Placida dendritica

History of Spread

Placida dendritica, an ascoglossan sea slug, feeding on green algae, was described from England in 1843, and has since been reported from both sides of the Atlantic and Pacific, and from Australia and New Zealand (Bleakney 1989; Thompson 1976). Bleakney found that morphology of the radula differed morphologically, with distinctive arrangements of denticles in North Atlantic, North Pacific, and Australia-New Zealand populations. Animals also differed greatly in body size, but did not show any other consistent morphological differences. Experiments by Trowbridge (1997) show that body size and morphology in P. dendritica vary geatly with the food alga offered, but the extent to which this affects the radula is not clear.

Several explanations have been offered for the cosmopolitan distribution of Placida dendritica. Bleakney (1996) regards this animal as an evolutionarily ancient species, which colonized the Atlantic from the Pacific 180 million years ago. Marcus (1961) suggested a Tethyan history, dispersing by that Tertiary equatorial sea. Atlhough this sea slug shows a high degree of plasticity, Trowbridge (1999) suspects that the many world populations may represent different, cryptic, species. The possibility of introduction was not discussed by these workers, except by Marcus: 'The species feeds chiefly on Codium, which does not belong in the ordinary growth on ship bottoms (Marcus 1961). However, the history of P. dendritica in the northwest Atlantic is suggestive of either of a recent invasion, or the response of an overlooked rare native herbivore to the invasion of the seaweed Codium fragile ssp. tomentosioides. The first collection of P. dendritica in the region was from North Carolina in 1959 , although it was been noted (with no details) from New England in an unpublished manuscript from the 1950s (Marcus 1961). Records are summarized below, from north to south:

Gulf of St.Lawrence- Placida dendritica was collected in 1970 at Logy Bay, on the Avalon Peninsula, on the green alga Derbesia marina (Bleakney 1989).

Gulf of Maine- Placida dendritica was collected in 1969 and 1970, in Bear River, off Annapolis Basin, Bay of Fundy, and in Chester Basin, on the southern Atlantic coast of Nova Scotia (Bleakney 1989). In Nova Scotia, P. dendritica fed on the green alga Derbesia marina, although it was expected to switch its diet to Codium fragile ssp. tomentosoides with the invasion of that species (Bleakney 1996). Further south in the Gulf of Maine, at Gosport Harbor on the Isles of Shoals (NH), P. dendritica was first noticed in abundance in 1996, feeding on the now abundant C. f. ssp. tomentosoides (Harris and Mathieson 2000).

Long Island Sound- Placida dendritica was first reported in 1967-1968, from Noank CT, feeding on C. f. ssp. tomentosoides(Clark and Franz 1968). At this time Clark and Franz had found only one population, and suggested that P. dendritica was a new arrival in the region.

Chesapeake Bay region- Placida dendritica was first collected in January 1974 in Swash Bay, an Atlantic coastal bays, in Northampton County VA. Eighteen specimens were found feeding on Bryopsis plumosa on an oyster tray (Vogel 1977). Since this is the only reported collection, we are uncertain of the establishment of P. dendritica in the Chesapeake Bay region. Since Codium fragile ssp. tomentosoides is now established in coastal waters adjacent to the Bay, this sea slug is probably present and could be common.

North Carolina- Placida dendritica was collected in Beaufort NC in 1959 (Marcus 1961).

Caribbean Sea- Placida dendritica was reported from the Florida Keys (Bleakney 1989) and Curacao (Marcus and Marcus 1970).

References- Bleakney 1989; Bleakney 1996; Clark and Franz 1968; Harris and Mathieson 2000; Marcus 1961; Marcus and Marcus 1970; Thompson 1976; Trowbridge 1997; Trowbridge 1999; Vogel 1977

Invasion Comments