Replacing natural habitats with artificial structures such as pier-pilings, jetties, and seawalls has important consequences to abundances of biota. It is, however, not often known whether these are direct (the novel habitat alters abundances of some species) or indirect (the novel habitat directly alters some aspect of the behavior or ecology of some species, which, in turn, alter abundances of other species). Marine animals in some modified habitats in Sydney Harbour provide experimental opportunities to test hypotheses to distinguish between direct and indirect processes. Covers of bryozoans and hydroids were greater on kelp growing on pilings than on kelp growing on natural reefs. The epifauna may be affected directly by the pilings or indirectly, i.e., the structure affects characteristics of the kelp which, in turn, influence covers of epifauna. Thus, differences in covers of epifauna on kelp can be due to: (1) factors associated with the primary habitats (pilings vs. reefs), (2) differences between characteristics of the kelp found in each habitat, or (3) an interaction between these factors (habitat and/or type of kelp). Kelp were experimentally transplanted between pilings and reefs, demonstrating that properties of the habitat directly affected covers of epifauna, which were not influenced by the type of kelp that grows on pilings or rocky reefs. Manipulative experiments to unconfound multiple components of habitats influencing disturbances to biota are needed to understand human impacts on natural systems.
The population biology and habitat of Haliclystus auricula was studied from samples taken every two months in southern Chile between November 2001 and November 2002. A total of 3790 medusae were collected and examined. The abundance of H. auricula varied seasonally. Maximum densities occurred in summer (1405/m2) and minimum densities during the winter, when the population gradually disappeared. Gonads were observed in medusae starting from a size of 2·0 mm. Stauromedusae were found most often on the red filamentous alga Ceramium rubrum (69%), followed by Gymnogongrus furcellatus (12%) and Ulva sp. (10%). There were highly significant differences in mean abundance of medusae on these three algae (P<0·05). The remaining 9% of medusae were found on other seaweeds. The smallest medusae (0·08–0·9 mm umbrella height) were found exclusively on C. rubrum. The highest densities were recorded in the midlittoral zone.
The diet of Haliclystus auricula was studied from the gut contents of Stauromedusae collected every two months in southern Chile between November 2001 and November 2002. A total of 3790 medusae were collected and examined. Stauromedusae prey consisted primarily of harpacticoid copepods (68·4%), followed by gammarid amphipods (15·4%), chironomid fly larvae (9·2%) and podocopid ostracods (5·9%). The remaining 1·1% of the diet was made up of empidid fly larvae, polychaete worms, isopods, juvenile decapod crustaceans and gastropods. Gut content was observed in 31% of the stauromedusae collected and only in medusae with an umbrella height greater than 0·4 mm. Frequency of medusae with prey items in their gut content increased with stauromedusae size. The type of prey most frequently found in the smallest medusae were copepods, whilst larger individuals contained mainly amphipods. A strong tendency for the length of prey consumed to increase with medusae size was found.
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