Isodictya palmata (Ellis & Solander, 1786)


Family : Isodictyidae

Form: 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 oculata or 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. Erect, digitate, up to 35 cm high, with variably compressed branches of ca. 2.5 cm wide, which may coalesce into plate-shaped areas.

Colour: Yellow-brown.

Consistency: Firm, spongy.

Surface: Surface finely and densely shaggy from projecting ends of skeletal fibres. A thin membrane is draped over the fibre ends.

Apertures: 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.

Contraction: None.

Internal characters

Skeleton: Anistropic reticulation of spongin enforced spicule tracts; ascending tracts thicker than interconnecting tracts.

Spicules: Cigar-shaped oxeas, occasionally styles or strongyles: 130-265 x 4-17 Ám. Microscleres : Palmate isochelae of a peculiar shape, with poorly developed lateral alae and plate-like inwardly projected falces: 17-35 Ám.

Distribution: Orkney, Faroes, between Faroes and Iceland, Norway; NE America.

Distribution Map: NBN map : National Biodiversity Network mapping facility, data for UK.

Identity: 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 Ám). Its chelae are arcuate, rather than palmate.

Editors: Christine Morrow, Bernard Picton & Rob van Soest.

 Picton, B.E., Morrow, C.C. & van Soest, R.W.B., 2011. [In] Sponges of Britain and Ireland