Exogone anomalochaeta Benham 1921

Exogone anomalochaeta Benham, 1921

Figures 1–2

Exogone anomalochaeta Benham, 1921: 24 –26, pl. 3, figs. 11–13; Benham 1927: 62, pl. A, figs. 9–10. Exogone (Exogone) anomalochaeta. San Martín & Parapar 1997: 291; Fukuda 2010: 115 –118, fig. 31; Barroso 2011: 68 –70, fig. 1.

Exogone heterosetosa. Monro 1939: 115 –116, fig. 9 (not E. heteroseta McIntosh, 1885). Exogone (Parexogone) tridentata Hartman-Schröder 1993: 143 –144, fig. Abb 20–21.

Material examined. Project ' Oceanprof '. 22°02’50.81”S, 39°52’24.10”W, 0–2 cm, 1050 m deep: 1 spec. (MNRJP 1151), 14 Dec 2002; 22°27’31.1”S, 40°09’23.5”W, 0–2 cm, 749 m deep: 1 spec. (MNRJP 1152), 18 Jun 2003. Project ‘ Habitats ’. 21°22’58”S, 40°19’44”W, 53 m deep: 1 spec., 21 Jul 2009; 21°42’53”S, 40°10’16”W, 98 m deep: 1 spec. (MZUSP 2905), 9 Mar 2009, and 1 spec., 7 Jul 2009; 22°17’42”S, 40°26’59”W, 104 m deep: 1 spec., 23 Feb 2009, and 3 specs (MZUSP 2908), 4 Jul 2009; 22°23’22”S, 40°34’59”W, 110 m: 1 spec. (MZUSP 2909), 25 Jul 2009; 22°46’54”S, 41°3’33”W, 78 m deep: 1 spec., 22 Feb 2009, and 1 spec., 0 3 Jul 2009; 22°52’1”S, 40°57’29”W, 92 m deep: 1 spec., 22 Feb 2009, and 1 spec. (MZUSP 2907), 3 Jul 2009; 23°10’5”S, 41°3’6”W, 107 m deep: 2 specs (MZUSP 2906), 0 2 Jul 2009; 23°11’28”S, 41°0’49”W, 117 m deep: 1 spec. (MZUSP 2904), 21 Feb 2009. Project ‘ AMBES ’. 19°33’20”S, 39°2’36”W, 420 m deep: 1 spec. (MZUSP 2912), 11 Dec 2011; 19°36’26”S, 39°10’17”W, 392 m deep: 6 specs (MZUSP 2911), 14 Dec 2011; 19°49’37”S, 39°35’41”W, 410 m deep: 1 spec., 14 Jan 2012; 20°14’19”S, 39°48’36”W, 381 m deep: 1 spec. (MZUSP 2914), 9 Jan 2012, and 5 specs (MZUSP 2915), 19 Jun 2013; 20°15’36”S, 39°46’15”W, 1040 m deep: 1 spec. (MZUSP 2913), 9 Jan 2012; 20°35’16”S, 39°53’47”W, 410 m deep: 58 specs, 8 Jan 2012, and 27 specs, 18 Jun 2013; 20°36’2”S, 39°51’35”W, 1000 m deep: 18 specs, 8 Jan 2012; 21°4’43”S, 40°8’31”W, 1024 m deep: 3 specs, 30 Dec 2011.

Additional material examined. Exogone anomalochaeta — Australia, Tasmania, Macquarie Island (54°37'S, 158°51'E): 17 specs (AM W617, syntypes), coll. H. Hamilton, det. W. Benham. Antartic, South Shetland Islands, Livingston Island: 2 specs (MNCN 16.01 /1517), coll. & det. G. San Martín, 0 7 Feb 1994; 24 specs (MNCN 16.01/ 1519), coll. & det. G. San Martín, 19 Feb 1994. Exogone heterosetosa — Australia, Western Australia, Goss Passage, north end of Long Island (28°28'18"S, 113°46'18"E), 8 m deep: 10 specs (AM W27057), coll. C. Bryce, 22 May 1994, det. G. San Martín, 2000. Exogone tridentata —Antarctic, Antarctic Peninsula, close to Base General Bernardo O´Higgins Riquelme (63°00.49'S, 57°09.45'W), 97 m deep: 2 specs (ZMH P-21218, holotype; ZMH P– 21219, paratype), coll. G. Hartmann, M. Siegel & G. Hartmann-Schröder, 1991, det. G. Hartmann-Schröder, 1992.

Description. Longest specimen examined 3.0 mm long, 0.3 mm wide, with 35 chaetigers. Palps narrower than anterior chaetigers, almost totally fused, with distal notch. Prostomium ovate, shorter than palps, with two pairs of eyes in open trapezoidal arrangement, almost transversally in line (Fig. 1 A); antennae inserted close to each other, on middle of prostomium or slightly posteriorly; lateral antennae ovate, papilliform; median antenna digitiform, elongate, distally tapering, reaching around half length of palps (Fig. 2 A–B, D). Peristomium shorter than subsequent segments, sometimes covering posterior part of prostomium, including posterior pair of eyes; peristomial cirri minute, rounded to ovate. Dorsal cirri ovate, longer than peristomial cirri, slightly shorter than lateral antennae, absent on chaetiger 2 (Figs 1 A; 2A–C). Ventral cirri similar to dorsal cirri but shorter (Fig. 2 C). Parapodial lobes conical (Fig 1 A; 2A–C, G–I). Compound chaetae as falcigers only; anterior parapodia with 5–9 falcigers each, midbody with 4–5, posterior parapodia with 3 falcigers each; falcigers with distally swollen, spinulated shafts; blades small, with inverted dorso-ventral gradation in length, ~4–8 µm long along body, dorsalmost blades sometimes not reaching beyond tips of shafts (Fig. 2 F), blades bidentate and spinulated, distal tooth shorter (Figs 1 B–C; 2F–I). Dorsal simple chaetae present on all parapodia, subdistally inflated, with slightly more sigmoid and more acute tip on posterior body chaetigers (Figs 1 E–F; 2G–I); ventral simple chaetae only present on posterior body, smooth, bidentate, tips resembling those of blades of ventralmost falcigers, with distal tooth shorter (Fig. 1 G). Anterior parapodia with 2 aciculae each, aciculae distally inflated, with apparently hollow tip, one straight, other slightly bent distally (Fig. 1 D); from proventricle onwards, single acicula per parapodium, similar to anterior ones, progressively stouter (Fig. 1 H). Pygidium with 1 pair of elongate anal cirri, much longer than dorsal cirri and up to twice as long as median antenna. Pharynx through 3 chaetigers, tooth close to anterior border; proventricle extending for 3.5–5 chaetigers, with ca. 20 rows of muscle cells (Fig. 1 A).

Remarks. The specimens herein examined match the original description, except for the length of the proventricle, which is longer in Brazilian specimens (3.5–5 chaetigers, as opposed to 2–3 chaetigers, as in the original description). In addition, although not mentioned in the text of the original description, Benham illustrated a specimen with dorsal cirri on chaetiger 2 (Benham 1921: Pl. 5, Fig. 11), while in Brazilian specimens, dorsal cirri are absent on chaetiger 2. Both these differences, however, could not be confirmed in the specimens examined from other localities, including the syntypes, all specimens being remarkably similar to the Brazilian ones.

Based on specimens from the Kerguelen Islands, Monro (1939) synonymized this species with E. heterosetosa McIntosh, 1885. However, based on the illustrations provided (Monro 1939: Fig. 9), all the material examined by Monro belongs to E. anomalochaeta and not to E. heterosetosa, which is a fairly different species (see San Martín 2005). Exogone heterosetosa differs from E. anomalochaeta by having the median antenna longer than the combined length of prostomium and palps, by having falcigers with proportionally larger blades, and also by having spiniger-like compound chaetae, which are absent in E. anomalochaeta. So, we consider both E. anomalochaeta and E. heterosetosa as valid species.

The type series of E. tridentata was also examined, but we could not find significant differences to E. anomalochaeta. Thus, E. tridentata is herein considered as a junior-synonymy of E. anomalochaeta.

This is the first formal report of Exogone anomalochaeta out of the Southern Ocean, although the presence of this species in waters off southern Brazil was already known from two unpublished PhD thesis (Fukuda 2010; Barroso 2011).

Geographic distribution and bathymetric range. Southern Ocean—Commonwealth Bay and Macquarie Island (type locality—from the intertidal zone to ~ 10 m deep) (Benhan 1921); South Shetland Islands, 19–184 m deep (San Martín & Parapar 1997). Brazilian specimens were collected from 749–1050 m deep.