420S, 12 ° 289420W, 341–360 m, collected on the RV Polarstern , 2 February 1998, station 77, Agassiz Trawl ( IRSNB); two males, 73 ° 229000S, 21 ° 109000W, 338– 333 m, collected on the RV Polarstern , 13 February 1996, station 11, Bottom Trawl ( IRSNB); two immatures, 73 ° 189000S, 21 ° 109000W, 457–459 m, collected on the RV Polarstern , 13 February 1996, station 12, Bottom Trawl ( IRSNB); two immatures, 71 ° 179000S, 12 ° 369180W, 415–416 m, collected on the RV Polarstern , 1 February 1998, station 197, Agassiz Trawl ( IRSNB) .

Distribution

Bellingshausen Sea (type locality close to Alexander Isl. ), Weddell Sea , Commonwealth Bay and Kermadec Trench (South Pacific), 90–4520 m .

Remarks

In their revision of the Stilipedidae, Holman and Watling (1983, p 41) retained A. mixta as a valid species although they speculated that, when additional material had been examined, A. mixta might later well be considered a junior synonym of A. dentata . Based upon their examined specimens (they only had a total of three specimens available of the two nominal taxa combined), two morphological traits were considered as diagnostic for the two species: the number of ST on the outer plate of the first maxilla and the length, relative to the outer plate, of the maxilliped palp. The former of these two characters appears to be highly variable within the two genera Alexandrella and Bathypanoploea (see Berge and Vader in press and below), and it is also found to vary among the examined specimens. One specimen (female 24 mm, RV Polarstern, 25 February 1998, station 264, see details above), was observed to have ST in a 9/3 arrangement, whereas it had a 10/4 arrangement in the next moult. The other character, the length of the maxilliped palp, is also variable to some extent, although not as much as the number of ST, but the palp is considerably longer in immature than in mature specimens. There are, however, no clear patterns of co-variation between these two characters, and it was hence not possible to retain the two taxa based on the available material (but see also Berge and Vader in press for a further discussion on the morphology of this species).

The examined specimens of this species all possessed a rudimentary molar and molar tuft (i.e. the small group of short setae located next to the reduced molar), very similar to that of the closely related genus Bathypanoploea (see Berge and Vader in press). The presence of a rudimentary molar also in this species, therefore removes the last distinguishing character between Alexandrella and Bathypanoploea . However, as the phylogeny of the group will be examined in a later paper, and because this relationship must be considered also in relation to Astyroides , no further nomenclatory changes are proposed herein.

As for A. australis (above), one of the examined females (female 24 mm, RV Polarstern, 25 February 1998, station 264, see details above) carried nearly fully developed young in the brood pouch. These juvenile specimens were removed from the brood pouch and examined. In general, the morphological differences between the juvenile and the adult female were similar to those found for A. australis (above). Both these two species are characterized by having either a smooth or only partly toothed incisor on the mandibles, but a fully toothed incisor was detected for both species in juveniles taken directly from the brood pouch. Furthermore, the dorsal armature on the immatures was conspicuously different and far less dominant, and the telson was shorter and more cleft in the immatures than in adults (see figures 1–3).

Figures

Figure 1. Alexandrella australis. T ad. and Lbr ad. from female 40 mm RV Eltanin, station 432. All other appendages from a juvenile specimen taken out of the brood pouch of the female.

Figure 2. Alexandrella dentata. Female 24 mm, RV Eltanin, station 264. Figure of incisor and lacinia mobilis (Mnd next moult) as seen through the current moult.

Figure 3. Alexandrella dentata. Juvenile taken from the brood pouch of female 24 mm (RV Eltanin, station 264).