An earlier study of ours demonstrated polygenic control of habitat preference between sympatric populations of an estuarine amphipod (Stanhope et al. 1992). Knowledge of the ecological history in this estuary suggested that two new and distinct habitat types (wood debris and Fucus) had been formed immediately adjacent to each other, in an area which was previously the habitat type common to the rest of the estuary (bank). This suggested the possibility that the ancestral population had been split into two resource specialists (disruptive selection on habitat preference). The genetic relatedness of these three populations (the proposed ancestor and the two proposed descendants) and six others occupying the same three habitat types were investigated on a regional geographic scale, using an extensive set of genomic DNA RFLPs. These data were combined with measures of habitat preference (including genetic tests) in the additional populations. A very strongly supported phylogeographic tree, unequivocally supported a shift in habitat preference in the wood‐debris population of this estuary. The data did not, however, support the hypothesis of disruptive selection on the ancestor (bank) with a consequent split into two resource specialists. Instead, it clearly indicated that the occupants of the Fucus habitat type were members of a habitat specific race, and thus their presence in this estuary reflected the expansion of an old resource base. Furthermore, the combined RFLP and habitat preference data, for all nine populations, indicated that the same polygenically based shift in habitat preference that had occurred in the original estuary had occurred independently, in another estuary, 700 km removed, that had similar environmental circumstances.
Earlier studies of population dynamics and population genetics of an estuarine amphipod (Stanhope & Levings, 1985;Stanhope, 1989), have suggested that amphipods residing in each of three separate and distinct habitat types within the same estuary exhibited strong habitat fidelity. These populations were separated by as little as 200 m of intertidal mud flat. Laboratory choice tests involving members of each population and the substrate that characterized each habitat type, demonstrated that the members of the two immediately sympatric populations (called Fucus and wood debris) exhibited strong substrate preference for their native habitat type. The third population (bank) showed a strong avoidance for a key feature of one of the habitats (Fucus) and a slight tendency to avoid the other (wood debris). Interpopulation hybrids between wood debris and bank amphipods revealed a highly significant dominance deviation in the F1, towards wood debris preference, which disappeared in the F,. Similarly, F1 generations of crosses between Fucus and bank showed a highly significant deviation towards Fucus preference that was lost in the F2. Crosses between Fucus and wood debris amphipods showed no significant preference for either substrate in the F1 or F2. The results of the interpopulation crosses are interpreted as evidence for polygenic control of habitat preference with dominant genetic effects in wood debris and Fucus amphipods for their native substrate. Knowledge of when and how the wood debris arose, the nature of the original environment, as well as the ancestry of the wood debris population, provide information on shifts in habitat preference in this estuarine invertebrate and suggest habitat fidelity has been a contributing factor in sympatric population subdivision.
An earlier study of population genetics of an estuarine amphipod provided evidence from genomic DNA analysis for a habitat-specific race of amphipods within the speciesEogammarus confervicolus. In some estuaries of the northeast Pacific, this race of amphipods exists sympatrically with other members of the species. Here we present evidence for a race-specific pheromone that appears to be the consequence of differential metabolism of the algae (Fucus distichus andPelvetia fastigiata) characteristic of the habitat occupied by this race. The race-specific pheromone identified in this study is a subtle modification of an already existing communication system: females of the habitat-specific race produce the pheromone characteristic of the species as a whole but have an ability not shared by other females of the species to modify this pheromone when raised on the algal substrate characteristic of their habitat. Only males of this race make a distinction between the more specific pheromone and the species pheromone. The formation of hybrids (conceived and raised on the algal substrate) between members of the habitat-specific race and the other members of the species disrupted the ability to produce and distinguish the race-specific pheromone; hybrids still produced a pheromone, but it was indistinguishable from that produced by the species as a whole. Behavioral assays and the results of reciprocal, interpopulation crosses indicated pheromone response in males had evolved with production; males however, did not have to be raised on the algal substrate to respond to the alternate pheromone. No evidence for maternal effects or sex linkage were detected in the results of the crosses; more specific indications of the genetics underlying pheromone production were not evident.
Feeding rates and prey-size selection of the predatory naticid snail Polinices lewisii (Gould) on the littleneck clam Protothaca staminea (Conrad) were studied on the west coast of British Columbia. Caging experiments indicated that few clams of the smallest size class (10–35 mm shell length) were drilled by Polinices lewisii; predation was heaviest on clams in the size range 35–50 mm. Feeding rates of P. lewisii did not vary significantly over the range of densities provided in field experiments, nor were they influenced by the close proximity of other moon snails. Feeding rates were relatively low. Summer feeding rates of P. lewisii indicated a correlation with respect to snail size, and results from the annual study indicated a seasonal trend towards reduced consumption during the winter months. Polinices lewisii, feeding on P. staminea, was estimated to eat 0.1% of its own dry mass per day. Information from recovered shells of drilled clams showed that the diameter of boreholes was correlated with snail length but only poorly correlated with shell thickness. Boreholes on valves of P. staminea were found mainly in the umbo region. No strong preference for either left or right valve was observed. Random sampling of field populations indicated that prédation by P. lewisii was a minor source of mortality in P. staminea populations at Ladysmith Harbour, British Columbia, Canada.
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