Maladaptation in a marginal population of the mosquito fish, Gambusia affinis.

At Armand Bayou, Texas, mosquito fish (Gambusia affinis) living in a small freshwater stream had fewer, larger young and made smaller reproductive efforts than mosquito fish living 150-300 meters away in large, brackish estuary. In experiment 1 the progeny of field-caught from both fresh and brackish water were raised 1/2 in fresh and 1/2 in brackish water for the 1st week of life, and then were raised 1/2 in fresh and 1/2 in brackish water for the rest of their life, at densities of 20 fish/tank (8 treatments: FFF, FBF, FBB, BFF, BFB, BBF, BBB). The major effects were caused by the environment in which the fish were raised after the 1st week; fish raised in fresh water had lower survival, were short term and were lighter at 86 days of age than fish raised in brackish water. In experiment II the progeny of fish from treatments FBB and BBB were raised in isolation from birth in either fresh or brackish water made up from distilled water (4 treatments: FF, FB, BF, FB). Eight of the fish reared in fresh water also received medication. Fish reared in fresh water had lower survival, were shorter, and were lighter than those reared in brackish water. The fish that had received medication did not differ in length and weight from the other fish reared in fresh water, but they did have higher survival rates. In experiment III the progeny of fish from a freshwater reservoir in Hawaii were raised in fresh or in brackish water. Fish reared in fresh water had the same survival rate, were the same length or slightly longer and weighed the same or slightly less than fish reared in brackish water. The fresh and brackish stocks differed significantly in the distribution of allele frequencies at 2 or 24 loci examined (PGD) (phosphogluconate dehydrogenase) and .alpha.-GPD(glycerol-3- phosphate dehydrogenase)); these differences were small. The primary cause of differences noted in the life-history traits of fresh and brackish populations in the field was not local adaptation (an evolutionary explanation) or developmental sensitivity to early environment (and ontogenetic explanation) but maladaptation to the freshwater environment (a physiological explanation).