“AUSTRODECUS CRENATUM, new species Fig. 12
Austrodecus breviceps.-Hedgpeth, 1950, p. 154. Austrodecus spec. 3, Stock, 1957, pp. 61-63, Figs. 31a, 32a-32b.
Material examined. Hero: 691-27 (29 males, 22 females, two juveniles, (paratypes, USNM 234630)); 691-28 (one male (holotype, USNM 234628), eight males, five females (paratypes, USNM 234629)).
Other material. Hero: 691-23 (one female); 691-32 (one male, three females); 691-33 (two males); 721776 (one male, one female); 721-777 (one female); 721-817 (one male); 721-1063 (one male); 731-1853 (one male); 824-13-1 (three males, two females); 82428-1 (two males, two females).
Marguerite Bay, Antarctic Peninsula, coll. D. C. Nutt, 20 Feb 1948, 64 m (reported by Hedgpeth, 1950, p. 154) (one male).
University of California, Davis—Lipps Collection: SOSC-L118 (one female, three juveniles).
Distribution. This new species, with the excellent series of 94 specimens listed above, is known from the type locality, Low Island, South Shetland Islands, from other material taken in localities from Marguerite Bay, Adelaide Island, and Anvers Island along the Antarctic Peninsula, and from Deception Island, and Greenwich Island, to King George Island, all in the South Shetland Islands. There are no records of it in Subantarctic localities; it is not known to cross the Scotia Sea as appears to have happened with A. glaciale and A. calcaricauda. Both of these species are found in localities along the Antarctic Peninsula and across the Scotia Sea in either the Magellanic area or off South Georgia Island. The recorded depths for the new species are from 21 to 360 m, although the depth record from station SOSC-L118 is from 1.5 m. This is the only hand collection record for the species, the others having been taken from deeper shipboard stations.
Diagnosis. Of the A. breviceps section. Size quite large in relation to other species of genus, trunk and lateral processes compact, closely crowded, both with tall slender tubercles at dorsomedian points. Ocular tubercle short, broad, eyes small, darkly pigmented. Proboscis and palps very short, proboscis only 0.8 as long as trunk width, palps sharply curved ventrally, terminal 2 segments joined anaxially, but do not form a subchelate structure. Abdomen very short. Ovigers extremely large, terminal segment spatulate. First coxae of all legs with tall paired dorsodistal tubercles.
Femora with multiple low dorsal tubercles bearing short setae, cement gland not evident. Second tibiae very short but almost as long as propodi. Claw very large, well curved, without auxiliaries.
Description. Species very large, robust for the genus, with a leg span of 8.2 mm. Trunk compact, lateral processes closely crowded, much wider than their length. Trunk with very large robust dorsomedian tubercles and tubercle of equal size at middorsum of each lateral process, each bearing 2-3 short setae. Ocular tubercle short, broad, without dorsoproximal bulge, eyes small, darkly pigmented. Proboscis stout, downcurved, very short, shorter than maximum trunk width. Abdomen very short, hardly extending beyond length of first coxae of fourth leg pair, tapering distally, without tubercles or setae.
Palps extremely short, slightly longer than very short proboscis, segments downcurved, with many papillae, few distal setae. First segment longest, a curved cylinder, third only about 0.6 length of first, clubbed distally, armed with 3 large curved endal spines each larger than next proximal spine. Terminal 2 segments short, with short setae, terminal segment placed anaxially on shorter penultimate segment but not forming a chelate structure.
Ovigers massive for genus, those of male extending out antero-laterally to palps, those of female only half as long and half diameter of male's. Terminal segment longest, spatulate, armed with field of very short distal spines and few papillae.
Legs moderately short, robust, armed with very few short setae. First coxae each bearing paired tall dorsodistal tubercles, the posterior one being slightly smaller than the anterior one, each armed with 2-3 setae. Second coxae only little longer than first, with several dorsal papillae. Third coxae shorter than first two, armed with few short ventral setae, without tubercles. Femora with few low dorsal tubercles and a larger dorsodistal tubercle bearing short spine. Cement gland of male not evident. First tibiae equal to femoral length, second tibiae shorter, but slightly longer than propodi. Tarsus very short, propodus moderately long, well curved, armed with 12-13 very short sole spines, and very large claw measuring slightly more than 0.6 times propodal length.
Measurements (in millimeters). Trunk length (palp insertion to tip fourth lateral processes), 1.6; trunk width (across first lateral processes ), 0.94; proboscis length, 0.77; abdomen length, 0.28; third leg, coxa 1, 0.26; coxa 2, 0.29; coxa 3, 0.24; femur, 0.67; tibia 1, 0.67; tibia 2, 0.53; tarsus, 0.13; propodus, 0.5; claw, 0.32.
Etymology. The species name (Latin: crenatum, meaning notched or crenulate) refers to the many trunk and lateral process tubercles which appear crenulate in lateral view.
Remarks. Stock [1957b] described and figured parts of this species, but did not name it, while outlining the differences between it and its similar congener, A. breviceps. The two are indeed very closely related and must have split off from a common parent species during the geological plate wanderings of their home shores to isolate two populations, or else they are the product of the west wind drift of Antarctic currents.
One would need to see both species side by side to note size differences, A. crenatum being the larger of the two species. This new species is, in fact, the largest and stoutest species examined during the course of this study of Austrodecus of the Antarctic and Subantarctic. Hedgpeth's  Marguerite Bay male specimen conforms in all respects to other males of the type lot, and Stock [1957b, p. 62, Figs. 31, 32] provided an excellent series of comparative figures of A. crenatum (as A. species 3) and A. breviceps, including the differences in size.
A major difference occurs in the penultimate palp segment, which in A. breviceps is quite long and forms a subchelate structure with the terminal segment. In A. crenatum it is quite short and does not form this subchelate shape. Other differences include the trunk and lateral process tubercles of Gordon's species which are much more setose than those of the new species. There is a small dorsoproximal tubercle on the abdomen of A. breviceps which is lacking on the new species, and very noticeable differences occur in the leg tuberculation and size between the two species. The legs of A. breviceps have shorter major segments, and these are very tuberculate and papillose on their dorsal surfaces and bear many more setae. The major segments of the new species' legs are longer, providing its legs with a more slender habitus, bear many fewer dorsal tubercles and setae, but have a much larger dorsodistal femoral tubercle than that of Stock's species. The ovigers of the new species are a little larger but differ mainly in having many more blunt distal spines than the ovigers of A. breviceps.
This species and A. breviceps are the only species examined for this report in which male ovigers were found to be consistently twice the size and diameter of those of females of the species. In all other species examined, the ovigers are almost equal in size or those of the female are only very slightly smaller in length and diameter. The significance of this is not evident because ovigers of either sex among Austrodecus species have never been found to carry eggs, but this difference is another recognition character where both sexes are taken in the same sample. Eggs of species in this entire family are deposited elsewhere, possibly forming a parasitic relationship with an unknown host after hatching” (Child 1994, p. 77-79)
- Child, C. A. 1994. Antarctic and Subantarctic Pycnogonida I. The Family Ammotheidae. Antarctic Research Series, Biology of the Antarctic Seas XXIII, Volume 63, pp. 1-48. http://invertebrates.si.edu/antiz/taxon_view.cfm?mode=bibliography&citation=1439
- Hedgpeth, J. W. 1950. Pycnogonida of the United States Navy Expedition, 1947-48. Proceedings of the United States National Museum, 100 (3260): 147-160, figs. 17-19. http://invertebrates.si.edu/antiz/taxon_view.cfm?mode=bibliography&citation=1469
- Stock, J. H. 1957. Pantopoden aus dem Zoologischen Museum Hamburg, 1 (=IV-VI). Mitteilungen aus dem Zoologishen Museum in Hamburg, 55: 81-106, 20 figs. http://invertebrates.si.edu/antiz/taxon_view.cfm?mode=bibliography&citation=1492