Discorhabdella littoralis, Maldonado, Carmona, van Soest & Pomponi, 2001

Maldonado, Manuel, Carmona, M. Carmen, Van Soest, Rob W. M. & Pomponi, Shirley A., 2001, First record of the sponge genera Crambe and Discorhabdella for the eastern Pacific, with description of three new species, Journal of Natural History 35 (9), pp. 1261-1276 : 4-1901

Discorhabdella littoralis n. sp.

Material collected. Holotype USNM-51472 , paratype HBOM-003:00952 .

Type locality. Pacific coast of Panama, Gulf of Chiriqui, Isla Montuosa (7 ° 27.992 ' N, 82 ° 15.629 ' W). Paratypes collected at a nearby site (7 ° 27.932 ' N, 82 ° 15.611 ' W).

Etymology. The name refers to the bathymetric distribution of this species, which is unusually shallow for the genus.

Description. Bright red-orange, approximately 5-30 cm in diameter, thinly encrusting colonies growing on rocky substrata. Oscula and ostia are not visible either in living individuals examined in situ or in fi xed individuals examined microscopically. The sponge surface is uniformly microhispid, with spicules projecting about 200 μm beyond the ectosome. Aquiferous canals were not visible beneath the ectosome despite the thinly encrusting habit (about 1 mm thick).

The skeleton consists of four spicule types:

(1) Ectosomal tylostyles: 130-180 μm ✕ 2.5 -4 μm; straight or slightly curved, with an oval tyle ( figure 3a ), and with a smoothly sharpened point surrounded by feeble spines ( figure 3b ).

(2) Choanosomal tylostyles: 117-300 ✕ 5-10 μm; somewhat curved, conical or fusiform, with a well-marked tyle that occasionally shows remains of tubercles ( figure 3c ).

(3) Pseudoastrose acanthostyles: 26-40 μm in length; with a moderately inflated tyle (10-18.5 μm diameter) that bears large spines, and with a short shaft, the spines of which are concentrated in a subterminal position ( figure 3d -h ). Spicules that are still in the process of formation have an axial canal ( figure 3g, h ) visible at the point of the spines (actines?) that radiate from the tyle.

(4) Sigmas: 13-15 μm ✕ 1 μm; symmetrical, C-shaped ( figure 3i ).

The skeletal arrangement consists of a stratum of densely packed acanthostyles that rests on a basal spongin layer from which the choanosomal tylostyles sparsely emerge in a hymedesmioid pattern. The ectosomal subtylostyles are arranged in bouquets around the points of the choanosomal tylostyles, which pierce the sponge surface. Sigmas are found mostly in the ectosome.

Habitat. This species is abundant at the collection site, growing on both horizontal and vertical, rocky substrata, from 10 to 30 m depth. This is the shallowest occurring species of Discorhabdella , a genus that was previously known only from deep waters. This species sometimes shares habitat with Crambe panamensis and, though the latter grows commonly on barnacles and thorny oysters, D. littoralis was never seen overgrowing those living substrata.

Taxonomic remarks. The acanthostyles of the new species are not the typical pseudoastros e acanthostyle s characterizing most species of Discorhabdella . However, evidence of a morphological relationship between the non-astrose acanthostyle of the new species and the typical astrose acanthostyle of other Discorhabdella species is provided by: (1) the polyaxonid pattern of axial canals in the acanthostyles of the former, (2) the con fi nement of its spines to a subterminal position in the shaft, and (3) the small size of the spicule. Several other skeletal traits strongly suggest a ffi nity between the new species and other species in Discorhabdella : (1) vestiges of tubercles in the tyle of some choanosomal tylostyles of the new species, (2) the peculiar arrangement of the microspines that surround the point of its ectosomal styles, and (3) the symmetrical, C-shaped form of its sigmas. Therefore, we interpret this new species to be a modi fi ed, shallow-water species of Discorhabdella that has lost its chelae and slimmed its acanthostyles. Such a skeletal weakening is similar to that described in western Mediterranean populations of Crambe crambe as a consequence of chronic silicon limitation in sublittoral habitats ( Maldonado et al., 1999). Indeed, the skeletal weakening in this sublittoral Discorhabdella relative to the epibathyal and bathyal species in the genus may have been forced by the same environmenta l limitation.

This view is also consistent with the suggestion of Boury-Esnault et al. (1992) that the pseudoastrose acanthostyle characterizing species such as Discorhabdella incrustans would have evolved, becoming thinner and longer, to lead to the nearly regular acanthostyle that characterizes Discorhabdella tuberosocapitat a. The fi nding of D. littoralis supports not only the hypothesis on acanthostyle evolution within the genus, but also substantially fi lls the gap between the extremes of this spicule’s morphology, proving an intermediate stage to retrace the change in the graded morphological series. An alternative direction in the gradation series, i.e. that the pseudoastrose acanthostyles would be derived from regular, diactinal acanthostyles by shortening in the main axis and hypertrophy of the tyle, is unlikely. It explains neither the skeletal a ffi nities with Crambe nor the presence of a polyaxonid pattern of axial canals in the tyle.

Figures

FIG. 3. Spicules of Discorhabdella littoralis n. sp.: (a) tyles of ectosomal subtylostyles; (b) feeble spines around the point of an ectosomal subtylostyle; (c) tyle of a choanosomal tylostyle, on which a slight tubercle (tb) is visible; (d-f) fully grown acanthostyles; (g, h) developing stages of acanthostyles, in which axial canals (ax) are visible in the tyle; (i) sigma.