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Ecology of the amphipods Parathemisto pacifica (Stebbing) and Cyphocaris challengeri (Stebbing) in the Strait of Georgia : mandible morphology, feeding habits, and food distribution Haro-Garay, Martha Jeannette

Abstract

In amphipods, morphological adaptations for feeding provide taxonomic evidence that helps to differentiate them into species. Intuitively, this indicates that the morphology of feeding structures may be related to the type of food the amphipods consume, and that their distribution would depend on the distribution of their food. These hypotheses were examined using zooplankton collections from three locations from the Strait of Georgia, British Columbia: the Fraser River plume, the estuarine plume and the Strait. The study investigated the mandible morphology, the stomach contents and the potential food for the amphipod species Parathemisto pacifica and Cyphocaris chalkngeri. Mandible morphology obtained with SEM photography and morphometric analyses were used to infer feeding habits. The mandible of P. pacifica has a wide molar indicative of microphagous habits and a cutting incisor related to carnivorous habits, while in C. chalkngeri, strong molar and sharp incisor in the mandible indicate carnivorous habits. The population dynamics indicated that the mean recruitment event for both species occurs in late spring. Recruits born then benefit from the available food during spring and summer, the most productive seasons in the Strait of Georgia. Temporal changes of zooplankton abundance and composition coincided, in general, with the seasonal increase of the river discharge volume; spatial variation coincided with the interaction of wind and tides with the Fraser River. The zooplankton species most commonly found in stomachs were the copepods Pseudocalanus minutus, Paracalanus parvus, Oithona sirrdUs, Oithona spinirostris, Oncaea borealis, Metridia pacifica, young stages of the euphausiid Euphausia pacifica, ostracods Conchoecia spinirostris and C. alata minor, and the pteropod Limacina helicina. These species had an overlapping distribution with the amphipod species studied. The presence of these microzooplankton species in the diet of amphipods suggests amphipods are an important link between nannoplankton (e.g. Dinophysis sp., Peridinium sp., Mesodinium sp.) and fish that consume amphipods (e.g. salmonids and clupeoids). This can be particularly influential in the trophic ecology of the Strait during the years of low abundance of the copepod Neocalanus plumchrus and the euphausiid Euphausia pacifica, the most abundant hervibores which salmonids feed on. The suggested trophic pathway is nannoplankton (20-30 u,m, e.g. flagellates, ciliates: Dinophysis sp., Peridinium sp. Mesodinium sp.) -> microzooplankton (20-200 urn, e.g. protozoans, tintinnids) -> mesozooplankton (2- 20 mm, e.g. copepods, cladocerans, ostracods) macrozooplankton (2-20 cm, e.g. amphipods, euphausiids) -> fish (e.g. herring, salmon). This type of food chain is considered typical of oceanic areas such as the Pacific subarctic. Its occurrence simultaneous to the phytoplankton (chain forming diatoms) -> zooplankton (copepods, euphausiids) -> fish (e.g. herring, salmon) suggests that the model most appropriate to describe the trophic dynamics of the Strait of Georgia would be the multivorous food web.

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