(Johnson, 1899)
Species Overview
Aka infesta (Johnson, 1899) (often referred to as Cliona labyrinthica ) is a deep-water excavating sponge visible only as tiny (0.5 mm) papillae sticking out of the calcareous substrate in which it has bored. It is common especially in deep-water coral beds along the coasts of Western Europe.
Taxonomic Description
Colour: Papillae are pale greenish white. Endolithic tissue (inside the substrate) more creamy white or yellowish.
Shape, size, surface and consistency: Tiny papillae of less than 1 mm in diameter stick out of the substrate, at variable distance, but not closely together. This makes detection of this species very difficult. In fact, only breaking the corals or shells and exposing the much larger inner excavations is often the only way in which the species is noticed. Excavation often several cm3 in size, divided up into lobate chambers of 5 mm or more across, connected by narrow channels. Most papillae have a pore-sieve type of apex, and are presumably inhalant, some are open at the end and are presumably exhalant. Papillae are fragile. Endolithic tissue is weak, slimy.
Spicules: Curved oxeas, sometimes a bit angularly bent near both apices: 80-170 x 2-12 µm.
Skeleton: In the papillae there is an ill-defined reticulation of spicule tracts, which is only well-developed in the apical part of the papilla; in the endolithic parts the spicules are few in number and not organized into a network.
Ecology: Found predominantly excavating Lophelia prolifera , a deep-water coral forming extensive beds off the coasts of Europe and West Africa. Occasionally also found in other calcareous objects, such as shells and calcareous sponges. Depth: 60-1500 m.
Distribution: Norway, Sweden, Ireland; Mediterranean, Azores, Cape Verde Islands.
Etymology: The name refers to the limestone-infestating habit
Type specimen information: Apparently no longer extant.
Remarks
A species which may be confused with A . infesta is Alectona millari which shows likewise tiny papillae sticking out of deep-water corals. However, the spicules are very different, and microscopic examination will easily tell them apart.
Most records of this species are under the name Cliona labyrinthica Hancock, 1849. This was described as excavating a Tridacna shell, which is a typical inhabitant of the tropical Indo-Pacific shallow waters. It is very unlikely that the labyrinthica and the present deep-water species are conspecific. The oldest description of such sponges from deep-water off Madeira, is that of Acca infesta Johnson, 1899. De Laubenfels (1936) realized that the name Acca was preoccupied and erected Aka as a replacement name. The genus Aka has been revived by Rützler and Stone (1986) to replace the name Siphonodictyon Bergquist, 1965 for excavating sponges belonging to the Haplosclerid family Phloeodictyidae.
A second species with properties similar to A . infesta was described by Stephens (1914) as Cliona coralliophaga ; papilla-size and excavations-size were identical, only the spicule sizes quoted by Stephens were much more variable and with a range up to 550 µm. It is assumed that some contamination from other sponges (e.g. Halichondria or Spongosorites) is the cause of this discrepancy; for the time being, this species is considered a junior synonym of infesta .
A . infesta is atypical among Aka species in having such tiny papillae and being a deep-water dweller. In skeletal aspects, it is close to several Aka species, including A . labyrinthica .
Source: Van Soest, unpublished data.