Date of Award
Doctor of Philosophy in Oceanography
Theodore A. Napora
The planktonic amphipods Parathemisto libellula and P. pacifica coexist in coastal waters from southeastern Alaska (56° N) to the south-eastern Chukchi Sea (70° N). Observations on distribution, reproductive cycles, diets, metabolism, and starvation tolerance contribute to understanding this case of congeneric coexistence.
Samples were collected monthly from September 1969 to October 1970 in southeastern Alaska, from May to September 1969 in the southeastern Bering Sea, and in September-October 1970 in the southeastern Chukchi Sea. Respiratory metabolism and starvation tolerance were studied in winter-caught amphipods from southeastern Alaska.
Both species consistently occurred above 300 min southeastern Alaska. In the southeastern Bering Sea, their distributions were separate. With P. libellula restricted to an area where summer bottom temperatures were less than 8.0° C. The P. pacifica found in the Chukchi Sea were probably expatriates from the Bering Sea.
In southeastern Alaska, P. libellula has a 1-year life cycle. Broods are released in early May. Juveniles initially live in the surface 50 m, but by late June (at a size of 10 mm) they migrate vertically. By late October they have a daytime depth of 200-300 m. Males mature in late winter at 19-21 mm and females at 21-25 mm.
In southeastern Alaska, P. pacifica reproduces throughout the year with a strong peak in early May. Juveniles initially live in the surface 50 m. Adults and subadults migrate vertically to a daytime depth of 100-200 m. Summer growth is rapid, and maturity is attained in 6-8 weeks at a size of 4. 5-6. 5 mm. Generation time lengthens in winter to 8-12 weeks, and size at maturity increases to 6-0-7.0 mm. Diets of the two amphipods pods are similar. About half the food items were calanoid copepods, and 30% were compound-eyed crustaceans (primarily euphausiids and amphipods). Parathemisto pacifica were more cannibalistic than P. libellula.
The temperature coefficient (Q10) for respiratory metabolism of P. libellula is 3-5 at temperatures below 5.0° C, and near 2 at temperatures above 10.0° C. At 7.5° C, the respiration rates for animals tested had a bimodal distribution--not all animals changed their metabolism at the same rate and/or temperature. Respiration rate-weight relationships are not affected by temperature. Respiration rates are inversely correlated with salinity between 30.7°/oo and 32.4°/oo This relationship may serve to maintain a constant metabolic rate during vertical migration.
The Q10 for respiratory metabolism of P. pacifica is near 2 at temperatures below 5.0° C; thus, P. pacifica cannot lower metabolic requirements during the winter as well as P. libellula.
Parathemisto libellula endured starvation for 56 days and P. pacifica for 36 days at 6-7° C.
Annual cycles of temperature and food in Alaskan coastal waters impose conditions outside optimum physiological ranges but within the tolerance of both species. Temperatures above 8.0° C may impose higher metabolic demands upon P. libellula than can be fully supported by the available food. Winter food may be inadequate for the metabolism of P. pacifica. Parathemisto libellula is better adapted to winters than P. pacifica, and P. pacifica better adapted to summers than P. libellula; consequently, in either time of year neither deominates long enough to exclude the other.
Wing, Bruce L., "ECOLOGY OF PARATHEMISTO LIBELLULA AND P. PACIFICA (AMPHIPODA: HYPERIIDEA) IN ALASKAN COASTAL WATERS" (1976). Open Access Dissertations. Paper 1409.