Latitudinal variation in the Dungeness crab,Cancer magister: zoeal morphology explained by incubation temperature

The difference in morphology between zoeae of Cancer magister Dana from Alaskan and Californian waters was documented to determine if the morphological variation is attributable to environmental influences. First-stage zoeae from Alaska have significantly longer carapace spines than zoeae from central California. The dorsal, rostral and lateral carapace spines were 14, 14 and 29% longer, respectively, in the Alaskan zoeae. The effect of temperature was tested on zoeal morphology as it is an obvious environmental difference between Alaskan and Californian waters. Ovigerous female crabs collected in southeastern Alaska in 1984 were held at 1°, 5°, 10° and 15° C until hatching occurred. Eggs were sampled seven times during the incubation period, and relative mortality, egg diameter and development stage were measured. All of the crabs and eggs at 1° C died before hatching occurred. Egg mortality averaged less than 2% in the other temperature treatments. Egg diameter increased significantly over the incubation period for all temperatures. Developmental rate of the embryos was inversely related to temperature. Hatching first occurred in 42 d at 15° C, 60 at 10° C and 160 d at 5° C. Newly hatched zoeae were collected and body length, dorsal, rostral and lateral carapace spines were measured. Significant differences existed between all temperatures for all spine lengths, with longer spines occurring at lower temperatures. Zoeal body lengths were also significantly different between the three temperatures. The results of this study question the use of spine lengths to distinguish similar larval species.