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.

Gill morphology, Thyasiroidea , New genus, Regab pockmark

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 ranging study by Dufour (2005). Dufour (2005) classified thyasirid gills into three categories based on the number of demibranchs, the degree of abfrontal extension and the extent of bacterial symbiosis. In all three types the filaments were either rod-like or lamellar, the lamellar form having greatest abfrontal extension and strongest association with chemosymbiotic bacteria. This lamellar gill (type 3 of Dufour 2005) has recently been described from two other heterodont families the Montacutidae (Oliver, Southward and Dando 2012) and the Basterotiidae (Oliver 2013).

A more complex gill, akin to the tubular form of many lucinids (Distel and Felbeck 1987) has been suggested to occur in Conchocele (Taylor and Glover 2010; Dufour 2005) and as such was partially described by Nakazima (1958) in C. disjuncta Gabb (= C. bisecta Conrad). In thyasirids the bacteria are extracellular with the exception of “Maorithyas” hadalis where an intra-cellular arrangement was described and where the frontal cilia are absent (Fujiwara et al. 2001).

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.

The laminar filament. (Type 3L) in Thyasira flexuosa, T. sarsi, Axinus cascadiensis, “ Conchocele ” excavata and the undescribed genus from Quatsino Sound

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).

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 of openings between the bands (Fig. 3A, openings white arrows) The vertical bands represent the frontal face of each filament and are strongly angulate with no trace of ciliation (Figs 3E, F). 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 transverse 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 (Fig. 6 (te), 7B white arrows). These faces (Fig. 7B black arrows) are composed of a double band of the bases of lateral frontal cirri. Although not well preserved there are indications of lateral cilia and the frontal cilia zone is a deep groove. With the frontal zone ripped off the tubes can be seen in cross-section (Fig. 7D) and long section (Fig. 7E) if cut laterally. The walls are composed mostly of bacteriocytes (bct), the lumen is open and attached to the walls are numerous spherical cells (sph) of unknown function but may be involved with the elimination of waste products as suggested for similar structures observed by 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 Oliver et al. (2013). 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).

Figure 1. 

Cross sections through the median area of five species of Thyasiroidea showing the demibranchs on one side in cross section. A Thyasira sarsi, B Thyasira flexuosa, C Axinus cascadiensis, D Conchocele bisecta, E Ochetoctena tomasi. lbp lateral body pouch; sbc supra branchial chamber.

Figure 2A–E. 

Scanning electron micrographs of the ctenidium of Thyasira sarsi; A gross transverse section, B longitudinal section of ventral portion of inner demibranch, C ciliated frontal surface, D transverse section through the frontal zone. E frontal surface with cilia removed. 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.

Figure 3. 

Scanning electron micrographs of the ctenidium of Conchocele bisecta; A frontal surface, openings of tubules arrowed; B frontal surface removed to reveal tubules; C abfrontal surface of laminar filaments; D longitudinal section; E edge of a frontal face; F section of the frontal face with projection on the inter filamentar junction arrowed. abf lam laminar abfrontal surface; fz frontal zone; ils inter lamellar space; ilj inter lamellar junction; lz laminar zone; tbz tubular zone.

Figure 4. 

Scanning electron micrograph of the ctenidium of Conchocele bisecta showing details of the frontal, tubular and laminar zones. fz frontal zone; ifj (abf) inter filamentar junctions, abfrontal; ifj (f) inter filamentar junctions, frontal; lz laminar zone; tbz tubular zone.

Figure 5. 

Scanning electron micrograph of the ctenidium of Ochetoctena tomasi, lateral view of a filament. abf (free) free abfrontal surface; att attachment point; fs frontal surface; lt lateral tubule; mt median tubule; sbc supra branchial chamber.

Figure 6. 

Scanning electron micrograph of the ctenidium of Ochetoctena tomasi, oblique section. fil frontal edge of a filament; lt (asc) lateral tubule of ascending arm; lt (dsc) lateral tubule of descending arm; mt median tubule; te tubule entrance.

Figure 7A–F. 

Scanning electron micrographs of the ctenidium of Ochetoctena tomasi, A frontal face, frontal surface intact on the left, removed on the right to reveal tubules; B frontal surface with entrances to tubules arrowed; C single filament showing lateral and median tubules; D cross section of a single tubule; E long section of adjacent tubules; F bacterial bundles within bacteriocytes and paddle tipped filaments in the lumen of the tubule. bct bacteriocyte; spb spherical body; ptf paddle tipped filaments; fs frontal surface; fs (rem) frontal surface removed; lt lateral tubule; mt median tubule.

Figure 8A–F 

Ochetoctena tomasi, A–B external and internal views of the shell of the holotype, NMW.Z.2014.014.00001; C–E scanning electron micrographs of the shell surface; F gross anatomy viewed from the left side after removal of the mantle. aa anterior adductor muscle; f foot; lbp lateral body pouch; id inner demibranch; od (r) remanant of outer demibranch; pa posterior adductor muscle.

Figure 9. 

Shells and anatomy of granulose thyasirids of the genus Parathyasira. A–E an undescribed abyssal species from the Arabian Sea, NMW.Z. 2014, A–B external and internal views of the shell; C–D scanning electron micrographs of the granulose microsculpture; E gross anatomy. F Parathyasira resupina, holotype and type of the genus, New South Wales, AMS C57834; G–I Parathyasira subcircularis, Rockall Trough, NMW.Z.2013, G external of shell; H gross anatomy; I micrograph showing radial granulose sculpture.

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” 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 but the gross structure of the gill remains undescribed. The allocation of this species to Maorithyas is incorrect as both shell and anatomy are not in agreement. The type species of Maorithyas, M. marama 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) illustrated 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).

Class Bivalvia Linnaeus, 1758

Subclass Heterodonta Neumayr, 1884

Order Veneroida H&A Adams, 1856

Superfamily Thyasiroidea Dall, 1900

Family Thyasiridae Dall, 1900