Mycale macilenta (Bowerbank, 1866)


Family : Mycalidae

Form: Thinly encrusting with irregular outlines when growing on rock. Can become a thick cushion when growing on living Queen scallops Aequipecten opercularis (though possibly this is a separate species).

Colour: Alive, the encrusting form is normally pale yellow or green. The form on Aequipecten varies from brown to purple or "vivid red". When preserved it is "yellow or brown".

Smell: None

Consistency: Soft, easily crushed.

Surface: Smooth, velvety.

Apertures: Oscules are clearly visible in living sponges, slightly raised above the general level of body, with transparent rims which collapse on collection. In thinly encrusting specimens transparent excurrent channels can be seen converging on oscules. Cushions on Aequipecten may only have a single oscule.

Contraction: Slight contraction occurs and oscules close.

Internal characters

Skeleton: Plumoreticulate, of ascending multi-spicular fibres of subtylostyles ("which do not divide or anastomose"), ending in loose brushes at the surface, with scattered megascleres lying in between. Rosettes of larger anisochelae frequent near the surface, with smaller anisochelae scattered throughout tissue. There is no specialised ectosomal skeleton. The surface is supported by the widely spaced, terminal brushes of the main skeleton, with a few single megascleres (sometimes these are absent altogether) scattered in the large interstices. Spongin is minimal.

Spicules: The megascleres are generally straight-shafted, slightly fusiform, subtylostyles (a), with barely formed elliptical heads, between 200-(270)-300Ám long. The microscleres include palmate anisochelae of three sizes. The largest (b) are grouped into rosettes in the ectosome (ca. 33-59Ám). The middle size are solitary and scarce (ca. 17-24Ám). Those of the smallest size (c) (11-15Ám) are very abundant and widely dispersed throughout the body. They have a distinctive lower tooth which is very short and curved in towards the shaft. The sigmata (d) group into two size categories (65-115Ám and 21-28Ám); the larger ones are always less frequent and both sizes may be scarce. Toxa (e) are variable in length (60-250Ám) and width, but usually frequent.

Habitat: Littoral to at least 40m, encrusting mainly on Pecten and Aequipecten. On rock faces and wrecks in both sheltered and exposed conditions. Apparently fast growing. "Also on Laminaria and hydroids."

Distribution: Common along the Channel coasts of "British Isles; France". Also "Belgium"; Atlantic coasts of "France; Spain"; and "Mediterranean". Common on Aequipecten in Strangford Lough and Scottish sea lochs. The encrusting form has been found in Strangford Lough entrance, in Lough Hyne and at St Kilda.

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

Identity: The most difficult of all the Mycale spp. to recognise, as its appearance and spiculation can be so variable. (This may indicate a complex of closely related species!) It most closely resembles Mycale contarenii (q.v.) but can be distinguished by the relative rarity of large sigmata, presence of small sigmata, and large and small toxa, the toxa not being grouped in bundles. Mycale similaris (q.v.), is also similar but the presence of trichodragmata in the latter helps to distinguish the two species, although these spicules are so small that they can easily be overlooked.

Voucher: BELUM : Mc1536. Lough Hyne, Cork.

Editors: D. Moss, B.E. Picton.

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