Tretodictyum amchitkensis, Reiswig & Stone, 2013

Reiswig, Henry M. & Stone, Robert P., 2013, <strong> New glass sponges (Porifera: Hexactinellida) from deep waters of the central Aleutian Islands, Alaska </ strong>, Zootaxa 3628 (1), pp. 1-64 : 20-25

Tretodictyum amchitkensis n. sp.

( Figs. 10 & 11 , Table 5)

Synonymy. Tretodictyum sp. nov. Stone et al., 2011: 20. Material examined. Holotype: USNM# 1196552 , ROV ' Jason II' from RV ' Roger Revelle', dive J2104, 05 August 2004, Amchitka Pass , 49.8 km WSW of Gareloi Island, Aleutian Islands , Alaska, 51º41.788'N, 179º35.030'W, 866 m, partial, dry & ethanol.

Description. The holotype ( Fig. 10A ) was a widely flared funnel, 16.8 cm tall, 25.1 x 16.9 cm in major and minor diameters, with a deep notch in one side and attached centrally to an oblique bedrock surface by a short narrow stem and basal disc. Most was recovered and conserved as three main dry fragments ( Fig. 10B ) and four smaller fragments in ethanol. The body wall is 5–6 mm thick in older parts and 3 mm thick at the growing margin. When tissues are intact, in either wet or dry state, the two surfaces are distinctly different to the naked eye. The outer or dermal side is markedly lineated by a system of longitudinal light ridges and dark grooves with ca 2 mm repeat spacing, radiating from the center (above the short stem) to the growth margin ( Fig. 10C ). These subsurface structures are covered by a transparent quadrangular spicule lattice raised above the skeletal frame and thus easily visible ( Fig. 10E ) with mesh spacing of 216–382–547 µm (n = 67). The inner or atrial surface has an opaque feltlike cover of more tightly packed spicules, not raised above the skeletal framework; it is penetrated by round apertures distributed rather evenly without pattern ( Fig. 10D ). The openings in dry fragments are sharply defined by the loose spicule feltwork; they are 0.95–1.48– 2.07 mm diameter (n = 50) ( Fig. 10F ). Where wet tissues remain intact in the ethanol fragments, margins of the soft tissue appear as white bands extending slightly inside the spicule margins ( Fig. 10G ). These visible apertures are clearly the terminal openings of exhalant canals; they are not the skeletal channels, here schizorhyses, although the channels are contained within the canals. When loose spicules are removed the margins of the schizorhyses are much less sharply defined than the water canals. Color of the dried fragments is light brown; fragments in ethanol are white.

The skeletal framework, after cleaning, is a typical three-dimensional tretodictyid system of longitudinal septa joined laterally through intervening low-density spaces. At the growing margin the vertical sheets of dense dictyonal framework, septa, undergo bifurcation to maintain even spacing between these elements ( Fig. 10H ). Septa are 0.71–1.10– 1.65 mm thick (n = 17) and gaps (grooves) are 0.52–0.79– 1.12 mm wide (n = 14); mean repeat period is 1.89 mm. The lateral dictyonal connectives joining adjacent septa are sparse ( Fig. 10I ) and scattered throughout the width of the wall. Indeed, the inner and outer surfaces of the cleaned skeletal framework in the marginal areas cannot be distinguished. The septa ( Fig. 10J ) are constructed of longitudinal strands, which in the middle run straight to the growing margin, but on both sides curve smoothly out the dermal and atrial surface to end on those surfaces. There are no specialized cortical layers. Radial beams connecting adjacent longitudinal strands are perpendicular to the strands and are weakly ranked to form curved lines of aligned beams; meshes are rectangular and nodes are not swollen. In older septa in more medial locations, dictyonal elements are inserted so septa become denser. Thin cortical layer development occurs on both surfaces resulting in a stronger framework; the two surfaces can now be distinguished since cortex development is slightly different. On the dermal side the cortex adds thicker lateral strands between septa ( Fig. 10K ) while on the atrial side oblique lattice plates circumscribe entrances to the schizorhyses ( Fig. 10L ). The channel system of schizorhyses is restricted to the lowdensity gap between septa; channels branch and anastomose within these spaces but there appears no provision for joining channels of adjacent gaps. Beams are ornamented with sharp conical spines throughout the framework, but very small areas may be smooth; nodes are not swollen; spurs are mainly rough, long and sharply pointed, although smooth, short and inflated ones can be easily found.

Megascleres are subhexactins to pentactins, regular hexactins, strongyloscopules and uncinates (dimensions given in Table 5). The subhexactins to pentactins ( Fig. 11A ) are surface spicules of both dermal and atrial sides. Rays are strongly tapered, uniformly ornamented by moderate spines, and end in blunt or pointed tips ( Figs. 11B, C ), often with reduction of diameter in steps. The short rounded distal ray or knob bears large spines projecting obliquely outward; the degree of reduction of the distal ray is variable ( Fig. 11D ) but it is never significantly developed. Regular hexactins ( Fig. 11E ) are parenchymal in position, and have six nearly equal cylindrical rough rays ending in abruptly pointed tips. Strongyloscopules ( Fig. 11F ) occur projecting slightly from both dermal and atrial surfaces. They have a rather sharp inflation at the neck-shaft transition and carry 3–4–8 strong straight tapering tines ending in discoid caps that are not larger in diameter than the upper tine shaft. In LM they may appear to be slightly tylote but they are clearly not so in SEM. The tines and head are densely covered by small reclined spines; the straight shaft is entirely but sparsely covered in small spines that become larger and more reclined near the sharply pointed tip. Uncinates ( Fig. 11G ) project in very tight bundles from all external surfaces. They are relatively large for tretodictyids, but exhibit the typical extremely small brackets and minute barbs that are almost invisible in LM.

Microscleres consist of mainly oxyhexasters (93%) and stellate discohexasters (7%), with a very few oxyhexactins (<1%) and onychohexasters («1%); all types are generally distributed. Oxyhexasters ( Fig. 11H ) are stout, each short primary ray bearing 2–4 widely spread, straight secondary rays that end in rather abruptly pointed tips. They appear entirely smooth in LM but prove to have a very few small reclined spines in SEM. The few oxyhexactins ( Fig. 11J ) have the same characters of the oxyhexasters but without ray branching. Discohexasters ( Fig. 11I ) are stellate with primary and secondary rays about equal in length. Each primary ray bears 4–7–10 secondary rays that end in caps, which like the scopules are not significantly larger than the supporting shafts; they could therefore be called strongylohexasters, a new spicule type name. All surfaces bear a dense cover of small reclined spines. The extremely rare onychohexasters (not figured, only two seen) are spherical, each primary bearing 3–4 secondary rays ending in a tuft of 3–6 slightly reclined marginal claws. Ray length proportions are like the oxyhexasters, but their rays are more heavily spined.

Etymology. The species name, amchitkensis , refers to the location of collection, Amchitka Pass.

Remarks. The new Aleutian Islands specimen with preponderance of oxyhexasters clearly belongs to the genus Tretodictyum . With its funnel-like body form it is excluded from four tubular species but is possibly related to three plate or cup-form species, T. minor ( Dendy & Burton, 1926) , T. montereyensis Reiswig et al., 2008 , and T. cocosensis Reiswig, 2010 . It differs from the Indian Ocean T. minor in having atrialia and possession of a moderate number of discohexasters, both of which are absent in the latter. It differs from the California T. montereyensis in having longer uncinates (846 vs 500 µm) and in presence of discohexasters, which are absent in the latter. It differs from the tropical E Pacific T. cocosensis in the shape of the subhexactin distal ray, the presence of only one class of scopules (two classes in the latter) and in its larger uncinates. These differences verify that the Aleutian Islands specimen is distinct from all presently known members of the genus, and it is here erected as holotype of a new species, T. amchitkensis .

Review of all video footage collected with the ROV 'Jason II' indicates that it is uncommon species, locally abundant in some areas, occurring singly on bedrock, mudstone, boulders and cobbles at depths between 704 and 1264 m.

Order Lyssacinosida Zittel

Figures

FIGURE 10. Tretodictyum amchitkensis n. sp., body and framework. A. Frame grab from video footage of the holotype in situ before collection, laser spacing 10 cm. B. Three largest dried fragments, one on left in top and side views. C. Dry external dermal surface showing ridges and grooves. D. Dry internal atrial surface showing open exhalant apertures. E. Closer view of dry dermal surface showing quadrangular spicule lattice over inhalant canals. F. Closer view of dry atrial surface showing uncovered exhalant aperture and dense spicule felt-work. G. Close view of wet atrial surface showing tissue ring around exhalant canal aperture. H. Cleaned marginal skeletal framework showing branching of vertical septa. I. Closer view of septa and intervening space showing transverse connecting strands. J. Marginal end of one septum in lateral view showing disposition of longitudinal strands (SEM). K. Dermal surface of older framework 5 cm from margin with thin surface cortex (SEM). L. Atrial surface of older framework 5 cm from margin with thin surface cortex (SEM).

FIGURE 11. Tretodictyum amchitkensis n. sp., spicules (SEM). A. Surface subhexactins and pentactin. B. Tangential ray end. C. Proximal ray end. D. Range of distal ray development of subhexactins. E. Parenchymal regular hexactin. F. Strongyloscopule, whole, magnified head, shaft tip and tine end. G. Uncinate, whole and enlarged middle segment. H. Oxyhexaster, whole and enlarged ray end. I. Discohexaster, whole and enlarged secondary ray tuft. J. Oxyhexactin.