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Biol. Bull. 201: 274-276. (October 2001)
© 2001 Marine Biological Laboratory

An Initial Study on the Effects of Signal Intermittency on the Odor Plume Tracking Behavior of the American Lobster, Homarus americanus

Corinne Kozlowski1, Kara Yopak2, Rainer Voigt2 and Jelle Atema2

Boston University Marine Program, Woods Hole, Massachusetts

Chemical signals are used by organisms for communication and location of food, mates, and shelters. The spatial and temporal distribution of these signals is shaped primarily by environmental conditions. Turbulent odor dispersal causes intermittency in the chemical signal even when the source emits continuously. Therefore, animals that use chemical cues to localize odor sources must overcome signal intermittency. The ability of lobsters to track continuously released odor plumes has been well-described (1). Lobsters may be using one or a combination of two possible mechanisms to locate an odor source: (a) odor-gated rheotaxis, which would cause the animal to move upstream, using the mean current for orientation once a chemical signal is detected (2), and (b) eddy-chemotaxis, which would require an animal to use the internal chemical and hydrodynamic fine structure of an odor plume to locate the source (3). The mechanisms lobsters use to overcome signal intermittency are still unknown. Male moths use a sequence of upwind surges and horizontal casting to locate a female releasing pheromone (4). In the presence of an odor source, tsetse flies perform a series of overshoots followed by 180° turns until they come within 1 m of the source (5). In more turbulent odor plumes, blue crabs decrease their locomotor activity and stop and turn more frequently (6). Here we explore how lobsters track odor plumes with a controlled increase in intermittency.

Lobsters (Homarus americanus), ranging in carapace length from 77.5 mm to 98.5 mm, were caught locally and kept in separate holding tanks with running seawater. Twice weekly the animals were fed about 2 g of squid. As in previous studies, this small amount was thought to increase their motivation to track an odor source, consisting of 100 ml squid rinse/l seawater. Each lobster was tested in a flume (1.8 m x 5.5 m x 0.5 m experimental arena) with a mean flow rate of 4.5 cm/s. Each lobster was blindfolded; a white dot of nail polish on the carapace served as a reference point to digitize the track. After a 20-min acclimation period, each lobster was placed into a shelter 6 m downstream from a jet source releasing odor at 100 ml/min through a nozzle with a 2-mm ID (Re = 200). The trial began once the lobster began exhibiting tracking behavior (antennule flicking and antennae waving) in the downstream patch field as visualized with dye; it ended once the animal was less than one body length away from the source, or after 20 min. Animals that tracked a continuous jet plume (Fig. 1A) were randomly tested with odor pulses (1 cm in length) with gaps of about 5-cm (Fig. 1B) and 10-cm (Fig. 1C) between them. Dye visualization showed that interpulse gaps were maintained for 2 to 3 m from the source; farther downstream, the pulses merged due to turbulent dispersal in the flume. Fresh seawater entered into the flume during each trial to minimize odor accumulation, and the flume was drained and refilled each night. All trials were videotaped and digitized using the Metamorph® Imaging System (Version 3.5, Universal Imaging Corporation) for analysis. Walking speed, heading, and turning angles were then calculated for each track.



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  		Figure 1. Individual tracks under plume conditions with continuous release (A, N = 10) and deliberate gaps of 5 cm (B, N = 7), and 10 cm (C, N = 4). Solid lines indicate approximate boundaries of the odor plume, as visualized with dye. Odor source is located at (x = 0 cm, y = 0 cm, z = 9 cm); shelter is located at (x = 550 cm, y = 0 cm).

 
Under all three plume conditions (continuous, pulsed with 5-cm gaps, and pulsed with 10-cm gaps), heading and turning angles remained constant with distance from the source for the successful tracks; 84.5% of heading angles ranged from -40° to 40°, and 86.9% of turning angles ranged from -20° to 20°. Source localization success decreased with increasing gap length: 10 out of 33 lobsters successfully tracked the continuous jet plume (Fig. 1A), while 7 of these 10 tracked the plume with 5-cm gaps (Fig. 1B), and only 4 tracked the plume with 10-cm gaps (Fig. 1C). Generally, lobsters emerged slowly from the shelter area (orientation phase, 3 to 6 m from the source). Under continuous plume conditions (Fig. 1A), walking speed then increased during the subsequent tracking phase (1 to 3 m) and decreased again as the animal approached the odor source (0 to 1 m). During the tracking phase and final approach, lobsters mostly stayed within the plume boundaries (20% of animals spent more than 5 s outside of the suggested plume area). Overall, the 10 lobsters that successfully tracked under continuous plume conditions (Fig. 1A) seemed to show a straighter approach to the source (higher linearity index; 68% had a linearity index equal to or greater than 0.9) than when tested under intermittent plume conditions (Fig. 1B, C) (57% had a linearity index equal to or greater than 0.9). In contrast, in plumes with deliberate gaps (Fig. 1B, C), walking speed remained constant with distance from the source and increased during the final approach; 81.8% of animals walked outside of the plume boundaries for at least 5 s during the tracking and final approach phases. Mean walking speed decreased with an increase in gap length. Most lobsters that did not locate the odor source in plumes with deliberate gaps walked along the wall or did not leave the shelter. Two lobsters showed tracking behavior downstream from the odor source (2) but then lost the odor plume upstream in the jet field.

These results suggest that successfully tracking lobsters use similar walking paths independent of signal intermittency. Counter-turning or casting behavior as described for moths (4) was rarely observed. However, tracking success dropped with increasingly intermittent signal conditions. It appears that lobsters require a minimal signal encounter rate to continue tracking the plume successfully to the source. The fact that lobsters stayed mostly within the odor plume boundaries further suggests that they use its internal fine structure for guidance.

First and second authors are listed alphabetically; both authors contributed to the experiment equally and in the same manner. This study was supported by NSF REU Grant (OCE-0097498) to CK and KY, and ONR Grant (N00014-981-0822) to JA.

Footnotes

1 Bowling Green State University, Laboratory for Sensory Ecology, Bowling Green, OH 43403-0212. Back

2 Boston University Marine Program, Marine Biological Laboratory, Woods Hole, MA 02543. Back

Literature Cited

  1. Moore, P. A., N. Scholz, and J. Atema. 1991. J. Chem. Ecol., 17:1293–1307.
  2. Baker, C. F., and J. C. Montgomery. 1999. Polar Biol., 21:305–309.
  3. Atema, J. 1998. Biol. Bull., 195:179–180.
  4. Vickers, N. J., and T. C. Baker. 1996. J. Comp. Physiol. A, 178:831–847.
  5. Bursell, E. 1984. Physiol. Entomol., 9:133–137.
  6. Weissburg, M. J., and R. K. Zimmer-Faust. 1994. J. Exp. Biol., 197:349–375.[Abstract]



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