The Influence of Odors on Food Search: Studies with Marine Decapod Crustacea
Field and laboratory tests were conducted for the kelp crab, Pugettia producta (Randall) and for the California spiny lobster, Panulirus interruptus (Randall) to determine the role of chemical stimuli in food search. Sensitive chemoreceptors were identified in gill chambers of Pugettia capable of inducing reflex cardiac inhibition. The gill chamber has not previously been described as a major chemoreceptive site in marine decapods. Stimulation of the branchial chamber with amino acids and sugars evoked cardiac inhibition that was significantly greater than that induced by sea-water controls, and chemical sensitivities were correlated with the natural diet of the animal. Primarily homolateral leg and claw food-orienting behavior were observed to follow stimulus injections into branchial chambers; however, stimuli were not strictly confined to this region. Consequently, unmistakable correlates have yet to be established between branchial chamber chemoreception and food search.
A separate study was conducted in which food search was activated by chemical stimuli in specimens of Pugettia, with no differences observed between laboratory and field tested animals. Food search was found to consist of two components: near field (non locomotor leg and chela probing) and far field (locomotion) search. Leg and chela probing were initiated at threshold concentrations one log unit or more below that of locomotion and were initiated prior to and in the absence of locomotion. The nutritional states of tested animals did not influence exhibited response hierarchies. These findings show that stimuli commonly considered to trigger distance chemoreception, i.e. non-particulate, usually low concentration chemical stimuli, fail to activate distant food search in Pugettia. The response hierarchy of P. producta emphasizes a search for nearby food items and is independent of the animals nutritional state.
Field experiments conducted for Panulirus used traps baited with either live intact or injured prey, or excised tissues, placed in containers allowing odor release but preventing contacts with entering animals. Individuals of six prey species failed to attract lobsters when alive and intact, but some of these became attractive once injured. Two excised tissues, abalone and mackerel muscle, were attractive to lobsters but were relatively nonattractive to crabs. Abalone muscle increased in attractivity following 1-2 days field exposure. Molecular weights of stimulants released by both weathered and fresh tissue were 10,000 daltons, with the 1-10,000 dalton fraction contributing significantly to attraction. The minimum effective concentration of abalone muscle effluent was 10-8- 10-10g/l in immediate trap vicinities, as estimated by use of a three-dimensional Fickian diffusion model. These values are extremely low, but within known detection limits for Panulirus. Concentrations of total primary amines released by abalone muscle never exceeded background levels at distances greater than 32 cm from tissues, arguing that chemical stimuli act over only a very short range.
Given the highly turbulent environments inhabited by Pugettia and Panulirus, it is the conclusion of this investigation that chemical stimulants induce only local searches for food. Chemical stimuli may modify, but do not appear to activate distant foraging. Locomotions activated by stimuli not specifically related to a food source are therefore deemed essential in bringing these animals to the immediate vicinities of food.