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Desmodasys abyssalis sp. nov.—first record of a deep sea gastrotrich from hydrothermal vents

Alexander Kieneke*†∫ and Julia Zekely‡

*Institut für Biologie und Umweltwissenschaften, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany. †Forschungsinstitut Senckenberg, DZMB, 26382 Wilhelmshaven, Germany. ‡Department of Marine Biology, University of Vienna, 1090 Vienna, Austria.

∫Corresponding author, e-mail: akieneke@senckenberg.de

A new marine gastrotrich species, Desmodasys abyssalis sp. nov. (Gastrotricha: Macrodasyida), is described from deep sea hydrothermal vents of the East Pacific Rise based on light microscopy and scanning electron microscope observations. Desmodasys abyssalis was collected from artificial tubeworm aggregations exposed to the intermediate flow zone (medium productivity) and occurred in low abundance. Desmodasys abyssalis is the first record of a gastrotrich inhabiting a deep sea hydrothermal vent habitat and is the first description of a deep sea gastrotrich.

INTRODUCTIONThe occurrence of typical meiofauna taxa such as Nematoda (e.g. Dinet et al., 1988; Vanreusel

et al., 1997; Zeckely et al., 2006a,b) or Copepoda (e.g. Conroy-Dalton & Huys, 1999; Lee & Huys, 2000; Tsurumi et al., 2003; Gollner et al., 2006, 2007) related to deep sea hydrothermal vents is well documented, but not a single record of a gastrotrich species belonging to the vent meiobenthos has been published so far. Furthermore, there is no record of a macrodasyid gastrotrich living in or at the deep sea benthal. However, Gastrotricha are known from the deep sea (Radziejewska, 2002; Gutzmann et al., 2004), but they all belong to the taxon Paucitubulatina (Martínez Arbizu, Radziejewska, personal communications). In the course of an ecological investigation of hydrothermal vents along the East Pacific Rise (Governar & Fisher, 2007), we found specimens of a new species of the gastrotrich subtaxon Macrodasyida. The specimens were present in small sediment accumulations between some artificial device aggregations that mimick vestimentiferan tube worm (Riftia pachyptila) colonies. With the fixed material provided, we describe the new gastrotrich species Desmodasys abyssalis based on conventional light microscopy and SEM. To date, two other species of Desmodasys have been described from boreal, sublittoral organic deposits of shell gravel and fragments of calcareous algae (Clausen, 1965, 2000).

MATERIALS AND METHODSSampling

The meiofauna was associated with artificial tubeworm aggregations, which were constructed from flexible, low-grade polyvinyl chloride hose to resemble small clusters of Riftia pachyptila Jones, 1985. These were deployed at three locations within and near a diffuse flow site, ‘Tica’ (9°50.447'N 104°17.493'W, 2500 m depth), which is located on the segment of the East Pacific Rise between the Siqueiros and Clipperton transform faults. Riftia pachyptila naturally occurs there, growing on bare basalt. After a period of one year the aggregations were lifted using the DSV ‘Alvin’. They were placed into separate boxes and mounted on an ‘elevator’. Reaching the surface, the ‘elevator’ was recovered on deck of the RV ‘Atlantis’. On board the artificial aggregations were disassembled and rinsed thoroughly with cold, filtered seawater. Specimens and additional trapped sediment were collected on 1 mm, 250 µm and 63 µm sieves, fixed in 10% formalin for 24 h and then transferred to 70% ethanol (for further information see Govenar & Fisher, 2007). Meiofauna fractions (1 mm to 250 µm and 250 µm to 63 µm) were examined at the University of Vienna for a total meiofauna community study. Gastrotrichs were found in two of the nine samples.

Specimen preparationFor light microscopic observation, specimens were transferred to a glycerol:water solution (1:9)

and after slow evaporation mounted in pure glycerol on microscopic slides. Coverslips were sealed with nail varnish. Observations of the specimens were carried out on a Leica DMLB compound microscope equipped with differential interference contrast (DIC). For documentation, we used an adapted digital camera (Color View I, Olympus) and the AnalySIS® software (soft imaging systems). Some of the LM-micrographs presented in this paper were combined from multiple images of different focal depth using the Helicon Focus® software.

A. Kieneke and J. Zekely Desmodasys abyssalis sp. nov. (Gastrotricha)

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The specimens for the scanning electron microscope (SEM) observations were dehydrated in an increasing ethanol series (using a special Teflon container closed by two copper grids usually applied to transmission electron microscopy), critical point dried and mounted on a round coverslip coated with a thin layer of special wax (TempFix). The coverslip was transferred to an SEM stub and specimens were coated with gold. Observation and documentation was done with a Zeiss DSM 940 scanning electron microscope.

We have analysed seven specimens prepared as LM whole-mounts and nine specimens prepared for SEM (total N=16). Positions of organs and structures are referred to as percentage body units (U) as it has been done in recent gastrotrich descriptions.

SYSTEMATICSOrder MACRODASYIDA Remane, 1925 [Rao & Clausen, 1970]

Family TURBANELLIDAE Remane, 1925Genus Desmodasys Clausen, 1965

Desmodasys abyssalis sp. nov.(Figures 1–4)

DiagnosisDesmodasys with a mean length of 250 µm up to a maximum of 300 µm (for the emended

diagnosis of the genus see Clausen, 2000). Due to a constriction at the level of the pharyngeal pores and another one at the furcal base, the body is subdivided into a head region, a trunk region and the caudal furca. Pharynx with a cylindrical buccal cavity, a posterior swelling and lateral projections that lead to the pharyngeal pores. Midgut continuously tapering towards the mid-ventral anus. Up to 10 frontal adhesive tubes in each ventrolateral tuft, a maximum of 14 tubes on each branch of the furca. Paired anterior testes with short, forward deflecting vasa deferentia that fuse below the midgut and discharge into the mid-ventral gonopore. Paired posterior ovaries and one or two large ova in a mid-dorsal position. A bulbous caudal organ posterior to the anus. Ventral locomotive cilia form a more or less homogeneous field reaching from the head region to the anus.

Type materialA single microscopic glycerol wholemount with the type specimen and two further paratypes is

deposited at the collection of Forschungsinstitut Senckenberg, Frankfurt/Main (inventory numbers: holotype, SMF 1; paratype 1, SMF 2; paratype 2, SMF 3). Two further slides with four specimens and an SEM stub with nine specimens are kept in the collection of the first author.

EtymologyThe species name abyssalis derives from the Greek word abyssal that means belonging to the

depth.

DescriptionHabitus. Like its two congeners, Desmodasys abyssalis is a compact macrodasyid gastrotrich with

a mean body length of 250 µm (Figures 1 & 2). The body has two distinct constrictions, one at the level of the pharyngeal pores at U28 (mean diameter: 45 µm) and another one at the base of the caudal furca at U92 (mean diameter: 26 µm). The head region (reaching from the mouth opening to the constriction at the level of the pharyngeal pores, see Figure 1) tapers from around U10 towards the frontal end, this has a 15 µm wide, blunt margin. The trunk region (reaching from the constriction at the level of the pharyngeal pores to the base of the caudal furca, see Figure 1) has a conspicuous maximum width at U50 (mean diameter: 63 µm). Caudally, the trunk of D. abyssalis continues in a bilobed, fin-shaped furca. Each branch of the furca has a length of nearly 30 µm (exclusive of adhesive tubes) and a width of 15 µm at its base tapering to a width of 5 µm at its tip. The angle between the furcal branches is 65° to 85° (Figures 3C & 4G).

Sensory devices. Distinct sensory devices such as pigmented eye spots or solid appendages (auricles or tentacles) were not observed in D. abyssalis. What we have found are two pairs of lateral, pear-shaped structures at U10 and U87, respectively (Figures 1, 3B, 4B & F). Each of these organs protrudes through the body wall with its slender, apical neck whereas the more voluminous, proximal part lies within or below the epidermal lining (Figure 3B: inset). In the descriptions of the other two species of Desmodasys, equal structures are indicated in the drawings and referred to as epidermal glands of the so-called banded secretion type (Clausen 1965, 2000). We think, these ‘glands’ of conspicuous shape are sensory organs and the filiform structures inside them may represent sensory cilia or microvilli.

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Adhesive tubes. Desmodasys abyssalis has a paired group of frontal adhesive tubes at U20 and a paired group of caudal adhesive tubes at each branch of the furca at U100 (Figure 1). There are no lateral-, dorsolateral- or ventral adhesive tubes. The frontal adhesive tubes are arranged in two prominent, ventrolateral tufts. Each tuft comprises a mean number of nine tubes. Shorter tubes (length: up to 10 µm) are situated medially in each tuft, whereas the longest ones (length: up to 25 µm) are situated laterally (Figures 3A, 4A,C,F).

The caudal adhesive tubes are bilaterally arranged along the inner margins of both branches of the caudal furca (Figures 3A,C & 4B,G). There is a mean number of twelve tubes per side. The length of the tubes increases from the base of each branch (length: up to 7 µm) to its tip (length: up to 20 µm). However, neighbouring tubes of each row weakly alternate in length (Figures 3C & 4G). In fixed specimens, adhesive tubes of both the frontal as well as of the caudal group are often curved distally (Figure 3A). This may be correlated to a certain flexibility of tubes in living animals. Each adhesive tube has a diameter of around 1.5 µm. Therefore, they have a very high length to width ratio and appear extremely long and thin.

Digestive tract. Desmodasys abyssalis has a terminal to slightly subterminal, roundish mouth opening, 3.5 µm in diameter (Figure 4C). There are no cuticular supportive structures such as hooks or ridges. The medium spacious, cylindrical buccal cavity reaches from the mouth to U5 and has a mean diameter of 7.5 µm. Caudally, it tapers and passes into the pharynx lumen. The buccal cavity wall bears two, caudally pointing pedicles (Figure 3B). The pharynx reaches a third of the length of the whole animal and terminates at U35 with the pharyngeo-intestinal junction. The pharyngeal diameter increases continually from the mouth (15 µm) towards the pharyngeo-intestinal junction (nearly 30 µm). Here, together with two lateral protrusions which lead to the pharyngeal pores at

Figure 1. Desmodasys abyssalis sp. nov. Habitus and internal organization, ventral view (locomotory cilia not displayed). an, anus; bc, buccal cavity; cat, caudal adhesive tubes; co, caudal organ; fat, frontal adhesive tubes; fgp, female gonopore; fo, putative frontal organ; mg, midgut; mgp, male gonopore; oo, mature oocyte; ov, ovary; ow, ovarial wall; ph, pharynx; pp, pharyngeal pore; sd, putative sensory device; tes, testis; vd, vas deferens.

Figure 2. Desmodasys abyssalis sp. nov. Ciliary pattern, ventral view. Reconstruction according to SEM images. an, anus; cop, caudal organ pore; lch, locomotory cilia of the head region; lct, locomotory cilia of the trunk region; mgp, male gonopore; pp, pharyngeal pore; sc, sensory

cilia; sd, sensory device.

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Figure 3. Desmodasys abyssalis sp. nov. Light microscopic images (differential interference contrast). (A) Habitus; (B) head region; inset: close-up of the putative sensory device; (C) caudal furca; (D) anus and caudal organ; (E–G) testis at different focal depth; (H) ovaries; note the ovarial wall (arrow); (J) putative frontal organ; (K) ventral view of the mid trunk region; distal portion of the vasa deferentia indicated by arrows; (L) lateral view of a whole specimen. Note the voluminous mature oocyte. an, anus; bc, buccal cavity; cat, caudal adhesive tubes; co, caudal organ; fat, frontal adhesive tubes; fgp, female genital pore; fib, fibres of the furca; fo, putative frontal organ; lm, longitudinal muscles; mg, midgut; mgp, male genital pore; oo, mature oocyte; ov, ovary; ph, pharynx; pp, pharyngeal pore; sp, spermatozoa;

spg, spermatogonia; vd, vas deferens.

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U28 (Figure 4D), the pharynx has a slight swelling (Figures 1 & 3B). The pharynx is continued by the midgut that expands from U35 to U83 where it terminates in the mid-ventral anus (Figures 3D & 4H). The intestinal diameter constantly decreases from the pharyngo-intestinal junction (nearly 25 µm) towards the anus (10 µm). There is no separate hindgut in D. abyssalis.

Reproductive system. Desmodasys abyssalis is a simultaneous hermaphrodite that contains a pair of mature bilateral testes and a pair of bilateral ovaries with one or two mature, vitellogenic oocytes (Figure 1). The testes have a maximum fronto-caudal extension from U32 to U48. The proximal

Figure 4. Desmodasys abyssalis sp. nov. Scanning electron microscopic images. (A) Ventro-lateral view of a complete specimen; (B) lateral view of the furca; (C) frontal view of the head region; (D) close-up of the pharyngeal pore; (E) close-up of the lateral body wall; note the high number of sensory cilia (arrows); granules at the ventral surface are insertions of locomotory cilia; (F) lateral view of the head region; note the locomotory cilia (arrowheads); (G) dorsal view of the furca; (H) ventral view of the rear trunk region. an, anus; cat, caudal adhesive tubes; cop, caudal organ pore; fat, frontal adhesive tubes; sd, sensory

device.

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portion is pointed frontally and forms a hollow organ filled with mature, filiform spermatozoa. The wall of each testis is provided by the solid spermatogonia (Figure 3E–G). Distally, each testicular lumen continues in a seminal duct (vas deferens) that runs caudally for a short distance and bends frontally at U48. Both seminal ducts fuse and open into the unpaired, mid-ventral male gonopore (Figure 3K). Seminal duct wall cells could not be determined.

The paired ovaries lie alongside the midgut and extend from around U65 to U83. Early oocytes are arranged as a longitudinal column of cubic cells with voluminous nuclei (Figure 3H). There might be a thin sheath of ovarian wall cells (Figure 3H, arrow). Oocytes mature in a frontal direction. Around U65 both ovaries fuse to form a uterus region where one or two mature, vitellogenic ova rest in a mid-dorsal position (Figure 3L). The uterus region extends frontally to U40. We have not detected a uterine wall. However, in some specimens we found a lateral pore on the left side of the trunk at U63 that we interpret as the female gonopore (Figure 3A).

A compact, ovoid caudal organ (copulatory organ) is situated caudal to the anus and extends from U85 to U92 (Figures 1 & 3D,L). It has a ventral protrusion which arises from the body wall and exhibits the caudal organ pore (Figure 4H). It has a length of 20 µm and a maximum width of 10 µm. A second accessory reproductive organ, the frontal organ (seminal receptacle), could be present in D. abyssalis. In some specimens, we found a sack-shaped structure on the right side of the animal between U63 and U76 (Figures 1 & 3J). There was no evidence of spermatozoa, foreign or auto sperms, inside the putative accessory reproductive organs.

Musculature. In the fixed specimens, we could only determine strong longitudinal muscle strands in the rear trunk region running alongside the caudal organ and the midgut (Figures 1 & 3D). Additionally, there are thin fibres running through the branches of the furca. Each fibre seems to be associated with an adhesive tube (Figure 3C). This might be a special musculature which is provided for movements of the tubes or the furca in toto.

Ciliation. Many of the cilia, locomotory as well as sensory, were lost during fixation, extraction or preparation of the specimens. However, we could detect cilia and the insertions of them on some of the specimens prepared for SEM (see Figure 4E & F). Taking these data, the ciliary pattern could be reconstructed (Figure 2). Desmodasys abyssalis has a completely ciliated ventral surface, there are no separated rows or tufts. The density of cilia at the ventral head region frontal to the adhesive tubes is nearly twice as high as on the trunk region (Figure 2). Sensory cilia are present along the flanks of the animal in distances of around 15 µm (Figure 4E).

Epidermal glands. Beside the two pairs of pear-shaped structures, that we interpret as sensory organs (see above), Desmodasys abyssalis has no visible epidermal glands.

Taxonomic remarksThe new described species definitely belongs to the genus Desmodasys because it possesses

all characteristics of that taxon. It has only two groups of adhesive tubes. One pair of frontal tubes is arranged in elongated, ventro-lateral tufts, and one pair of caudal tubes is situated at the medial margins of the furca, shorter caudal adhesive tubes characteristically alternate with the longer ones. Additionally, Desmodasys abyssalis sp. nov. has a typical constriction at the level of the pharyngeal pores and lacks a medial, caudal cone. However, the general appearance of Desmodasys abyssalis sp. nov. is fairly similar to those of the two other Desmodasys species, Desmodasys phocoides Clausen, 1965 and Desmodasys borealis Clausen, 2000, but it has some differences that distinguish it clearly from its congeners: (1) both D. phocoides and D. borealis with lengths of 400 and 430 µm (measurements of the holotypes) are much larger than mature D. abyssalis with a mean body length of 250 µm; (2) D. abyssalis lacks the two dorso-laterally arranged columns of epidermal glands; (3) testes are much smaller in D. abyssalis than in the other Desmodasys species. In D. abyssalis, they just reach 15% of the whole body length, whereas in D. phocoides and D. borealis they are longer than a quarter of the body length; (4) the buccal cavity is a little more spacious in D. abyssalis and the pharynx increases in width towards the pharyngo-intestinal junction as opposed to the pharynx of D. phocoides and D. borealis that have a constant width; (5) D. abyssalis has a lower number of adhesive tubes in the anterior group (maximum number of 10 tubes) than D. phocoides (maximum number of 14 tubes) and D. borealis (maximum number of 16 tubes); (6) ecologically, D. abyssalis is the only member of the genus inhabiting a chemoautotrophic deep sea environment.

However, D. abyssalis does share some features with the other Desmodasys species. The somewhat conical frontal end of D. abyssalis resembles that of D. phocoides. These two species also share, with a maximum of 14 and 15 tubes per furcal branch, an almost equal number of caudal adhesive tubes. Desmodasys abyssalis and D. borealis are both characterized by a more fin-shaped furca in contrast to the less prominent caudal lobes of D. phocoides.

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Ecology and distributionDesmodasys abyssalis was present in two of three artificial tubeworm aggregations in the

intermediate flow zone (for the general experimental design and results see Govenar & Fisher 2007). In sample M1 (medium productivity zone) there was a total number of 45 animals (1.5 animals per 10 cm2 tube surface), in sample H2 (high productivity zone) there were only two animals (0.06 per 10 cm2). Measurements of water temperatures at ‘Tica’ in 2002 yielded a value of around 10°C in the intermediate flow zone.

DISCUSSIONThe presence of a macrodasyidan gastrotrich, Desmodasys abyssalis, at a hydrothermal vent along

the East Pacific Rise is a significant discovery and one that opens up new possibilities for further exploration of deep sea meiofauna. Gastrotrichs are generally considered soft-bodied and delicate, yet, the current study reveals that processing of deep-sea specimens is still possible, and can even yield excellent material for new species descriptions. With this discovery, it will be necessary to amend current diagnoses of gastrotrich ecology and biogeography and include specialized deep sea environments as part of their habitat.

Clausen (2000) suggests two main adaptations of Desmodasys species to its habitat: (1) the numerous and long adhesive tubes in Desmodasys might be a special feature for living in exposed biotopes like the coarse organic deposits on which D. phocoides and D. borealis were found; and (2) as the hitherto described species of Desmodasys are only known from two boreal localities in Norway, these species are regarded to be cold water forms. We think these features can be considered as pre-adaptations for settling the deep sea hydrothermal vent biotope. Here, Desmodasys abyssalis lives on the tube walls of Riftia pachyptila, a hard substratum that is exposed to the hydrothermal flow. Additionally, water temperatures along the intermediate flow zone of a hydrothermal vent field may be comparable to the temperature condition at boreal coasts.

While the food spectrum of gastrotrichs comprises microalgae, diatoms and bacteria, we presume the chemoautotrophic, sulphide-oxidizing bacteria, which are the most abundant microbes at the surface of Riftia-tubes (Lopez-Garcia et al., 2002) to be the sole nutrition resource for Desmodasys abyssalis. As endemic hydrothermal vent fauna is often well adapted to the toxic sulphide exposure (Childress & Fisher, 1992), we expect that D. abyssalis possesses mechanisms to protect its tissues from the sulphide inhibition of aerobic cell respiration. The major presence of D. abyssalis at the medium productivity zone may indicate a small range of physical, chemical and biological factors which are tolerated by this species. However, we cannot preclude that hydrothermal vents are not the sole habitat of D. abyssalis.

We hope that future studies on the hydrothermal vent biocenosis will consider the Gastrotricha as well. Perhaps there are further undiscovered species that will give more insights to this newfound habitat for marine Gastrotricha.

This work would not have been possible without the expertise and assistance of the captain and crew of the RV ‘Atlantis’ and the pilots and crew of the DSV ‘Alvin’ in shipboard and submersible operations in 2002 (AT 7-26) and 2003 (AT 11-03) cruises to the East Pacific Rise. Financial support was given by the Austrian Science Foundation grant FWF (P16774-B03 to M.B.), US National Science Foundation grant (OCE-0002729 to C.R.F.) and the International Office Vienna and Promotion Grants from the University of Vienna as well as the CheSS TAWNI Award (all to J.Z.). A.K. was financially supported by the German Science Foundation (DFG) through a research grant (AH 94/3-1 and -2, MA 2557/4-1 and -2). Furthermore, we want to express our gratitude to B. Govenar and C.R. Fisher, who designed and constructed the artificial tubeworm aggregations and shared their sampled fauna with us. Two anonymous referees significantly improved the manuscript.

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Submitted 26 June 2007. Accepted 22 August 2007.