Estuaries, such as the Great Bay estuary in N.H. are characterized by large fluctuations in temperature and salinity (for more details on the estuary click here). Animals living in an estuary, like lobsters, have evolved physiological and behavioral mechanisms to optimize their survival in this complex and occasionally stressful habitat. We have been carrying out field and laboratory studies for 8 years on this topic. The field studies are described in the Field Studies section of my Home Pages. The laboratory studies include investigations of their: 1) behavioral responses of lobsters to drops in salinity; 2) salinity receptors; 3) ability to behaviorally thermoregulate; 4) thermal receptors; 5) ability to osmoregulate; 5) locomotion and biorhythms and; 6) cardioregulatory physiology. Each of these will be discussed briefly below.
Lobsters tend to avoid low salinity water. We have demonstrated this using several different behavioral assays (Jury et al., 1994a). Interestingly, we find that male and female lobsters differ somewhat in their aversive responses. Males appear to be less responsive, which might explain why there are more male lobsters the further one moves up into the estuary (Figure 2; see (Howell et al., 1999).
Currently, Dan O'Grady, a graduate student in the laboratory, is exploring two related questions: How does a drop in salinity influence the overall locomotory activity of lobsters? Do juvenile lobsters differ from adult lobsters in their sensitivity to drops in salinity?
When many animals, including lobsters, are exposed to novel stimuli, their heart rate changes (see movie below). We have developed a cardiac assay so we can determine the sensitivity of lobsters to salinity and temperature. Fine wire electrodes are placed near the heart and gill bailers (scaphognathites) to record heart and ventilation rate. These electrodes are connected to an impedance device that detects changes in the impedance of the tissues as they move. The signal is then amplified and displayed on a Macintosh computer using MacLab software (http://www.adinstruments.com). We then alter the environment and look for changes in heart rate as indicators that lobsters sense the change.
Typically, when we slowly drop the salinity of the water in the test chamber lobsters respond with an increase in heart rate when the salinity reaches about 26 ppt, a drop of 6 ppt from the normal salinity of 32 ppt. Then, as the salinity continues to drop, lobsters exhibit a dramatic slowing of their heart rate, or a bradycardia, when the salinity reaches 21 ppt. A typical response is shown in the recordings below. An abstract from the paper we submitted on this topic is available on the abstract page (Dufort et al., 2001).
It appears as if the receptors involved in sensing salinity are located in the branchial chamber, although they have yet to be identified.
When lobsters are placed in a thermal gradient tank (see Figure 4) they will move to a location in the tank with a temperature of approximately 16-17°C (Crossin et al. 1999). In general, male lobsters prefer slightly warmer temperatures than females. These responses in the laboratory appear to correlate well with their movements in thermal gradients in the estuary. Steve Jury has created a model which will predict the movements of lobsters in an estuary based primarily on their responses to temperature. Preliminary results suggest that it accurately predicts both seasonal movements, and the difference in sex ratios.
The cardiac assay described above has been used to determine the sensitivity of lobsters to changes in temperature. They are very sensitive, detecting changes of <1 °C (Jury and Watson, 2001). Attempts to locate the receptors they are using to detect changes in temperature have been challenging. While it appears as if the receptors are external, removal of the antennules, antennae and legs do not effect their ability to sense changes in temperature. This is a very exciting area for future investigations.
Lobsters are not strong osmoregulators. In fact a better term to describe them is hyperosmoconformers. As the salinity is dropped, their blood osmolarity also drops, but it always stays hyperosmotic to the ambient water. If changes are gradual, they can withstand 5-10 ppt for several days, but further decreases, or prolonged exposure to <10 ppt can be lethal (Figure 5).
As part of his Master's Degree Steve Jury quantified the energetics of osmoregulation in Homarus americanusand demonstrated that male lobsters use less energy to achieve the same level of hyperosmoregulation as female lobsters (Jury 2t al., 1994b)). This suggests an energetic advantage for males inhabiting the upper reaches of an estuary. Christina Rockel (now Houchens and now with a Master's Degree), has documented differences in the osmoregulatory capabilities of coastal and estuarine lobsters. Her data suggests that during acclimation to different salinities there is a change in the enzymes that pump ions across the gill membrane (Na+/K+ ATPase). Due to these adjustments estuarine lobsters can survive better at low salinities, as illustrated above in Figure 5. We are presently trying to visualize these pumps, quantify the changes and determine why male and female lobsters have different abilities to osmoregulate.
Lobster movements in the field have a great impact on their distribution and abundance. In the laboratory we have been quantifying their normal patterns of locomotion, as well as factors that influence the distance they travel. Our major tool for this type of study is a lobster "racetrack", which is illustrated below (Figure 6). We have also been developing a lobster treadmill, which is also pictured below (Figure 7).
The lobster treadmill has allowed us to investigate the relationship between locomotion and changes in heart and ventilation rate. These results were presented by Dan O'Grady at a recent International Lobster Conference. The figure below shows one example of how heart and ventilation rate change during a bout of walking.
In the laboratory or field, under natural light/dark conditions, lobsters are most active at night (see Figure 9). This rhythm persists in constant darkness for several weeks. Chris Chabot at Plymouth State College has demonstrated this quite clearly using lobster running wheels he designed especially for this project.
Recently, Dan O'Grady demonstrated that if the salinity in the tank is lowered, locomotion decreases. These data will be a major part of his Master's Thesis.
Temperature also has a profound influence on locomotion, but it is not a linear relationship (Figure 10). Locomotion is very limited below 8-10 °C and then above this temperature it jumps to a new level. Thus, it appears as if lobsters are always in one of two behavioral states, active or inactive.
The heart rate of lobsters is regulated by circulating hormones and the cardioregulatory nerves. By lesioning the cardioregulatory nerves it is possible to distinguish between the two. This method is described below.
In two recent papers we have shown that both the transient cardiac response to temperature (Jury and Watson, 2001) and the cardiac response to drops in salinity are mediated by the cardioregulatory nerves (Fig. 11; Dufort et al., submitted).