Arctica islandica , black clam

Background

Arctica islandica , the black clam, is found on both sides of the North Atlantic. In the western Atlantic the species ranges from the Arctic to Cape Hatteras. Species information is from Cargnelli et al . (1999). Black clams prefer medium to fine grain sand or sandy mud, burrowing to just below the sediment surface. They inhabit fully marine waters from the intertidal zone to 482 m, but usually between 10-280 m. In eastern Maine black clams are found in relatively shallow water though never intertidally. South of Cape Cod they occupy deeper, more offshore waters. Adults are sessile and form dense beds. Black clams are filter feeders. They spawn from spring to fall with fertilization happening in the water column. Larvae are planktonic for 55-60 days after which they settle on soft substrate as sessile juveniles for approximately 9-12 years. Black clams are harvested commercially from the southern Gulf of St. Lawrence to New Jersey.

Temperature limits, critical thresholds, vulnerability, and barriers to adaptation

Sea Surface temperatures in the current distribution of the black clam range from a February minimum of -2.1 oC to an August maximum of 28.2 oC.

Thermal criteria for Arctica islandica are from Cargnelli et al. 1999 and references therein. Black clams are restricted to cooler waters which rarely exceed 20 oC. The optimal temperature has been estimated from field distributions as 6 o-16 oC, however a lethal temperature range of 13 o-16 oC has been reported, with 21 oC clearly being lethal. Their inshore distribution appears to be limited by the summer 16 oC isotherm. Experiments on a clam from Nantucket Shoals showed cessation of growth below about 6oC. Larvae in the laboratory can survive temperatures from 8.5 oC to at least 20 oC but best growth is from 13 o-15 oC. During the warmest months larvae were found most concentrated between 14 o-18 oC off Massachusetts. Juveniles can survive temperatures as high as 20 oC and can grow as low as 1 oC. Shell growth rate increases the most from 1 o- 6oC with a decreasing rate to at least 12 oC.

Black clams were somewhat above average rank in the sensitivity analysis. This is attributable to their mobility as planktonic eggs and larvae, which may aid dispersal to suitable habitat, and to their eurythermal nature. There appears to be little difference among life stages in potential vulnerability to warmer waters resulting from climate change as all stages are viable up to approximately 20 oC. However, adults may be more sensitive to temperatures above 16 oC.

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Impacts

A 4 oC rise in global temperature will impact the future distribution of black clams in the western Atlantic. Results from all models and scenarios are generally similar and show a potential loss of thermal habitat from the southernmost part of the study area northward to between North Carolina and New Jersey depending on the model. Model CCSR also shows loss in Labrador waters to varying degree with scenario. No gain of thermal habitat is predicted as the northern range limit of this species is north of our study area. Harvesting should not be negatively affected except perhaps in New Jersey waters. If distribution changes with global warming follow present inshore-offshore patterns, then the phenomenon of offshore distribution in southernly populations may move north of Cape Cod into the Gulf of Maine.

References

L. M. Cargnelli, S. J. Griesbach, D. B. Packer, and E. Weissberger. 1999. Essential Fish Habitat Source Document: Ocean Quahog, Arctica islandica , Life History and Habitat Characteristics. NOAA Technical Memorandum NMFS-NE-148.

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