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1 Amherst College, Amherst, MA
2 Boston University Marine Program, Woods Hole, MA
* Corresponding author: atema{at}bu.edu
Serotonin (5-HT) and octopamine have been implicated in regulating the behavioral phenotype of lobsters by altering levels of aggression in agonistic interactions (1). High-dosage injections of these amines consistently evoke extension (5-HT) and flexion (octopamine) postures in both lobsters and crayfish. These postures have been likened—probably erroneously—to aggressive and submissive postures, suggesting a specific role for these neuromodulators in social behavior (2). Dopamine injections into freely moving lobsters have also induced motor activity, including extension of claws, legs, and tail, originally interpreted as an agonistic posture (3). However, the relationship between neuromodulator treatment and overall crustacean behavior is not simple, with responses varying with the stimuli presented and the aminergic manipulations performed (4,5). Serotonin is known to regulate feeding behavior in other animal species (6), but its effects on lobster feeding behavior and responses to odors have not yet been investigated. As lobsters are known to rely heavily on chemical signals for food and social information (7), we examined the reaction of H. americanus to food and social odors after the animals had been injected with serotonin and two other biological amines common to the lobster central nervous system (CNS).
Seven adult intermolt male lobsters (81–93 mm carapace length) were injected with serotonin, octopamine, dopamine (Sigma Chemicals), or lobster saline. Each injection of amine was administered in random order over 4 consecutive days of testing, and consisted of 0.3 mg of neuromodulator per kg of animal dissolved in 1 ml lobster saline. Injections were made intramuscularly into the first abdominal segment to the right of the ventral nerve cord, following Peeke et al. (4). After injection, each lobster was placed in an observation tank (36 x 46 x 72 cm) with a constant flow of unfiltered seawater. Each tank had a plastic, two-entrance shelter in front of the window and a constantly flowing air-lift water circulation system with funnel interruption (7). Food odor and body odor were injected into the funnel, which delivered an irregular flow of tank water into the shelter at a mean rate of 4.8 ml/s. Lobsters were not fed for the 4-d duration of the experiment. Tanks were kept dark during observations except for a 25-watt bulb covered in black plastic with a small hole cut through that allowed a narrow beam of light to illuminate the shelter interior.
Body odor stimuli were prepared by collecting water samples from one male and one female lobster isolated in 10 l of aerated standing seawater for at least 3 h. Food odor consisted of store-bought clam juice. Fifteen minutes after drug injection, lobsters were presented with 0.5 ml of each stimulus at 5-min intervals, and the resulting behaviors and their durations were recorded to the nearest second. The order of stimulus introduction was held constant over the four days of injections but varied randomly among the animals. We selected behaviors from Atema and Cowan (7) based on frequency and quantifiability. They included "locate source," turning toward the glass at the front of the shelter and probing the stimulus inflow tube; "check entrance," walking over to, and standing still at, one of the two entrances; and "dactyl clasping," a typical feeding response marked by opening and closing the dactyls of the first two pairs of walking legs. Stimulus introductions were repeated every 15 min for the first hour, and then again 120 and 210 min post-injection to test for a time delay in response. The study was conducted blind for six of the eight animals tested, with the observer unaware of the amine injected from day to day.
Lobsters injected with serotonin and presented with clam juice displayed a mean duration of dactyl clasping that was twice that observed after saline injections. Statistical analysis showed that this difference in duration was significant (ANOVA post-hoc t test, t = 6.69, P = 7e-11). The serotonin effect peaked at 60 min post-injection, when the duration of clasping was over 5 times that of the saline control; the effect then returned to initial levels by t = 120 min (Fig. 1). The t = 60 min response was also significantly longer than the initial response at t = 15 min (t = -2.69, P = .0082). In contrast to serotonin, dopamine induced a shorter duration of clasping in response to clam juice at all times examined; the mean response was less than 50% that of saline. This difference was statistically significant (t = -2.68, P = .0078). Octopamine-induced clasping in response to clam juice varied widely between animals, and the overall difference in duration for octopamine injections compared to saline was not significant (t = 1.254, P = 0.21). Dactyl clasping was not observed in response to body odor. The occurrence of "locate source" and "check entrance" behaviors, infrequent for all three chemical stimuli, appeared unrelated to treatment condition.
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Financial support is acknowledged from NSF-REU (OCE-0097498 site awarded to Boston University Marine Program). We thank Sean Sapcariu for overseeing blind injection schedules, Molly Steinbach for design advice, and Gabi Gerlach for her aid in statistical analysis.
Literature Cited
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