“TUBULAR GILLS” Extreme gill modification in the Thyasiroidea with the description of Ochetoctena tomasi gen. et sp. nov. (Bivalvia: Thyasiroidea)

Three dimensional tubular structures of the ctenidium of some thyasirid bivalves are described for the first time. The classification of the thyasirid gill is modified accordingly into five types based on the number of demibranchs, reflection of the filaments and shape of the filaments, either rod, laminar or tubular. The tubular structure is seen in its most modified form in a chemosymbiotic abyssal species from the south-east Atlantic, which is described here as Ochetoctena tomasi gen et sp. nov.


Introduction
Chemosymbiotic bivalves typically host their autotrophic bacteria in tissues of the ctenidium (gill). As a consequence gill morphology is modified and from a gross perspective these gills are thick and fleshy in comparison with those adapted for filter feeding (Taylor and Glover 2010). The fleshy nature of the gills of thyasirids and lucinids was recognised long before their function was discovered and was shown to be a result of elongation of the abfrontal zone of the filaments (Allen 1958). Bacterial symbiosis associated with this modification was confirmed by a number of studies in the 1980s ( see review by Taylor and Glover 2010). In the heterodont families Lucinoidea and Vesicomyidae the association is apparently obligate but the Thyasiridae display both symbiotic and asymbiotic taxa. Variation in thyasirid gill structure was first demonstrated by Southward (1986), and later in a wider Recently I was sent two thyasirids from the Regab pockmark (Gulf of Guinea) to identify and name following their inclusion in the bacterial study of Rodrigues and Duperron (2011). From a cursory examination it is immediately apparent the gill filaments are not laminar but that the gill is a three dimensional network of tubules.
Using scanning electron microscopy this paper describes the gross structure of these "tubular" gills and compares them with those described by Dufour (2005), especially the lamellar (Type 3) gill associated with chemosymbiotic taxa. Both Conchocele and the Regab thyasirid have been shown to be chemosymbiotic (Imhoff et al 2003;Rodrigues and Duperron 2011). Comparisons with the lucinoid gill are made and finally the Regab thyasirid is described as Ochetoctena tomasi gen. et sp. nov.

Methods
All specimens had been previously fixed in ethanol or formaldehyde of unknown concentrations. For scanning electron microscopy, gill tissues were excised and cut transversely and longitudinally using a thin razor blade. Tissues were dehydrated in 100% ethanol overnight and critically point dried with liquid CO 2 as the intermediate fluid in a Quorum K850 critical point dryer. Dried samples were mounted and gold coated before examination using a Jeol Neoscope SEM. Both demibranchs are present, the outer extending over approximately half of the inner (Figs 1A-C). The filaments are fully reflected forming a descending and ascending lamella. The filaments are extended abfrontally giving each a laminar form. The supra-branchial chamber is represented by the large dorsal space (sbc) between the descending and ascending laminae. Ventrally the abfrontal regions extend and fuse to form continuous inter lamellar septae of varying patterns (ils) (Figs 1A-C, 2A, B). Using Thyasira sarsi as typical of this group the micro-structure is as follows. All parts of the abfrontal surfaces and the septae (ils) are lined with polygonal bacteriocytes (Fig.  2B). The frontal face of the filaments are fully ciliated (Figs 2C, D) with a median band of frontal cilia (fc) bordered on both sides by a band of lateral frontal cirri (lfc) and behind these a band of lateral cilia (lc). If removed the ciliated surfaces can be recognised by a remaining intricate pattern of scars on the epithelial surfaces ( Fig. 2E, arrowed). Immediately behind the ciliated bands there is a network of inter-filament junctions (ifj) (Fig. 2D).

Institutional
In "Conchocele" excavata only alternate filaments fuse to form inter lamellar septae and the middle portions of these septae do not bear bacteriocytes (Fig. 2F).

The tubular/laminar filament (Type 4T/L) in Conchocele bisecta
Both demibranchs are present, the outer and inner of approximate equal size (Fig. 1D). The filaments are fully reflected forming descending and ascending lamellae and appear to be narrowly separate along their entire length. At low magnification the filament is seen to be extended abfrontally but rather than appearing to be laminate the frontal region appears as a series of small blocks but these are not apparent abfrontally. Under the SEM the frontal surface appears as a series of vertical bands with rows  Nakazima (1958) assumed that the cilia had been lost through poor preservation but if this was the case the intricate pattern of ciliary 'roots' would remain and be visible under the SEM (see Fig. 2E for comparison). The lateral margins of the frontal bands are smooth ( Fig. 3E) but between each band a small projection can be seen sitting on the inter filament junction (Fig.  3F, arrowed).
With the frontal bands ripped off a regular tubular structure is seen ( Fig. 3B) with inter filament junctions (arrowed). Viewed from the abfrontal face of a lamella the regular tubular structure is not apparent and replaced by laminar filaments (Fig. 3C abf-lam). Widely separated but torn inter lamellar junctions (ilj) are visible in Fig.  3C, and best seen in cross-section in Fig. 3D (ilj). In cross-section (Figs 3D, 4) it can be seen that the frontal zone (fz) is narrow consisting primarily of the angular bands. The tubular zone (tbz) is a little over half the thickness of the lamella with the remainder being laminar (lz). Inter-lamellar junctions maintain a series of inter lamellar spaces (Fig. 3D, ils). Behind the frontal zone and between the laminar and tubular zones are inter filament junctions (ifj).
The tubes are lined with densely packed bacteriocytes as is the surface of the laminar zone, the inter lamellar junctions do not bear bacteriocytes. Preservation was not sufficient to acquire detailed images of the bacteria.

The tubular filament (Type 5T) in Ochetoctena tomasi
Both demibranchs are present but the outer is about half the depth of the inner (Fig. 1E). The normal filament structure is not apparent even under low magnification but appears as a series of small blocks (Fig. 8F). In trans-verse view (Figs 5, 7C) the ascending and descending arm of each filament is seen to be made up of a series of tubules (lt), abfrontally these are fused to form a median tube (mt). There is a small dorsal supra branchial chamber (Fig. 5 sbc). In lateral section the tubes can be seen to open between the frontal faces of the filaments  Reid and Brand (1986). Sporadically there are bundles of filaments ( Fig. 7F ptf), 15 -20 μm in length, each with a swollen, paddle-shap tip. These are similar in form to structures described as hyphomicrobial cells observed in Syssitomya by . This interpretation is open to question given the observation of paddle shaped artifacts observed on bivlace cilia by Beninger, Potter & St-Jean (1995). Here these structures are not attached to the walls of the tubules and if they are cilia they are probably not in situ.
The bacteria in the bacteriocytes are small, subspherical measuring approximately 0.65 μm in diameter (Fig.  7F bct).

Discussion
The results presented here reveal further modification of the thyasirid gill towards a complex three-dimensional structure. The gill types described by Dufour (2005) can now be added to with two additional types; "tubular/laminar"  Figure 2F "Conchocele" excavata, longitudinal section of ventral portion of inner demibranch. abs abfrontal surface; fc frontal cilia; lc lateral cilia; lfc lateral frontal cirri; ifj inter filamental junction; ils inter lamellar septum; sbc supra branchial chamber.   and "tubular". The known gill types can be summarized as follows (Table 1) with a revised nomenclature based on that of Dufour 2005. The revised nomenclature incorporates the shape of the filaments and the number of demibranchs. Filaments that have no or little abfrontal extension are termed rods and these equate to Dufour Types 1 and 2. For Dufour Type 2 those with single demibranchs and non-reflexed filaments are termed "Type 2b (R)" while those with both demibranchs are termed "Type 2a (R)". The letters "L" and "T" refer to the laminar and tubular structures respectively. For each gill type the corresponding genera are listed, but some remain unallocated due to lack of data.
The extent of frontal ciliation is noted but the present observations give rise to difficulties in interpretation of the functioning of the tubular/laminar and tubular gills. All Type 3 (L) gills have complete ciliation of frontal and lateral cilia with eulateral cirri as also seen in the Lucinoidea (Taylor and Glover 2010). Here, it has not been possible to show any similar ciliation in Conchocele (Type 4T/L). Without ciliation it is difficult to comprehend how the currents to drive the water flow through the gill tubules are generated. Bernard (1972) records ciliary currents over the gill but gives no details of the ciliation. The lack of the basal structure of the cilia and cirri usually seen when the cilia are removed suggests that the Conchocele gill is atypical but its functioning remains enigmatic and requires observation on live material. It seems improbable that the small projections sitting on the frontal interfilamentar junctions, even if motile, could drive water currents into the tubules and could not produce the currents indicated by Bernard (1972).
Although there is an indication of frontal ciliation in Ochetoctena this is reduced and in both this genus and Conchocele they may be unable to create sorting and feeding currents on the gill.
Thyasirids typically hold their bacteria extra-cellularly (Dufour 2005) but the taxon Maorithyas hadalis is reported to hold the bacteria intra-cellularly (Fujiwara et al 2001). This unique condition suggests a further gill type   Fleming, 1950 is illustrated by Oliver and Sellanes (2005) and can be seen to be similar to Thyasira sensu stricto in both shell and anatomy. This tubular structure increases the surface area of the bacteriocyte zone and creates a more rigid network further facilitating the movement of water from the infra-branchial chamber to the supra-branchial chamber. Similar structures are present within the Lucinoidea; Dando et al (1985) noted the inter-lamellar bridges in Myrtea spinifera and Distel and Felbeck (1987) illustrat-ed a complex tubular structure in Lucinoma aequizonata. Such complex structures are found throughout the Lucinoidea but this gill differs in consisting of a only a single demibranch (Taylor and Glover 2010). The tubular structure is yet another convergent morphological feature shared by chemosymbiotic thyasirids and lucinids.
The gill structures described here can potentially impact on the systematics of the Thyasiroidea. The observations on the gills of Conchocele bisecta and "C" excavata indicate that these taxa are not congeneric and this is part of the subject of another paper that also describes a new genus containing species previously assigned to Conchocele (Oliver and Frey, in press).  Definition. Equivalve, Equilateral. Outline subcircular, lunule margin depressed, posterior margin with a single weak sinus. Posterior sulcus shallow but prominent. auricle absent. Escutcheon narrow, shallow. Ligament deeply sunken. Hinge edentulous. External sculpture of commarginal lines and growth stops, overall with microscopic conical, calcareous spines these randomly distributed with some linked by low ridges . Ctenidia of two demibranchs, filaments tubular, "Type 5T".
Distribution. Known only from the Regab pockmark off west Africa.  Etymology. Named for my son Tomas.
Discussion. The shell of Ochetoctena has a weak posterior sulcus and the escutcheon is excavated but lacks any auricle to support the sunken ligament. In this it differs from Thyasira sensu stricto where there is a well developed auricle and from Conchocele which is oblique with a very strong posterior sulcus. Species lacking an auricle are often placed in the genus Parathyasira (Oliver and Killeen 2002;Payne and Allen 1991). The type species of Parathyasira is P. resupina Iredale, 1930 (Fig. 9F) and has a shell microsculpture of radial rows of calcareous spines. This character is also seen in the Atlantic species P. granulosa (Monterosato, 1874), P. subcircularis Payne & Allen, 1991 (Fig. 9G-I) and an un-named species from the Arabian Sea (Figs 9A-E). The spines in these species are angular with strongly developed basal connections between the rows (Figs 9C-D). In contrast the spines in Ochetoctena are not arranged in rows, are conical and the basal connections are weak or absent (Figs 8C-E). The genus Spinaxinus is characterised by having a spiny microsculpture but this is entirely periostracal (Oliver and Holmes 2006;. Ochetoctena is the only thyasirid known to have ctenidia of the Type 5T structure; Conchocele has a partly tubular gills of the Type 4TL morphology. The ctenidia of these Parathyasira species show poorly developed abfrontal extension resulting in a flimsy open structure. Dufour (2005) reported that the gills of P. granulosa are of her type 2 (type 2aR above) suggesting that the genus Parathyasira is characterised by a weak symbiont partnership.
There are, therefore, shell and anatomical synapomorphies that separate Ochetoctena from all other known thyasirids.