Acanthanas pusillus, Anker & Poddoubtchenko & Jeng, 2006

Anker, Arthur, Poddoubtchenko, Denis & Jeng, Ming-Shiou, 2006, Acanthanas Pusillus, New Genus, New Speceis, A Miniature Alpheid Shrimp With Spiny Eyes From The Philippines (Crustacea: Decapoda), Raffles Bulletin of Zoology 54 (2), pp. 341-348 : 342-347

publication ID	https://doi.org/ 10.5281/zenodo.4508871
persistent identifier	https://treatment.plazi.org/id/03F587BF-FFE2-FF8F-D4B5-42200B9CDBE3
treatment provided by	Carolina
scientific name	Acanthanas pusillus
status	sp. nov.

Treatment

Acanthanas pusillus View in CoL , new species

Type material. – Holotype: 1 post-ovigerous female, CL 2.30, TL 6.80 (NMCR-27001), north of Doljo , Panglao I., Philippines, Panglao 2004 Marine Biodiversity Project Sta. B 36, 9 ° 35.9'N 123 ° 44.5'E, 24 m, reef wall, from corals, 1 Jul.2004 [specimen parasitized by bopyrid isopod, Hemiarthrinae; bopyrid detached and deposited under NMCR-27002] GoogleMaps . Paratypes: 1 ovigerous female, CL 2.10, TL 5.90 ( ZRC 2005.0089 View Materials ), type locality and data; 1 ovigerous specimen with appendix masculina, CL 2.45, TL 6.95 (MNHN-Na 15777), Bingag, Panglao I., Philippines, Panglao Marine Biodiversity Project Sta. B 16, 9 ° 37.6'N 123 ° 47.3'E, 20 m, from coral rubble on sand and gravel, 17 Jun.2004 GoogleMaps .

Description. – Body bulky, stout ( Fig. 1a, b View Fig ), not compressed laterally, glabrous, not setose. Rostrum triangular, elongated, about twice as long as broad at base ( Fig. 2a View Fig ), slightly ascendant in lateral view ( Fig. 2b, c View Fig ), tip acute, reaching from middle of second segment to end of third segment of antennular peduncle ( Fig. 2 View Fig a-c), rostral carina slight, disappearing posteriorly to rostrum. Orbital teeth acute, shorter than rostrum, directed slightly mesially ( Fig. 2a View Fig ). Pterygostomial margin protruding, bluntly angular or rounded ( Fig. 2b, c View Fig ). Branchiostegial margin without marked emargination and devoid of setae. Cardiac notch well developed ( Fig. 1b View Fig ). Eyes concealed in dorsal and lateral view, except for most anterior portion ( Fig. 2a View Fig ), exposed in frontal view ( Fig. 2e View Fig ); cornea well-developed, anteromesial margin with relatively slender subacute tooth; base of eyestalk with stout acute projection ( Fig. 2d, e View Fig ); both corneal tooth and basal projection conspicuous in lateral view ( Fig. 2b, c View Fig ). Ocellar beak not conspicuous. Epistomial sclerite without marked acute tooth.

Antennular peduncle moderately stout ( Fig. 2 View Fig a-c, f), second segment slightly broader than long, much shorter than first segment, subequal to third segment ( Fig. 2a, f View Fig ); stylocerite falling short of distal margin of second segment, distally acute ( Fig. 2 View Fig a-c); ventromesial carina with large acute tooth as illustrated ( Fig. 2f View Fig ); lateral flagellum biramous, with secondary ramus short and partly fused to main ramus, bearing a few aesthetascs ( Fig. 2f View Fig ); proximal portion basal to bifurcation composed of three or four segments ( Fig. 2f View Fig ). Antenna with basicerite moderately stout, ventrolateral margin with strong acute tooth ( Fig. 2b View Fig ); dorsomesial margin with subacute tooth and blunt projection ( Fig. 2b, h View Fig ); scaphocerite broad, short, ovate ( Fig. 2a, h View Fig ), anterior margin of blade convex, reaching or extending slightly beyond strong distolateral tooth ( Fig. 2h View Fig ); carpocerite small, reaching to distal 3/4 of scaphocerite ( Fig. 2b, g View Fig ).

Mouthparts typical for family. Mandible ( Fig. 3a View Fig ) with twosegmented palp; molar process stout; incisor process expanded, distolateral margin with at least 12 minute teeth, distal margin with 10 larger, subtriangular, acute teeth, middle teeth largest ( Fig. 3a View Fig ). Maxillule ( Fig. 3b View Fig ) with bilobed palp, dorsal and ventral lobe each with one stiff seta; ventral endite with strong elongated spine-like setae. Maxilla ( Fig. 3c View Fig ) with narrow scaphognathite; endopod small, unsegmented; dorsal endite not distinctly subdivided into two halves. First maxilliped ( Fig. 3d View Fig ) feebly developed caridean lobe on exopod; endopod apparently without segmentation; epipod narrow, ear-shaped. Second maxilliped ( Fig. 3e View Fig ) with relatively small oval-rounded epipod; endopod and exopod typical for family, without specific features. Third maxilliped ( Fig 3g, h View Fig ) pediform, relatively stout; lateral plate ear-shaped, distally acute ( Fig. 3f View Fig ); antepenultimate segment flattened, with inconspicuous suture proximal to exopod; penultimate segment about twice as long as wide at base; ultimate segment slightly tapering distally, armed with three strong apical spines ( Fig. 3f View Fig ); exopod reaching distal margin of antepenultimate segment, with at least one flexible seta; arthrobranch absent.

First pereiopods (chelipeds) ( Fig. 4 View Fig a-d) equal in size, symmetrical in shape, carried extended or simply flexed, not particularly modified or enlarged; coxa mesially with broad, rounded, weakly protruding lobe furnished with setae; basis with small exopod; ischium unarmed; merus not particularly swollen, distally widening, about four times as long as wide at base, dorsal margin ending in subacute distal tooth; carpus subcylindrical, distally widening, about three times as long as wide at base ( Fig. 4c View Fig ); chela simple, moderately slender, not enlarged, with fingers slightly shorter than palm ( Fig. 4a, d View Fig ); palm smooth, linea impressa and adhesive discs absent; fingers slightly gaping when closed ( Fig. 4d View Fig ), cutting edges unarmed, without snapping mechanism, finger tips acute.

Second pereiopod slender ( Fig. 4e View Fig ); ischium subequal to merus; carpus five-segmented, segment length ratio approximately equal to 4: 1: 1.2: 1.2: 1.5; chela simple, fingers longer than palm, tips acute ( Fig. 4e View Fig ). Third pereiopod moderately slender ( Fig. 4f View Fig ); ischium with one spine on ventrolateral margin; merus about 1.2 times longer than ischium, unarmed; carpus slightly more than half length of merus, unarmed; propodus more slender and longer than merus, with five spinules on ventral margin and two slender distoventral spinules proximal to dactylus ( Fig. 4f View Fig ); dactylus slender, slightly curved, about 0.3 length of propodus, biunguiculate, secondary unguis subterminal. Fifth pereiopod ( Fig. 4g View Fig ) generally slightly similar to third pereiopod; ischium with one spine on ventral margin; merus and carpus unarmed; propodus with two or three slender spinules on ventromesial margin, slender spine proximal to dactylus, and several rows of setae along distal ventrolateral margin ( Fig. 4g View Fig ); dactylus biunguiculate, similar to that of third pereiopod.

Abdominal pleuron II greatly expanded ( Fig. 1 View Fig ); posterolateral angles of pleura rounded (somites I-II), rounded-angular (somites III-IV) or acute (somite V) ( Figs. 1 View Fig , 4k View Fig ); somite VI with subtriangular articulated plate at posteroventral angle ( Figs. 1 View Fig , 4k View Fig ), posterior projection subacute ( Fig. 4k View Fig ); preanal plate posteriorly rounded. First pleopod with endopod small, shorter than half-length of exopod. Second pleopod of one ovigerous specimen (MNHN-Na 15777) with appendix masculina inserted slightly above appendix interna, tip of appendix masculina reaching tip of endopod, furnished with at least four slender spines ( Fig. 4h, i View Fig ); second pleopod of another ovigerous specimen (ZRC 2005.0089) and postovigerous specimen (NMCR) with appendix interna only ( Fig. 4j View Fig ); protopod with subacute projection distally ( Fig. 4h View Fig ). Uropods sligthly exceeding telson ( Fig. 4l View Fig ); lateral lobe of sympodite distally with two acute teeth, mesial tooth much smaller than lateral tooth ( Fig. 4l View Fig ); exopod with diaeresis bearing blunt tooth near lateral margin, almost straight from this tooth to mesial margin ( Fig. 4l View Fig ); lateral spine slender, reaching posterior margin of exopod ( Fig. 4l View Fig ).

Telson ( Fig. 4l View Fig ) moderately slender, tapering distally; about twice as long as broad at base; proximal margin about twice as long as posterior margin; dorsal surface with two pairs of spines both situated in posterior half, at some distance from lateral margin; posterior margin slightly rounded with two pairs of slender posterolateral spines, mesial more than twice length of lateral ( Fig. 4l View Fig ); median portion between spines with about six elongated plumose setae; anal tubercles absent.

Gill formula as given for genus. Number of eggs relatively low (8 in smaller ovigerous female, 15 in larger ovigerous male); eggs relatively large (diameter 0.58 x 0.45 mm to 0.88 x 0.50 mm).

Coloration. – Ground colour beige-ochre, mottled with more or less intense red and pink, especially on the abdomen, where red chromatophores may form diffuse transverse bands; some carapace areas appearing golden; appendages and tail fan mostly colourless (description based on two freshly dead specimens) ( Fig. 5 View Fig ).

Size. – This species is one of the smallest alpheid and caridean shrimps: the smallest and the largest ovigerous specimens measure only 2.10 mm CL (5.90 mm TL) and 2.45 mm CL (6.95 mm TL), respectively.

Etymology. – The Latin adjective pusillus (tiny, dwarf) obviously refers to the very small size of this shrimp.

Habitat. – Coral reef wall, in crevices of dead corals, and in coral rubble on sand and gravel, at depths of 20-24 m.

Parasite. – The holotype was parasitized by a hemiarthrine bopyrid isopod (female), attached under the abdomen ( Fig. 1a View Fig ).

Distribution. – Presently known only from the type locality, Panglao, off southwestern Bohol, Philippines.

Remarks. – Acanthanas belongs to a group of genera characterized by the absence of arthrobranch on the third maxilliped and the presence of an articulated triangular plate on the sixth abdominal somite. These include Athanas Leach, 1814 ; Arete Stimpson, 1860 ; Aretopsis De Man, 1910 ; Athanopsis Coutière, 1896 ; Pseudathanas Bruce, 1983 ; and two undescribed genera (A. Anker, pers. obs.). Other features shared by these genera are the presence of small exopods on the first and sometimes the second pereiopods (except in Pseudathanas ), and the presence of a rostrum and orbital teeth (except in one of the two undescribed genera, A. Anker, pers. obs.).

Acanthanas appears to be most closely related to Athanas . However, the presence of two spine-like projections on the eyestalks and two pereiopodal epipods (instead of three in Athanas ) immediately separates Acanthanas from Athanas . Further distinguishing features of Acanthanas are the more concealed eyes; the presence of a subtriangular projection on the distodorsal margin of the cheliped merus and on the distal margin of the protopod of the second pleopod; the presence of a peculiar blunt tooth on the dorsomesial region of the antennal basicerite; and the absence of spines on the ischium of the chelipeds (usually present in Athanas ).

Acanthanas may be separated from Arete by the development and shape of the chelipeds; the presence of two acute projections on the eyes; and the carpus of the second pereiopod bearing five segments (vs. four in Arete ). The remaining “athanoid” genera, namely Aretopsis , Athanopsis and Pseudathanas , are more distantly related to Acanthanas , new genus, and may be easily separated from it by numerous features on the chelipeds, frontal margin, uropods etc. ( Bruce, 1969, 1983; Miya, 1980; Berggren, 1991).

The incisor process of the mandible of Acanthanas pusillus is conspicuously expanded and distally furnished with minute to small-sized teeth. This feature, however, may not be of generic importance since a similar condition is also found in Athanas areteformis Coutière, 1903 , A. verrucosus Banner & Banner, 1960 and A. borradailei (Coutière, 1903) (A. Anker, pers. obs.). Another interesting and yet to be explained feature of Acanthanas is the presence of an appendix masculina in one of the two ovigerous specimens. Similar observations were made in Athanas ornithorhynchus Banner and Banner, 1973 ( Marin et al. 2005.) and Aretopsis amabilis De Man, 1910 ( Nomura, 1986) . In Arete spp., the presence of an appendix masculina in egg-bearing individuals is related to protandric hermaphroditism ( Suzuki, 1970; Gherardi & Calloni, 1993). However, the small sample size of Acanthanas – three specimens – does not allow further assumptions. The relative small number of large (compared to the body) eggs may indicate an abbreviated larval development.

The chelipeds of Acanthanas appear to be among the least specialized within the family Alpheidae . They are barely enlarged and end in a very simple claw with unarmed fingers. Similar chelipeds occur in females of some Athanas species (e.g., A. marshallensis Chace, 1955 ).

The finding of Acanthanas pusillus may have been possible because of the very intensive sampling over a relatively extended period (five weeks). Even so, only three specimens of this interesting “micro-shrimp” were collected, showing that very small-sized (nearly amphipod-sized) shrimps may be easily overlooked, escape or avoid being found by dwelling inside the smallest coral crevices.