Tretodictyum montereyensis, Reiswig & Dohrmann & Pomponi & Wörheide, 2008

Reiswig, Henry M., Dohrmann, Martin, Pomponi, Shirley & Wörheide, Gert, 2008, Two new tretodictyids (Hexactinellida: Hexactinosida: Tretodictyidae) from the coasts of North America, Zootaxa 1721 (1), pp. 53-64 : 55-58

Tretodictyum montereyensis n. sp.

Figs. 2–5 , Table 1.

Material examined. Holotype: CASIZ 175317 , coll. C. Baxter using MBARI ROV ' Ventana', dive 105, north wall of Monterey Bay Canyon, central California, 36°48'30"N, 122°05'20"W, 92–369 m, 08 Dec. 1989.

Diagnosis. Tretodictyum with ridge/groove system covered by a thin fused dermal cortex with thick dictyonal strands oriented in all directions; large rough pentactins and occasional subhexactins serve as both dermalia and atrialia. Microscleres are mainly oxyhexasters, oxyhemihexasters, and oxyhexactins, but rare onychohexactins also occur.

Description. The holotype and only specimen consists of two fragments ( Fig. 2 ), the larger of which was a curved piece 25 mm long, 12 mm wide and 10 mm thick. It was clearly a small fragment from a larger plate or bowl-shaped specimen, calculated as 6.5 cm diameter from projection of the curvature. Both surfaces are smooth and lack prostalia, but appear hirsute when viewed at 50x due to the presence of perpendicularly oriented uncinate bundles. The convex surface, assumed to be dermal, is pierced by small irregular apertures, 90–800 µm diameter while the concave surface, assumed to be atrial, is likewise smooth but pierced by apertures 480–800 µm which are difficult to distinguish from the irregular and variable-sized meshes of the dictyonal network on this surface. Larger inhalant and exhalant canals oriented perpendicular to the outer surfaces are visible through the tissues of both surfaces. The ridge-groove system expected to be present on the basis of spiculation of the specimen, is not visible on either surface. The preserved fragments are white; the texture is stony-hard but the interior is fleshier than that of other dictyonal hexactinellids.

Dictyonal framework. The dermal framework ( Fig. 3A ) consists of an irregular network of very thick dictyonal strands running in all directions overlying a likewise irregular network of smaller-meshed, thinner beams, 3–4 dictyonalia in thickness. Some thick surface strands form the boundaries of short channels ( Fig. 3A ) that lead into a subsurface layer of longitudinal channels ( Fig. 3E ), which appear to be homologues of the surface groove system common in other tretodictyids. The dermal framework (a cortex) is supported on a series of longitudinal septa (primary framework), which delimit the underlying longitudinal grooves. The septa, homologues of the longitudinal ridges in other tretodictyids, are composed mainly of longitudinal dicty- onal strands which curve and spread towards both dermal and atrial surfaces ( Fig. 3C ); a few very thick longitudinal strands occur here. The framework of the atrial surface ( Fig. 3F ) is similar to that of the dermal surface, consisting of very thick strands oriented in all directions, and a network of thinner beams under that surface. Both surfaces are thus covered by a very strong framework, which is not primary in origin and thus is secondary and cortical in nature.

The channel system is schizorhysial and can be directly compared to that of other tretodictyids but modified here by addition of a thin dictyonal cortex over the surface ridge-groove system. Channels extending from the bottom of the grooves enter into the lower dense dictyonal network and there join to a labyrinthic system of interconnected large channels. These eventually open through the atrial framework by ill-defined terminal elements of the schizorhysial system.

The framework meshes are highly variable due to irregular spacing of dictyonal nodes, especially in thick strands ( Table 1f). Beams also vary considerably in width as evident in the SEMs ( Fig. 3 ) and measurements ( Tab. 1f). Nodes are never swollen and beams are ornamented with small patchily but evenly distributed (within patches) fine spines ( Fig. 3B, D, G ). Spurs on surface dictyonalia are rough, thin, tapered and sharply pointed.

Spicules. Spicule forms are shown in Figs 4 and 5 and dimensions are given in Table 1. Megascleres are pentactins, occasional subhexactins, strongyloscopules and small uncinates in both dermal and atrial surfaces. Dermalia and atrialia are pentactins and rare subhexactins. They are entirely coarsely rough, and have cylindrical rays that end in abruptly sharp or blunt tips. Irregular step-like reductions in ray diameter are common. The proximal ray may be longer or much shorter than the tangential rays. Scopules occur as a single type as thin (young) and thick (mature) stages; they are entirely rough in SEM but spination can only be resolved on the head with light microscopy. Tines, 3–4 in number, are very slightly splayed and end simply in rounded or indistinct button tips ( Fig. 4C ). The shaft ends proximally in a sharp tip without inflation or conspicuous increase in spination. Small uncinates occur in bundles oriented vertical to the surface. In light microscopy shallow brackets can be seen with high magnification oil immersion objective, but small barbs ( Fig. 5F ) can only be seen with SEM.

Microscleres consist almost exclusively of oxy-tipped forms (hexasters, hemihexasters, hexactins) but onychohexactins rarely occur – only one was found in SEM preparation ( Fig. 5D ). Oxy-tipped forms have 1– 3 terminals, which are sparsely and finely spined (seen only in SEM) and end in slightly curved tips. The onychohexactin is more coarsely spined than oxy-tipped spicules and its ray tips bear a few recurved claws ( Fig. 5D ) or stellate buttons from which the claws may have been broken. In both oxy- and onychohexactin forms, the point of demarcation of primary and secondary rays is often marked by an irregular bend.

Etymology. The species name, montereyensis , refers to the location of collection, Monterey Bay, California.

Remarks. This species has been briefly described as Tretodictyum sp. in Lee et al. (2007). Two characters of the framework of this specimen are atypical for members of Tretodictyum – the oblique-running, thickened strands in dermal and atrial surfaces and the submersion/overgrowth of the dermal ridge-groove system. Had loose spicules not been available, it would not have been recognizable as a tretodictyid.

The new form differs from the four known species of Tretodictyum in these framework characteristics as well as other features. The species T. minor (Dendy & Burton) , from the Andaman Islands, has the body form of laterally fused tubes, lacks atrialia, has dermalia much thinner than the new form, scopules with up to 9 tines (only 4 in the California form), and lacks oxyhemihexasters and oxyhexactins. Tretodictyum pumicosum Ijima , from Indonesia, has dermalia in the form of hexactins with well-developed distal rays. Tretodictyum schrammeni Ijima , also from Indonesia, has a branching body form and beam ornamentation of spines in transverse rows. Tretodictyum tubulosum (Schulze) , from Japan, also has branched body form and no atrialia. The Tretodictyum reported from the NE Atlantic and Mediterranean ( Boury-Esnault et al. 1994) as T. tubulosum is similar to the Japan form, but probably represents an undescribed species.

Figures

FIGURE 2. Two body fragments of Tretodictyum montereyensis n. sp. in transverse section, dermal surface upwards, with part of the schizorhysial channel system seen internally.

FIGURE 3. Acid-cleaned dictyonal framework of Tretodictyum montereyensis n. sp. A. dermal surface with entrance apertures to schizorhyses. B. dermal beams and spurs. C. longitudinal section of septum. D. beams and nodes of the internal framework. E. oblique view of ridge/groove system overlain by dermal cortex (dermal surface left). F. atrial surface with indistinct terminations of the schizorhysial channel system. G. beams and spurs of the atrial framework. (A–D, F–G: SEM; E: LM)

FIGURE 4. Spicules of Tretodictyum montereyensis n. sp. A. surface pentactin. B, C. narrow and thick stages of scopule. D. uncinate with magnified central section. E. oxyhexaster. F. oxyhemihexaster. G. oxyhexactin.

FIGURE 5. SEM micrographs of spicules of Tretodictyum montereyensis n. sp. A. oxyhexaster. B. oxyhemihexaster. C. oxyhexactin. D. onychohexactin with magnified tip. E. thick scopule head. F. central part of small uncinate showing brackets and very small barbs.