Comparisons of Secondary Production, Life History, and Mouthpart Functional Morphology Between Two Populations of the Amphipod Gammarus minus

Abstract

In this study, features of ecology, behavior, and

functional morphology related to feeding activity of

two populations of the amphipod Gammarus minus

were compared. The two populations occupied

different habitats, and I attempted to determine

whether differences observed between the two

populations were related to habitat. Annual

production and life history of the populations were

compared and their relationship to factors such as

temperature, water-chemistry, and quantity of

available organic matter were examined. Mouthpart

and foregut morphology were compared between the

two populations and between immature and mature

amphipods with light and scanning electron

microscopy. Measurements of structure were

analyzed by linear regression. A behavioral study,

comparing feeding of immature and mature G. minus,

was conducted in the laboratory. Annual production

of G. minus occupying a habitat characterized by the

presence of watercress, gravel substrate, and constant

temperature (Site 1) was 3.9 g/sq.m (95% C.I.: 3.2-

4.5), while that in the habitat characterized by leaf

detritus and fluctuating temperatures (Site 2) was 1.8

g/sq.m (95% C.I.: 1.6-2.1). Breeding occurred

throughout the year at Site 1, but there was a yearly

cycle at Site 2. The greatest numbers of the smallest

size classes of amphipod were present at Site 2 when

the quantity of ash-free dry mass (AFDM) of wood

and bark was greater than AFDM of leaf detritus. Of

nine mouthpart and foregut structures studied, three,

the number of cuspidate setae on outer plates of

maxillipeds, the length of the dactyl on maxilliped

palps, and the number of hook setae on the foregut

ampullae, were found to be correlated with body

length. Of these, rates of increase in maxilliped setae

numbers and hook setae numbers were greater for

immature than mature animals, and the number of

hook setae for a given sized animal was generally

greater for animals at Site 1 than Site 2. Animals

presented with ground-up leaf material in the

laboratory exhibited twenty-one recognizable

behaviors. The frequencies of six behaviors were

found to be statistically different between immature

and mature animals. The differences suggest that

immatures may prefer a food type or size other than

that provided in the experiment.