Picton & Goodwin, 2007
Crella (Pytheas) plana Picton & Goodwin (2007) is a peach-coloured Crella recognizable by raised areolate porefields and a tangential crust of acanthostyles. It differs from most other Crella species possessing both acanthostyles and acanthoxeas in having a reversed position of these spicules, with acanthostyles in the surface crust and the acanthoxeas scattered in the choanosome. In addition the acanthostyles are also echinating the substrate, a condition typical for the subgenus Pytheas.
Shape, size, surface and consistency: Thick encrusting sponge with numerous pore sieves, these have raised rims. Forms quite small patches on bedrock, maximum patch diameter ranged between 2 and 6 cm.
Spicules: Megascleres: Acanthoxeas: 105-165 by 2-8 µm. Fusiform oxeas entirely spined with large robust spines. Many of the spicules are bent in the middle. Most are between 4 and 8 µm in width but a few much thinner ones are present. Fusiform acanthostyles: 120-175 by 2-8 µm. Very similar in appearance to the acanthoxeas, fusiform with large, robust spines. Most are between 4 and 8 µm in width but a few much thinner ones are present. One end tapers abruptly to a sharp point, the other is rounded but not tylote. Basal acanthostyles: 185-230 by 4-8 µm, tapering acanthostyles with a tylote head, entirely spined but the spines are much sparser and slightly smaller than those of the acanthoxea. Tornotes: 310-370 by 5-7 µm, smooth oxeas, many slightly fusiform, pointed at both ends but one end tends to be more sharply pointed; some have mucronate ends. Microscleres: Chelae: 15-18 µm, arcuate.
Skeleton: The choanosomal skeleton consists of plumose bundles of tornotes between 50 and 80 µm in diameter rising from a basal layer of spongin in which acanthostyles are embedded, shaft upwards. Acanthoxeas and fusiform acanthostyles are scattered through the choanosome and chelae are also present in small numbers. The ectosome is a multiple layered crust of tangential acanthostyles, up to 120 µm thick.
Ecology: On bedrock exposed to tidal streams, 32-42 m.
Etymology: From the latin plan meaning flat. In contrast to other Crella species which tend to form colonies in the shape of low mounds C. plana patches are steep sided with a level top.
Type material: Holotype: [Mc2746]. Sample in IMS, section and spicule preparation, Rathlin Island, Duncan's Bo (55°18.718'N 006°15.123'W; water depth: 37-42 m); coll. by B.E.P. and C.E.G., 8 September 2005.
The spiculation does not match any existing Crella species, this species is unusual in possessing both acanthoxeas and fusiform acanthostyles in the choanosome. It is most similar in spiculation to Crella (Pytheas) schottlaenderi Arndt, 1913 but this does not have choanosomal acanthoxeas. In some of the tissue sections (Mc2746) embryos were visible, these contained smaller versions of the spicules in the main tissue (acanthoxeas 50-75 by 1 µm) and additionally fine sigmas (12-14 µm), these embryonal spicules were not present on the SEM preparations so it was not possible to examine them in detail, it is possible that they are embryonal chelae.
Source: Picton & Goodwin, 2007