Acanthodasys caribbeanensis sp. n., a new species of Thaumastodermatidae (Gastrotricha, Macrodasyida) from Belize and Panama

Abstract We describe one new species of Acanthodasys (Gastrotricha, Macrodasyida, Thaumastodermatidae) collected from sublittoral sites around Carrie Bow Cay, Belize and Isla Colón in the Bocas del Toro archipelago, Panama. Though eight species of Acanthodasys are currently recognized, no species has yet been reported from the Caribbean. Acanthodasys caribbeanensis sp. n. is characterized by the lack of lateral adhesive tubes, the presence of ventrolateral adhesive tubes, and with cuticular armature in the form of both spineless and spined scales. The spineless scales are not elliptical as in other species of Acanthodasys, but are instead variable in shape and closely resemble the spineless scales of species of Diplodasys. Spined scales bear uniancres up to 50 µm long and are the largest reported in the genus. Uniancres are arranged dorsally around the mouth rim and distributed in five distinguishable columns. Adult size varies from 325–625 µm long.


Introduction
Th e phylum Gastrotricha contains some of the smallest (0.1mm-3mm) benthic marine invertebrates. In general, marine gastrotrichs are restricted to the interstitial pore spaces between sand grains, making them overlooked in most tropical biodiversity investigations. Knowledge of gastrotrich biodiversity in the Tropical Northwestern Atlantic (aka wider Caribbean) is almost entirely limited to species lists or descriptions of novel taxa from only a handful of sites in the fi ve Caribbean ecoregions: the Gulf of Mexico (Todaro 1994;Todaro et al. 1995), South Florida ecoregion (Th ane-Fenchel 1970;Schoepfer-Sterrer 1974;Decho et. al. 1985;Evans and Hummon 1991;Evans 1994), Bahamanian ecoregion (Renaud-Debyser 1963), Lesser Antilles ecoregion (Kisielewski 1984;Hummon 2010) and Central Caribbean ecoregion including Colombia (Hummon 1974) and Panama (Hochberg 2008). Additional unpublished records of marine gastrotrich diversity and biogeography can be found in Hummon (2009).
Here, we document a new species of Acanthodasys (Macrodasyida, Th aumastodermatidae) from two Central Caribbean islands, Carrie Bow Cay on the Belizean barrier reef complex and Isla Colón in the Bocas del Toro archipelago, Panama. Investigations on both islands were conducted as part of a Smithsonian sponsored biodiversity project to barcode the local marine meiofauna.

Methods
Gastrotrichs were collected from a variety of sublittoral sites around Carrie Bow Cay, Belize (Jan.13-28, 2010) and Bocas del Toro, Panama (June 6-19, 2010). Research was performed at the Smithsonian fi eld station on Carrie Bow Cay, Belize and Smithsonian Tropical Research Institute, Bocas del Toro, Panama, respectively. Sediments were collected by SCUBA in bags and buckets and brought back to the fi eld stations for subsampling. Extraction of gastrotrichs was performed with the following protocol: 1) approximately 100 cm 3 of sediment was combined with ca. 900 cm 3 of 7.5% MgCl 2 in a 1 L Erlenmeyer fl ask and allowed to rest for 15 mins; 2) the fl ask was gently shaken and the supernatant was decanted over a 48 μm mesh; and 3) the mesh was gently washed with seawater into a petri dish. Specimens were sorted with a Leica EZ4 stereomicroscope, transferred to a glass slide, and viewed with a Zeiss A1 compound microscope equipped with DIC (diff erential interference contrast). Light micrographs were captured with a Sony Handycam digital camera. Measurements of individual specimens were performed with an ocular micrometer. Lengths and positions of organ systems are described in terms of percentage body units, where total body length from anterior (U00) to posterior (U100) is 100 units.
Specimens were prepared for museum archival using the following protocol: fi x in 2.5% gultaraldehyde in 0.1M phosphate buff er saline (PBS; pH 7.4) for at least one week; rinse with PBS for 1 hr; expose to 1% OsO 4 in 0.1M PBS for 1 min (to increase constrast); rinse in PBS for 15 min; dehdryate through an ethanol series; transferr to propylene oxide for 30 min; and embed in epon on a glass microscope slide (coverslipped and placed in an oven a t 60ºC for 24 hrs). Type specimens are deposited in the National Museum of Natural History, Smithsonian Institution, Washington, DC.
Abbreviations: PIJ, pharyngeointestinal junction; TbA, anterior adhesive tubes below mouth rim; TbL, lateral adhesive tubes; TbP, posterior adhesive tubes on caudal pedicles; TbVL, ventrolateral adhesive tubes. Material examined. Ten specimens. Th ree prepared for archival. Diagnosis. Acanthodasys with an adult body length from 375 μm to 625 μm long. Maximum body width at mouth/PIJ/midpoint of body is 30/62/100 μm. Pharynx to 238 μm long. Cuticle of spineless and spined scales. Spineless scales of various shape. Spined scales bear uniancres to 50 μm long, distributed as one dorsal column fl anked by two dorsolateral columns and two lateral columns. Uniancres also extend dorsally across mouth rim. Ten epidermal glands per side. Up to nine TbA per side inserting directly on body surface at mouth rim. TbL absent. At least 43 TbVL per side beginning at PIJ and extending onto the outer edges of the caudal lobes. Two TbP insert terminally on caudal lobes, two insert medially. Hermaphroditic, with paired testes and single glandular caudal organ. Rosette organ on left dorsolateral side of body; single ovary present.  Etymology. Named after the region of origin, referring to the Caribbean (Latin adjectival ending: ensis; caribbeanensis).

Order
Description. Th e description is based on the holotype (adult, 625 μm long), with ranges given from specimens measured in vivo. Body strap-shaped and 425-625 μm long (Fig. 1A). Mouth region narrow; body infl ates at U04, narrows again at U08, and widens toward the trunk. Widths of mouth at U04/pharynx region at U08/PIJ at U36/trunk at U50/caudal base at U92 are 30/67/105/125/20 μm. Pharynx 238 μm long with pharyngeal pores at U32. Dorsal rim of oral hood, ca. 10 μm wide, free of cuticular spines and fringed with numerous mobile cilia and few stiff sensory hairs to 10 μm long. Epidermal glands to 18 μm diameter and to 10 per side. Ventral locomotory cilia present as a complete fi eld beginning at ca. U08 and extending to the caudum (Fig. 2B). Cuticular armature. Cuticular spines evident at low magnifi cation and arranged around the periphery of the head, along the trunk and on the caudal lobes (Figs 1, 2). Spined-and spineless scales present. Approximately 10-12 oral uniancres line the dorsal periphery of the head behind the "naked" region of the oral hood (Fig. 1B). Uninacres slightly staggered in position; uniancres increase in size from medial position (10 μm long) to a lateral position along head (U06, 23 μm long). Uniancres in lateral and dorsolateral positions continue along the trunk as individual columns. Th e trunk contains a total of fi ve columns of uniancres: one dorsal, two dorsolateral, and two lateral columns (Figs 1A, 2A). Sixteen uniancres in dorsal column begin at U08 and extend to U80, increasing in size from 20-50 μm long down the length of the body, and with a slight bend or posterior curvature at the apex (Fig. 3B). Dorsolateral columns of ca. 14-16 spines that increase in length along the trunk, from 17-35 μm long and with a slight posterior curvature or distinguishable bend (Fig. 2D); column ends at U85. Lateral columns of 23-26 spines that increase in length along the trunk, from 20-35 μm, and mostly with a straight shape but oriented in a slight posterior direction; column ends at U87. Five small uniancres present on each caudal lobe, 8-20 μm long, in somewhat dorsolateral position (Fig. 1C). Cuticle between uniancres present as spineless scales. Scales of various shape; most dorsal scales have an obtuse triangular shape with the apex pointed posteriorly; some ventral scales same shape as dorsal scales; others of varying shapes (Figs 1B, 2E, 3A). Center of each scale with a triangular or eye-shaped depression. Scales ca. 15 μm wide and arranged in 5 columns on dorsal surface; ventral surface also with scales but the number of columns was undetermined.
Adhesive tubes. Anterior adhesive tubes (TbA), 8 per side to 12 μm long, distributed along the mouth margin in a single row (Fig. 2B,C). TbL are absent. TbVL arranged in bilateral columns, to 23 μm long, begin at PIJ and extend posteriorly on to the lateral edge of the caudal lobes; tubes on caudal lobe edges each ca. 15 μm long (Fig. 2B). TbP distributed on caudal lobes as two terminal tubes to 16 μm long and two medial tubes on the inner edge of the caudal lobes to 12 μm long.
Digestive tract. Mouth terminal to 30 μm wide. Pharynx to 238 μm long with pharyngeal pores at base around U32. Intestine narrow and tapering toward posterior end. Anus at U88.
Among the eight described species, only A. aculeatus Remane, 1927 andA. arcassonensis Kisielewski, 1987 possess spineless scales distributed among their uniacres as found in A. caribbeanensis sp. n. However, the new species diff ers from both described species in several signifi cant ways: the head profi le diff ers between A. caribbeanensis sp. n. and A. arcassonensis; the uniancres are arranged in distinguishable columns in the new species (Figs 1, 2); the uniancres are larger in A. caribbeanensis sp. nov (up to 50 μm long) compared to all other species (e.g, A. arcassonensis to 11 μm long; A. fi borosus to 20 μm); the spineless scales are not elliptical in the new species but instead variously shaped; and the new species possesses adhesive tubes in a ventrolateral (TbVL) as opposed to a lateral (TbL) position. Th is latter characteristic is noteworthy because the arrangement of TbVL of A. carribeanensis sp. n. is more similar to the arrangement observed in species of Diplodasys, the sister taxon of Acanthodasys within the Diplodasyinae (Ruppert 1978), than to other species of Acanthodasys (e.g., compare to A. fl abellicaudus Todaro, 2009 andD. sanctimariae Hummon &Todaro, 2009).
Interestingly, there are several aspects of the cuticle of A. caribbeanensis sp. n. that appear unique and worth further mentioning. First, the uniancres of the new species are the largest described spines (see Fig 1A; up to 50 μm long on an adult specimen 625 μm long) in the genus and the largest spines known from any species of Macrodasyida. Second, the spineless scales of A. caribbeanensis sp. n. are more similar in appearance to the spineless scales of species of Diplodasys than they are to the scales descibed for other species of Acanthodasys. For example, the scales of D. minor Remane, 1936 have a similar shape to those of A. caribbeanensis sp. n., and several species have scales with a wide depressed region in their center like the scales of A. caribbeanensis (e.g., see D. ankeli ssp. pacifi ca Schmidt, 1974; comparison of species' scales in Kisielewski 1987). Note, however, that the scales of A. caribbeanensis sp. n. appear to lack any obvious ribbing or texture (Fig. 3A) that often characterizes the scales of species of Diplodasys.
Another peculiar feature of Acanthodasys caribbeanensis sp. n. is the relatively wide size distribution of reproductive adults. We found specimens ranging from 325 μm long in Panama to 625 μm long in Belize, and all specimens showed clear evidence of reproductive maturity i.e., paired testes and/or a solitary rosette organ were present (Fig. 3C). According to Ruppert (1978), the rosette organ is a glandular ring around the female gonopore, but whether it is present in specimens prior to the maturation of female gametes is not mentioned. According to our surveys, specimens of A. caribbeanensis sp. n. may have a rosette organ but no obvious ova or testes. As evidence, we have prepared a paratype (Cat. No. USNM 1145898) of a smaller adult specimen, ca. 400 μm long (slightly curled), with a clear rosette organ but without evidence of eggs or testes. Unfortunately, the rosette organ can be diffi cult to see in live specimens, but becomes apparent if animals are fi xed briefl y in OsO 4 (see methods); the OsO 4 greatly enhances the contrast of many tissues including the epidermal glands (see Fig. 3C).