Description
The ascoglossan sea-slug Placida dendritica is generally regarded as a single cosmopolitan species, found on both sides of the Atlantic and Pacific oceans, ranging from tropical to cold-temperate waters, and varying greatly in size. The chief hard structure in this animal, the radula, does vary geographically. Mophological features of P. dendritica show distinct patterns in the Pacific and the North Atlantic (Bleakney 1989). Experiments by Trowbridge (1997) show that body size and morphology in P. dendritica vary geatly with the food alga offered, but it is is not clear whether this affects the radula. Genetic studies would be useful to determine whether P. dendritica is a single species, and to determine the history and relationships of different populations.
Taxonomy
Kingdom | Phylum | Class | Order | Family | Genus |
---|---|---|---|---|---|
Animalia | Mollusca | Gastropoda | Ascoglossa | Stiligeridae | Placida |
Synonyms
Invasion History
Chesapeake Bay Status
First Record | Population | Range | Introduction | Residency | Source Region | Native Region | Vectors |
---|---|---|---|---|---|---|---|
1974 | Unknown | Unknown | Cryptogenic | Regular Resident | Western Atlantic | Unknown-Marine | Shipping(Fouling Community) |
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
Ecology
Environmental Tolerances
For Survival | For Reproduction | |||
---|---|---|---|---|
Minimum | Maximum | Minimum | Maximum | |
Temperature (ºC) | ||||
Salinity (‰) | ||||
Oxygen | ||||
pH | ||||
Salinity Range | poly-eu |
Age and Growth
Male | Female | |
---|---|---|
Minimum Adult Size (mm) | ||
Typical Adult Size (mm) | 20.0 | |
Maximum Adult Size (mm) | 20.0 | |
Maximum Longevity (yrs) | ||
Typical Longevity (yrs |
Reproduction
Start | Peak | End | |
---|---|---|---|
Reproductive Season | |||
Typical Number of Young Per Reproductive Event |
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Sexuality Mode(s) | |||
Mode(s) of Asexual Reproduction |
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Fertilization Type(s) | |||
More than One Reproduction Event per Year |
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Reproductive Startegy | |||
Egg/Seed Form |
Impacts
Economic Impacts in Chesapeake Bay
The sea-slug Placida dendritica has no known economic impacts in the Chesapeake Bay region.
Economic Impacts Outside of Chesapeake Bay
Placida dendritica is a grazer on the troublesome invasive seaweed Codium fragile ssp. tomentosoides. However, high abundances of the slug do not seem to affect abundance of the seaweed, but do increase fragmentation, and potentially, dispersal and vegetative reproduction of the plant (Harris and Mathieson 2000). Thus, this sea slug is not a good candidate for biological control of Codium spp.
References- Harris and Mathieson 2000
Ecological Impacts on Chesapeake Native Species
The sea-slug Placida dendritica is an herbivore on green algae. In its only reported collection in the Chesapeake Bay region, it was found feeding on Bryopsis plumosa (Vogel 1977). Effects of its grazing on this alga have apparently not been studied.
References- Vogel 1977
Ecological Impacts on Other Chesapeake Non-Native Species
The sea-slug Placida dendritica is known from only one collection in the Chesapeake Bay region, where it was found grazing on the presumed native alga Bryopsis plumosa (Vogel 1977).
Elsewhere in the Northwest Atlantic, P. dendritica has become a major grazer on the introduced Codium fragile spp. tomentosoides (Clark and Franz 1968; Harris and Mathieson 2000). The sea slug feeds by puncturing the cells, and sucking out the cytoplasm. High abundances of the slug do not seem to affect abundance of the seaweed, but do increase fragmentation, and potentially, dispersal and vegetative reproduction of the plant (Harris and Mathieson 2000).
References- Vogel 1977; Clark and Franz 1968; Harris and Mathieson 2000
References
Bleakney, J. Sherman (1989) Morphological variation in the radula of Placida dendritica (Alder & Hancock, 1843) (Opisthobranchia: Ascoglossa/Saccoglossa) from Atlantic and Pacific populations, The Veliger 32: 171-181Bleakney, J. Sherman (1996) Sea slugs of Atlantic Canada and the Gulf of Maine., , Halifax. Pp.
Clark, Kerry B.; Franz, David R. (1968) Occurrence of the saccoglossan opisthobranch Hermaea dendritica Alder & Hancock in New England, Veliger 12: 174-175
Harris, Larry G.; Mathieson, Arthur C. (2000) Patterns of range expansion, niche shift and predator acquisition in the Gulf of Maine, In: Pederson, Judith(Eds.) Marine Bioinvasions. , Cambridge. Pp. 46-56
Marcus, E. (1961) Opisthobranch mollusks from California, The Veliger 3: 1-85
Marcus, Ernest (1961) Opisthobranchia from North Carolina, Journal of the Elisha Mitchell Scientific Society 77: 141-151
Marcus, Ernst, Marcus; Eveline d.B.R. (1970) Opisthobranchs from Curacao and faunistically related regions, Studies on the Fauna of Curacao and Other Caribbean Islands 122: 1-129
Thompson, T. E. (1976) Biology of Opisthobranch Molluscs, , London. Pp.
Trowbridge, Cynthia D. (1995) Establishment of the green alga Codium fragile ssp. tomentosoides on New Zealand rocky shores: current distribution and invertebrate grazers, Journal of Ecology 83: 949-965
Trowbridge, Cynthia D. (1997) Dietary induction of opisthobranch morphology: Placida dendritica (Alder & Hancock, 1843) on different green algal diets, Journal of Molluscan Studies 63: 29-38
Vogel, Rosalie M. (1977) Shell-less opisthobranchs of Virginia and Maryland, , Williamsburg, VA.. Pp.