Acetes sibogae australis, a planktonic sergestid shrimp was collected at night with light traps suspended about 2 m deep in the harbour off Townsville, north Queensland, as well as in shallow water off the nearby beach and transferred to aquaria at the Australian Institute of Marine Science.Experiments were conducted in unaerated aquaria (15 x 22 x 60 cm) and water was changed every 2 days. Each aquarium was illuminated from both sides, by fluorescent light and the backs and bottoms of the tanks were painted black to make the transparent shrimps more visible. The front pane was marked at 5 cm intervals to estimate swimming speed and distance. Between 8 and 12 shrimps were placed in each aquaria and they were fed on Artemia, prior to commencement of experiments.During experiments, diced banana prawns (Penaeus merguiensis) or freshly killed and crushed Acetes, soaked in fluorescein dye were dropped into the aquariums and left to sink to the bottom, leaving thin trails of highly visible dye in the water. The movement patterns and swimming speed of the shrimps in response to the presence of trails was recorded. Additional experiments were carried out using meat and meat extract. Sensitivity to these food odours was also monitored over the time that individuals were exposed to the scent trails.The sensitivity of Acetes sibogae australis to specific chemical stimuli also was tested with pieces of blotting paper soaked in saturated solutions of 17 different L-amino acids.The importance of vision in following trails was tested by providing undyed meat and undyed blotting paper soaked in millipore filtered prawn juice as food stimuli. To determine whether Acetes sibogae australis generally swims downward when following a vertical trail or if they follow a chemical gradient, dyed meat tied to a string near one side of the aquarium was lowered and then slowly pulled it up near the opposite side and the response of the shrimps recorded.Experiments were also carried out to determine the anatomical location of the chemoreceptors on the shrimp. The role of the antennules was investigated by testing the behavioural response of shrimp to dyed stimuli, 18 hours after their antennules were excised. Other possible receptor sites, such as the setae in antennae were identified after ultrastructure analysis.
