Heat Effects on a Marine Snail

Littorina littorea enters a reversible state of heat coma at temperatures below the lethal temperature of the species. The threshold of heat coma can be shifted as much as 8.5° C by acclimation; a linear relation exists between heat coma temperature and acclimation temperature over the experimental range.Deganglionated columellar muscle preparations of 5° C and 25° C acclimated Littorina littorea showed an approximately linear decline in tetanic tension when tested from 5° C to 40° C. There was no statistically significant difference in the amplitude of muscle contraction or of tension-length curves of 5° C- and 25° C-acclimated muscles at any experimental temperature; this excludes failure of muscle contraction as the cause of heat coma. Speed of contraction was similar and relaxation slower in muscles from 5° C-acclimated snails than in muscles from 250 C-acclimated snails when measured at 10° C but not at 25° C.Attempts to ascertain by the use of drugs and nerve degeneration experiments whether muscle contraction was elicited by direct on indirect stimulation were inconclusive, although the effects of TTX were consistent with the hypothesis that stimulation was by nerves.Lethal temperature in L. littorea shows a shift of only about 1° C to 2° C as a result of temperature acclimation. The ability of the columellar muscle to contract following exposure of the snail to extreme temperatures for one hour was similarly shifted only 1° C to 2° C by acclimation.Measurement of spontaneous activity of the central nervous system of L. littorea showed good agreement between temperature of cessation of spontaneous activity and temperature of heat coma in 5° C- and 25° C-acclimated snails. Analysis of nervous activity showed that increase in activity with rising temperature was based upon both a greater frequency of firing of single units and activation of new units. Single unit frequency appeared to show no acclimation effect except in the temperature of cessation of activity.