Placida dendritica (Alder & Hancock, 1843)

Author & date of last revision: Ian Smith on 20 January 2013

Synonyms:Hermaea dendritica Alder & Hancock, 1843;

Description

Body (excluding appendages)
Up to 11mm long. Translucent greenish white with distinctly visible internal, green to olive brown, many branched, dendritic, digestive gland [image 1]. Dorsal anus to anterior of pericardium.
Cerata
Cerata arranged in up to eight groups of 2-4, the larger ones in each group positioned more dorsally than the small ones (Seaslugforum). Surface smooth. Cerata often conceal much of body. Transparent, revealing branched, green to olive-brown, internal digestive gland extending to the apex, which lacks cnidosac. Colour of digestive gland fades to yellowish-brown if starved. Many white pigment spots on surface. Number and size of cerata varies with diet.
Rhinophores
Enrolled tube, slit along anterior (when held upright). Smooth, greenish translucent white, with spots of white pigment, most concentrated distally (Seaslugforum). Distinctly visible internal green dendritic digestive gland.
Head
Greenish white with green dendritic digestive gland. Eye at base of, and to side of, rhinophore.
Foot
Sole translucent greenish-white showing spheroids of ovotestis [2]. Anterior smoothly rounded with no central indentation,no propodial tentacles and only a slight lateral expansion. Can extend beyond posterior of cerata; total length of animal about 8 times width of foot.

Key identification features

Body translucent whitish with dendritic green markings [1].
Rhinophore enrolled, widely slit open down anterior, with dendritic green markings.
Length about 8X foot width when fully extended.

Similar species

Hermaea bifida

Body translucent white with orange-red to maroon-red surface pigment.
Rhinophore an orange-brown to mahogany-red tube, slit down one side [5Hb].
Length about 20X foot width when fully extended [1Hb].

Ecology and behaviour

ELWS and shallow sublittoral where enough light for foodplants: Codium [13Ev] and Bryopsis plumosa [9Ev] . Very well camouflaged on Codium where it feeds by preference on tips.

In a study in Oregon, USA, P. dendritica on Codium spp. were generally smaller than those on Bryopsis corticulans, and, when similar sized individuals were compared, those on Codium had more numerous and shorter cerata, wider bodies and smaller more numerous radular teeth than those on Bryopsis. Diet changes induced morphological changes (Trowbridge).

Simultaneous hermaphrodite. Spawn in a coil of about one and a half turns [3], with up to 850 ova, deposited May-June on Codium. Shelled veligers drift as plankton before transforming into adults.

Distribution and status

Norway to Mediterranean, New England (USA), Australasia, S. Africa and Pacific coast of N. America. (GBIF map) Scattered Irish and British records, mostly from south coast England and Scottish islands (interactive UK distribution map, NBN ) .

References and links

Thompson, T.E. 1976. Biology of opisthobranch molluscs 1. London, Ray Society.
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.

Trowbridge, C.D. 1997. Dietry induction of opisthobranch morphology: Placida dendritica (Alder & Hancock, 1843) on different green algal hosts J.moll.Stud. 63, 29-38

Current taxonomy; World Register of Marine Species (EML).

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Photo:

Placida dendritica. After Alder & Hancock, 1845-55, "British nudibranchate mollusca", as Hermaea dendritica.

Photo:

Placida dendritica. Length 10mm. Sole translucent showing spheroids of ovotestis. Anterior smoothly rounded with no central indentation,no propodial tentacles and only a slight lateral expansion. LWS. Orkney. August 1976.

Additional images

GALLERY of larger images of this species.
All images copyright; see 'image details'.

Recorded UK distribution

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