Isodictya doryphora Brondsted 1927

Isodictya doryphora (Brøndsted, 1927)

(Fig. 14, Tab. 10)

Isodictya doryphora (Brøndsted, 1927): Burton 1929: 397.

Synonymy:

Desmacidon doryphora Brøndsted, 1927: 2 –3, figs. 1, 2.

Material. 2 specimens from station 048-1 (SMF 11779, 11834), 602.1 m, 70° 23.94' S, 8° 19.14' W, 12.01.2008.

Description. Observed specimens (Fig. 14 A) characterized by specific surface with thin, rather soft processes, which attain a length of about 5 mm. Very densely on the entire surface, with distances of about 1 to 2 mm between them. Growth form otherwise irregular, somewhat defined by structure of substrate the sponge is growing on. Associated with bryozoans. One specimen (SMF 11779) almost white in alcohol, the other one dark brown, almost blackish (SMF 11834, Fig. 14 A).

Skeleton: Skeleton (Fig. 14 B) basically constructed of thick tracts of oxeas, running towards the surface, there forming long surface processes. Tracts interconnected by secondary tracts, consisting of fewer spicules and less consistent throughout the sponge. Secondary fibres usually run in right angles to primary fibres. Epidermis formed by some unorganized oxeas and mainly chelae. Chelae also found free within the choanosome in high abundance. Tissue comprising a good deal of incorporated sand grains and some overgrown Bryozoans.

Spiculation (Tab. 10): Main spicules oxeas (Fig. 14 C), slightly bent in the middle, 700 to 900 µm long and 30 µm wide. Specimen SMF 11834 with wider variance in oxea length, and especially a higher proportion of shorter oxeas (down to 525 µm). Among observed specimens overall mean values quite similar. A second less abundant kind of oxeas (Fig. 14 D) present. These smaller and especially thinner than general oxeas, about 500 to 700 µm long and 10 µm wide possibly representing juvenile stages of larger ones. Palmate isochelae (Fig. 14 E–H) of common shape, without plate-like extensions which sometimes occur in the inner falxes of the chelae in this genus. Chelae 55 µm long.

Remarks. The species was originally described as Desmacidon doryphora by Brøndsted (1927). Nonetheless, the reassignment of the species to Isodictya according to Burton (1929) is undisputable. The species shows significant differences from Desmacidon, as defined by Van Soest (2002b) based on a re-examination of the holotype. The most characteristic difference between the two species/genera are the chelae: Desmacidon has anchorate isochelae whereas I. doryphora has palmate isochelae, which are very similar e.g. to those of Isodictya toxophila Burton, 1932 (see below). The skeleton in Desmacidon shows a less distinct differentiation in primary and secondary tracts, but builds a strong reticulation of coalesced thick tracts. In the holotype, as described by Brøndsted (1927), no additional smaller oxeas occurred. Still, in our opinion (as discussed above), this cannot be considered a real evidence for a taxonomic differentiation, as these spicules are rather few in our specimens and are very likely to be mere juvenile growth forms of the larger, common oxeas.