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(Ellis and Solander, 1786)

Species Overview

Isodictya palmata (Ellis and Solander, 1786) is a large palmate deep-water sponge, occurring in the boreal and sub-arctic North Atlantic. Smaller specimens may resemble Haliclona (H.) oculata or H. (H.) simulans, but from these it may be readily distinguished by the arrangement of the oscules in groups along the edges of the flattened branches.

Taxonomic Description

Colour: Yellowish gray in alcohol.
Shape, size, surface and consistency: (Isodictya palmata 3) (Spongia digitata Esper) (Spongia lobata) (Pachychalina caulifera) Erect, digitate, up to 35 cm high, with variably compressed branches of ca. 2.5 cm wide, which may coalesce into plate-shaped areas. Surface finely and densely shaggy from projecting ends of skeletal fibres. A thin membrane is draped over the fibre ends. Oscules up to 5 mm in diameter, arranged along the edges of the compressed branches and scattered in groups on the plate-like fused branches.
Spicules: (Isodictya palmata Koltun) Megascleres: cigar-shaped oxeas, occasionally styles or strongyles: 130-265 x 4-17 µm. Microscleres: Palmate isochelae of a peculiar shape, with poorly developed lateral alae and plate-like inwardly projected falces: 17-35 µm.
Skeleton: Anistropic reticulation of spongin enforced spicule tracts; ascending tracts thicker than interconnecting tracts.
Ecology: Deeper water.
Distribution: Orkney, Faroes, between Faroes and Iceland, Norway; NE America.
Etymology: palmatus (Latin) = (hand-)palm-like, referring to the shape.
Type specimen information: Type probably lost.

Remarks

This is a distinctive species combining a reticulation of oxeas with palmate isochelae. On the American east coast the very similar Isodictya deichmannae (De Laubenfels) is recognized on the basis of a predominance of styles rather than oxeas (cf. Hartman, 1958a). In view of the fact that chelae and habit are identical, and oxea-bearing specimens also occur sympatrically it is possible that deichmannae is conspecific rather than a separate species.
In Boreal-Arctic deep water another Isodictya- like species is recognized, viz. Dendoricella flabelliformis (Hansen) (= Desmacidon clavellata Arnesen, 1903), differing from Isodictya palmata in the shape (clavate-flabelliform, no branches) and the size of the oxeas (400-600 µm). Its chelae are arcuate, rather than palmate.
Recently it was proposed to remove the genus from its usual assignment in the Poecilosclerida to the Haposclerida, but here we take a conservative view and retain the genus in the Poecilosclerida.
Source: Hajdu et al., 1994

Isodictya palmata