Aristias excavatus, Kilgallen, Niamh M., 2010

Kilgallen, Niamh M., 2010, A new Antarctic species of Aristias Boeck (Crustacea, Amphipoda, Aristiidae), with remarks on the genus in Antarctica, Zootaxa 2426, pp. 43-53 : 45-51

Aristias excavatus sp. nov.

( Figs 1–4 )

Type material. Holotype, male 10.25 mm, NIWA 58157, Ross Sea, 76º45.54’S, 167º49.33’E, 724–752 m, taken from the sponge Myxodoryx hanitschi (Kirkpatrick, 1907) (Demospongiae, Poecilosclerida , Myxillidae ) ( NIWA 36399), bottom fishing trawl, TAN 0802/ 70, 15 February 2008.

Etymology. The specific name, excavatus, Latin , refers to the excavate anterior margin of the lateral cephalic lobes.

Diagnosis. Lateral cephalic lobes broadly rounded, distinctly excavate along anterior margin; eyes apparently absent. Epistome and upper lip bounded by slight slit, upper lip projecting in front of epistome. Gnathopod 2 coxa well developed. Pereopod 3 coxa slightly expanded distally. Pereopods 5–7 with small spurs on anterodistal corner of propodus; coxa 5 and 6 producing posterior lobe, narrowly rounded ventrally. Epimeron 3 posterior margin smooth. Uropod 3 rami subequal in length; outer ramus article 2 more than half the length of article 1. Telson deeply cleft to 65% of length, lobes subtriangular.

Description of male holotype. Head deeper than long; lateral cephalic lobes broadly rounded, distinctly excavate along anterior margin; rostrum absent; eyes apparently absent. Antenna 1 peduncle article 2 about half the length of article 1; flagellum missing from specimen. Antenna 2 article 3 short, subequal in length and breadth; article 4 the longest; flagellum 7+ articulate, broken at seventh article; calceoli absent. Mouthpart bundle subquadrate. Epistome and upper lip fused, bounded by slight slit; upper lip produced in front of epistome. Lower lip apically finely setose, apices of lobes pointed. Mandible molar a sparsely-setose flap; palp attached centrally, with a row of A-2 and short D-3 setae, and 3 short E-3 setae, article 3 slightly falcate, about 80% the length of article 2; incisor smooth on right mandible, slightly crenate at inner margin of left; lacinia mobilis present only on left mandible in the form of a small peg; accessory setal row absent. Maxilla 1 inner plate broad, strongly setose with at least 8 pappose setae along margin; outer plate presumed to be in a modified 7/4 arrangement, as diagnostic for the family [orientation on slide obscuring clear view of setaltooth number and arrangement]; palp 2-articulate with 3 strong robust setae and distinct serration along the apical margin. Maxilla 2 inner plate broad, outer plate narrower. Maxilliped inner plate broad, apically tapering and crowned with 1 stout robust seta; outer plate large, reaching to midway of palp article 3, apically rounded and with 3 simple setae, inner margin with row of slender robust setae; palp well developed, broad, 4- articulate, dactylus lacking unguis.

Gnathopod 1 parachelate; coxa vestigial, hidden by coxa 2; basis long, lacking setae; ischium short, subequal in length and breadth; carpus shorter than propodus; propodus tapering distally, posterior margin very slightly concave, minutely serrate; dactylus bifid. Gnathopod 2 minutely chelate, coxa well developed, subequal in length to coxa 3; ischium very long; carpus very long, posterior margin straight; propodus long, posterior margin straight. Pereopod 3 coxa slightly expanded distally; merus expanded anteriorly; carpus short, 1.5 × longer than broad; propodus long, subequal in length to merus, with small spur on posterodistal corner. Pereopod 4 coxa longer than broad with weak posteroventral lobe; merus expanded anteriorly; carpus short, 1.5 × longer than broad; propodus long, subequal in length to merus, with small spur on posterodistal corner. Pereopod 5 coxa posterior lobe strongly produced, narrowly rounded ventrally; basis longer than broad, weakly crenate posteriorly, posterodistal corner rounded and produced into slight lobe; merus slightly expanded posteriorly; carpus short, 1.3 × longer than broad; propodus long, slightly longer than merus, with weak spur at anterodistal corner. Pereopod 6 coxa posterior lobe strongly produced, narrowly rounded ventrally; basis longer than broad, crenate posteriorly, posterodistal corner rounded and produced into slight lobe; merus expanded posteriorly; carpus short, 1.3 × longer than broad; propodus long, slightly longer than merus with weak spur at anterodistal corner. Pereopod 7 coxa not bilobate; basis longer than broad, crenate and expanded posteriorly, posterodistal corner broadly rounded and produced into lobe; merus expanded posteriorly; carpus 1.3 × longer than broad; propodus long, longer than merus, with weak spur at anterodistal corner. Gills from gnathopod 2 to pereopod 6.

Epimeron 1 anterodistal corner subquadrate. Epimeron 2 posteroventral corner subquadrate. Epimeron 3 posterior margin smooth, posteroventral corner subacute, not produced. Urosomite 1 with dorsal depression followed posteriorly by rounded hump. Uropod 1 peduncle subequal in length to rami, strongly armed with robust setae minutely and serrate laterally; rami subequal in length, armed with robust setae and minutely serrate apically. Uropod 2 subequal in length to rami, with lateral row of robust setae; rami subequal in length, minutely serrate along one margin. Uropod 3 peduncle slightly shorter than rami, not armed with robust setae; rami subequal in length; outer ramus 2 articulate with article 2 more than half the length of article 1; inner ramus minutely serrate along inner margin. Telson subequal in length and breadth, deeply cleft to 65% of length, lobes subtriangular, each bearing 1 lateral and 1 apical robust seta.

Ecology. The specimen was found with a sample of frozen sponge. It is not known whether the amphipod was living on the surface of, or living within the pore space of the sponge. However, as other members of the family Aristiidae are known to be inquilinous, it is likely that that is also the case for this species.

Remarks. Aristias excavatus sp. nov. is the twenty-eighth known species of Aristias . Of these, nine are blind, either lacking eyes completely, or having rudimentary eyes that lack ommatidea. Of these blind species of Aristias , only A. excavatus and A. adrogans have excavated lateral cephalic lobes. The new species differs from A. adrogans in the shape of the epistome/upper lip complex, of which the upper lip is unprojecting and separated from the epistome by a small notch in A. adrogans , but is projecting and separated from the epistome only by a slit in this species. Also, that species has a uropod 3 inner ramus which also distinctly shorter than the outer ramus, as well as being quite broad and tapering just at the apex, whereas A. excavatus has rami that are subequal in length, with the inner ramus uniformly tapering from base to tip. Aristias excavatus is the first known blind Antarctic species and thus differs from the two other species recorded as having an Antarctic distribution, A. antarcticus Walker, 1906 , and A. collinus K.H. Barnard, 1932 . Additionally, it differs from A. collinus by having an angular upper lip which projects in front of the epistome, whereas that of A. collinus is rounded and not projecting. Aristias excavatus further differs from A. antarcticus by the distinct excavation on the anterior margin of the lateral lobes, which is lacking in that species; the length of the uropod 3 inner ramus, which is subequal to the outer ramus in A. excavatus , but distinctly shorter than it in A. antarcticus ; and the shape of the telson lobes, which are subtriangular in this species as oppose to subrectangular in A. antarcticus .

Systematics of the Antarctic species of Aristias . The systematics of the Antarctic species of Aristias is in disarray, with much of the material described by different authors being attributed to A. antarcticus despite distinct morphological differences between material from different localities ( Tab. 1 ). Walker (1906, 1907) recorded and illustrated Aristias antarcticus from 37 m depth at Winter Quarters Bay, McMurdo Sound. Unfortunately his description did not indicate the shape of the epistome and upper lip complex, which is an important character in distinguishing between the species of this genus. Schellenberg (1926) described material of what he attributed to A. antarcticus from south Victoria Land, but his figures differ from those of Walker (1907) in having an excavate lateral cephalic lobe (much like A. excavatus sp. nov., although Schellenberg’s specimen is figured with well developed eyes), and telson lobes that are narrowing distally, unlike the uniformly subrectangular lobes of Walker’s specimen. Schellenberg states that the epistome of his material is straight and separated from the upper lip by a pit (“Das Epistom ist gerade und durch eine Grube von der Oberlippe getrennt”), and the hind margins of epimera 2 and 3 are straight and finely serrated (“Der Hinterrand des II und III Epimers is gerade und fein gesägt”). K.H. Barnard (1932) recorded Aristias antarcticus from South Georgia and the South Shetlands, attributing his material to that species based on the shape of urosomite 1 and the width of the carpus of pereopods 5–7. He illustrated the upper lip of his specimens as being angularly produced in front of the epistome. In the same publication, Barnard also described a new species, Aristias collinus , which he stated differed from A. antarcticus in the rounded, unproduced upper lip, and the shape of urosomite 1, which was much more deeply depressed than that Walker’s species. He also described the telson as having the lobes “angularly bevelled off distally” (Barnard 1932: 44) as in Schellenberg’s (1926) figure of A. antarcticus . Nicholls (1938) also recorded six specimens of A. antarcticus from Commonwealth Bay and the Davis Sea, but, figures 6b and 6d of his illustrations of the urosome appear to be quite distinct from each other and probably represent two different species (the urosome depicted in Nicholls’ fig. 6c is probably the same state as depicted in his fig. 6b, telescoped under the pleon). In Nicholls’ (1938) figures, the epistome is separated from the upper lip by a distinct notch. Finally, Bellan- Santini & Ledoyer (1974) recorded four specimens of A. antarcticus from Île Australia, Kerguelen Islands. From the illustrations, it appears that this material is very similar to A. antarcticus sensu Walker , differing only in the shape of the telson and perhaps pereopod 7, which seems to be more slender. However, these differences are negligible at best so it is possible that this represents a valid record.

When all of these subtle morphological differences are taken into account, it seems as though at least two of these records, those described by Schellenberg (1926) and Nicholls (1938) (in part), probably represent new species. Thus, all previous records of A. antarcticus require confirmation.

Figures

FIGURE 1. Aristias excavatus sp. nov., holotype male habitus, 10.25 mm, NIWA 58157. Scale bar = 2 mm.

FIGURE 2. Aristias excavatus sp. nov., holotype male, 10.25 mm, NIWA 58157. Scale bars: a = 0.2 mm (antenna 2), b = 0.2 mm (mouthparts).

FIGURE 3. Aristias excavatus sp. nov., holotype male, 10.25 mm, NIWA 58157. Scale bars: a = 0.4 mm (gnathopods), b = 0.4 mm (pereopods).

FIGURE 4. Aristias excavatus sp. nov., holotype male, 10.25 mm, NIWA 58157. Scale bars: a = 0.4 mm (pereopods), b = 0.2 mm (uropods and telson).

Tables

TABLE 1. Recorded Antarctic material of the genus Aristias, showing differences between diagnostic character states.

Species	Locality	Depth (m)	Lateral cephalic lobes, anterior margin	Upper lip	Epimeron 3 posterior margin	Uropod 3 inner ramus	Telson lobes
Aristias antarcticus sensu Walker, 1906	Winter Quarters Bay, McMurdo Sound	36	Straight	Not described/ illustrated	Smooth	Shorter than outer ramus	Sub-rectangular
Aristias antarcticus sensu Schellenberg, 1926	Observatory Bay, Kerguelen	Not given	Excavate	Separated from the epistome by a notch	Finely serrated	Not described/ illustrated	Sub-triangular
Aristias antarcticus sensu K.H. Barnard, 1930	McMurdo Sound & off Cape Adare	82–457	Not described/ illustrated	Not described/ illustrated	Smooth	Not described/ illustrated	Sub-rectangular
Aristias antarcticus sensu K.H. Barnard, 1932	South Shetlands & South Georgia	60–342	Excavate (in some specimens according to the author)	Projecting in front of epistome, separated by a slit	Not described/ illustrated	Not described/ illustrated	Sub-triangular
Aristias antarcticus sensu Nicholls, 1938	Common- wealth Bay & Davis Sea	45–439	Appears to be straight, but not figured in entirety	Not projecting in front of epistome, separated by a notch	Smooth	Shorter than outer ramus	Sub-triangular
Aristias antarcticus sensu Bellan-Santini & Ledoyer, 1974	Kerguelen	24	?Excavate	Not described/ illustrated	Smooth	Shorter than outer ramus	Sub-triangular
Aristias collinus K.H. Barnard, 1932	South Shetlands	200–342	Not described/ illustrated	Rounded, not projecting in front of epistome, separated by a slit	Smooth	Not described/ illustrated	“Angularly bevelled off distally”
Aristias excavatus sp. nov.	Ross Sea	724–752	Excavate	Projecting in front of epistome, separated by a slit	Smooth	Subequal to outer ramus	Sub-triangular