Schulze, 1880
Species Overview
Plakina monolopha Schulze, 1880 is a whitish or pale yellow, soft crust of several mm in thickness. Its surface shows characteristic pits and slits on low magnification. It is a southern species recorded infrequently from south and west coasts of Europe.
Taxonomic Description
Colour: White, delicately rose, ochre-yellow, brown or rose-brown.
Shape, size, surface and consistency: Flat crusts with irregular roundish or lobate borders. Thickness several mm. Frequently the crusts have irregular holes as if lobes have encircled a bare part of the substratum. Optically smooth surface, but under low magnification a system of gently curved slits or pits is visible. When pried from the substrate a thin veneer is left due to a system of lacunae deep in the choanosome near the substrate. Consistency soft.
Spicules: (Plakina monolopha spics) (Plakina monolopha Topsent) Small calthropses, triods and diods, no proper microscleres (although the size of the "megascleres" is small). Calthropses (lophocalthrops or lophotriaene) have one lophate ray with two to five proliferations; triods are equiangular or Y-shaped; diods have a clear central irregular knot. Rays of calthropses and triods: 25-30 x 2-3. Diods: 70-90 x 3-4 µm.
Skeleton: Alveolar arrangement around the large eurypylous choanocyte chambers. Lophocalthropses usually concentrated at the surface.
Aquiferous system: The large eurypylous choanocyte chambers have a radial arrangement around a large excurrent canal, convening into a system of lacunae just above the substratum. These drain into large oscules. Incurrent openings are situated in deep grooves with narrow communication with the environment.
Ecology: Under stones and boulders in shallow water, 10 m.
Distribution: SW Ireland, west coast of France and Spain; Mediterranean.
Etymology: The name refers to the single lophate ray of the calthrops; in other species of Plakina two or three rays may be lophate.
Type specimen information: No type material in BMNH.
Remarks
With low magnification the characteristic surface depressions can be easily observed. The only species with which it may then be confused is Plakortis simplex . This is easily distinguished by the lack of (lopho-)calthropses.
The colour of this species is related to exposure to light: the more light the darker the colour.
Source: Schulze, 1880.