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Effect of temperature on the behavior of stage IV American lobster (Homarus americanus) larvae.

ABSTRACT The behavior of stage IV American lobster (Homarus americanus) larvae in relation to various water temperatures was observed in laboratory. Time budgets were documented in 2009 in trials using a single larva per experimental enclosure in relation to three temperature exposures (10[degrees]C, 15[degrees]C, and 20[degrees]C). Larvae acclimated at a given temperature (15[degrees]C and 20[degrees]C) were used. The same treatments were used in 2010, where one larva was observed within a group of five individuals per experimental enclosure. Larvae took less time to hide when they were acclimated to 15[degrees]C and then exposed to 20[degrees]C. Video observations showed that larvae were stressed (repeated tail flicks) when treatments simulated important heat shocks (e.g., larvae acclimated at 20[degrees]C and exposed to 10[degrees]C). Results for single larva were similar to those from groups of larvae. The tail-flicks display was also observed in 2010 with larvae acclimated at 15[degrees]C and then exposed to 10[degrees]C. Observations from both years showed that the time budget was similar regardless of the larval cohort used in trials (different brood stocks and times of the season). Overall, results confirm that water temperature affects the behavior of lobster larvae. Acclimation to a given temperature, however, tends to show that larvae may acclimate to cold water. This study provides interesting results to better understand the ecology of American lobster, particularly at the time of settlement. These results should also be of interest for the lobster industry, which looks to improve stock enhancement procedures through, for instance, lobster-stocking programs.

KEY WORDS: Homarus americanus, stage IV larvae, behavior, temperature, acclimation

INTRODUCTION

The American lobster (Homarus americanus) is a common species found along the east coast of North America from Labrador to South Carolina (Observatoire global du Saint-Laurent-OGSL 2012). In Canada, lobsters are found in high abundance in the southern Gulf of St. Lawrence (sGSL) as well as off the southwest coast of Nova Scotia. The life cycle of lobster has a long-pelagic phase during which larvae are exposed to currents that may affect, as they reach stage IV, their distribution upon settlement. At that stage (competent stage), lobster larvae are known to leave the pelagic habitat for the benthic habitat (Barshaw & Rich 1997, James-Pirri & Cobb 2000, Paille et al. 2002). Once on the bottom, larvae rapidly seek a shelter to limit their vulnerability to predators (Botero & Atema 1982, Boudreau et al. 1990, Palma et al. 1998).

Marine invertebrates, including crustaceans, are affected by many environmental parameters. The American lobster, for instance, is able to detect minute temperature variations. Jury and Watson III (2000) showed that adult lobsters could discriminate temperature oscillations as low as 0.15[degrees]C, as shown by variations in heart rates. Water temperature may affect lobster growth (Cobb et al. 1983, Boudreau et al. 1991), size or age at sexual maturity (Waddy & Aiken 1991), and locomotion (Jury & Watson III 2000, Magnay et al. 2003). Temperature may also affect lobster reproduction by modifying (1) mating period and duration, (2) molt synchronization, (3) the egg production cycle and attachment, (4) incubation success, and (5) larval release (Waddy & Aiken 1991). In addition, larvae released from eggs that developed at warm temperatures contain higher energy reserves than those released from eggs at cold temperatures (Sasaki et al. 1986). In the natural habitat, high larval survival rates are observed early during the summer when water temperature increases rapidly (Ennis 1995).

The response of lobster larvae to temperature, including survival, may vary in relation to their developmental stage. Except for stage II larvae, stages I, III, and IV larvae are tolerant to high temperatures, particularly when exposed for short periods (<6 h) (Gruffydd et al. 1975). MacKenzie (1988) observed that stages I and II larvae reared at 10[degrees]C had a mortality rate of 10% and 39%, respectively, compared with those reared at 12[degrees]C (<1% and 13%, respectively). Stages III and IV larvae had a high mortality rate (>70%) when reared at 10[degrees]C relative to those reared at 12[degrees]C. These results suggest that mortality may be great when larvae develop under low temperature conditions, particularly when individuals reach competency.

Low water temperature may also affect larval behavior as well. Annis (2005) showed that only 2% of stage IV larvae swim to the bottom at temperatures less than or equal to 12[degrees]C. This observation supports results from MacKenzie (1988) on the occurrence of a 12[degrees]C thermal threshold for stage IV larvae. Water temperature may also influence other behaviors of stage IV larvae, such as exploration on the sea floor, shelter selection (cryptic behavior), and the number of times individual larvae travel between the bottom and the water column before settling. Any delays in settlement may increase the vulnerability of larvae to pelagic and benthic predators (Botero & Atema 1982).

The American lobster is one of the most important marine resources in Canada. Revenues in 2008 exceeded CA$ 920 million (Peches & Oceans Canada 2008). Though many fishery restrictions exist to protect the lobster stocks, the number of lobster landed in the sGSL decreased in the 1980s. Landings have since improved due to conservation measures and favorable ecosystem conditions except in the Northumberland Strait where they have continued to decline (Comeau et al. 2004, Department of Fisheries and Oceans 2007, Comeau et al. 2008). In response to this decline, the lobster industry initiated a lobster enhancement program to increase stock abundances using current knowledge and technologies. The literature shows that a period of 11 days is sufficient to reach stage IV larvae at 22[degrees]C (Castro & Cobb 2005), as compared with 54 days at 10[degrees]C (Ennis 1995). Larvae obtained from hatcheries for enhancement programs are usually reared at temperatures between 18[degrees]C and 20[degrees]C for cost-effectiveness. Stocking, however, may sometimes be done under cold (10-12[degrees]C) field conditions and it is suspected that thermal shocks may affect larval survival and behavior, particularly their cryptic behavior. It is thus important to document the relationship between temperature and lobster larval behavior.

The objectives of this study were to: (1) describe the behavior of stage IV lobster larvae in relation to relevant water temperatures; (2) verify if a thermal shocks occurs such that larval behavior is impacted; (3) verify if acclimation to a given temperature could minimize a potential thermal shock; (4) verify if the behavioral response of larvae at different temperatures is similar when observations are done on single larvae as compared with larvae within a group of larvae (effect of interactions between individuals). It is predicted that the general behavior of lobster larvae will be affected by water temperature such that larvae observed in a high temperature environment will (1) spend a longer period hidden, (2) reach the substrate rapidly, and (3) have a low level of stress. In the event that water temperature has an effect on general larval behavior, acclimation at a given temperature should minimize this effect. Larvae acclimated to a given temperature will spend longer periods hidden, be faster to reach the substrate, and have a lower level of stress. Results from this study should provide some ecological information on the effect of water temperature on the behavior of lobster larvae whereas swimming toward the bottom and finding a shelter and settling, which will provide information to the lobster industry to improve enhancement techniques.

MATERIALS AND METHODS

Rearing Conditions of Stage IV Larvae

Stage IV American lobster larvae were obtained from the Coastal Zones Research Institute (CZRI) in Shippagan (NB), Canada. The CZRI used berried females captured from various locations in the sGSL. Once released, larvae were collected from holding tanks at the water surface using a small mesh net and placed in 1,200 1 tanks at 20-22[degrees]C at an initial density of 10 larvae/1. These tanks had a circular double drain and were supplied with water at a rate of 5 1/min. An aeration system produced an intense bubbling in the water column to obtain a homogeneous distribution of food and to avoid cannibalism between larvae. Larvae were fed a mixture of dried food (Salt Creek brine shrimp flakes; Artemac) and frozen Artemia (Hikari, Kyorin Ltd.) daily. The photoperiod used was 16 L:8 N h to simulate summer conditions for our region (47[degrees]N). Once stage IV was reached (12 days after hatching), individuals were immediately separated from the other stages to avoid cannibalism. Larvae were then transferred by car from the hatchery to the Universite de Moncton (Moncton, NB) for the experiments (travel time of 3 h).

Behavioral Observations on Single Larvae

The experiments were done in a laboratory in July and August 2009 using only stage IV larvae. Once in Moncton, larvae were maintained in four enclosures (50 X 40 X 40 cm height) filled with artificial seawater (28%-30%) using a density of 100-120 larvae/enclosure. Larvae were acclimated for a period of 4 days at two different temperatures: 15[degrees]C (experimental group) and 20[degrees]C (control). The 20[degrees]C temperature represents the temperature used in hatcheries to rear larvae. The 15[degrees]C temperature represents an intermediate water temperature that lobster larvae may experience during the summer in their natural habitat. Photoperiod, diet, and feeding periods used during the experiments were the same as those at CZRI. Once acclimated, a larva was captured and placed in a treatment enclosure and exposed to a given temperature (10[degrees]C, 15[degrees]C, and 20[degrees]C). These temperatures may be observed in bottom waters during the summer in the sGSL (Chasse et al. 2006, Comeau 2006). Each treatment was done in triplicate for a total of 18 trials per day of observation (three temperature exposures X two acclimation temperatures X three replicates). Seven batches of stage IV larvae were used during the summer for a total of 126 trials. These batches were used to quantify changes in the larval behavior of lobsters that could be related to an increase in water temperature during the summer period as well as to the development of berried females in the hatchery.

Each treatment enclosure (as described above) contained a sterilized 1-cm thick mixed gravel layer on the bottom and a shelter made from three stones (ca 20 x 19 x 8 cm height). A small amount of water (0.175 1) from the acclimation enclosures was added in the treatment enclosure before the first trial to insure that the first larva exposed to a treatment had the same chemical signals (congeneric odors) as larvae used in the subsequent treatments within the same enclosure. Water in the experimental enclosures was not changed between trials for a given batch of larvae as larval behavior may be modified by chemical signals (Moore et al. 1991).

[FIGURE 1 OMITTED]

For each trial, a larva was introduced to the enclosure using a funnel attached to a plastic pipe to mimic the releasing technique used by the industry during enhancement in the field. The larva was videotaped following the release using a Sony Handycam DCR-SR47 over a 1 h period. Six treatment enclosures were used simultaneously with each being videotaped independently. A total of 18 videos were filmed every observation day. The 18 treatments were randomly assigned during the day. All observations were done between 9 am and 4 pm. Videotapes were subsequently viewed and analyzed to quantify time budgets (Table 1).

[FIGURE 2 OMITTED]

Behavioral Observations on Larvae Within a Group of Larvae

The experiment was repeated between July and August 2010 using groups of five larvae. The goal of this experiment was to investigate and describe the time budget of single larvae within groups of larvae in enclosures. Although the experiments were similar to those done with single larvae, a larger diameter pipe was used to release five larvae simultaneously in the treatment enclosure. Because hatching occurred earlier in 2010 than in 2009, observations on the last larval batch were obtained for only two replicates per treatment (total n = 120).

Data Analysis

All videotapes were viewed to determine larval time budgets and provide a general description of the behaviors displayed by stage IV lobster larvae. Four main behaviors were observed: hiding, swimming, exploring, and surfacing (swimming directly at the water surface). Because multicollinearity was evident for time budget data, multivariate analyses (e.g., MANOVA) using all behaviors within the same statistical model were not done. The following dependent variables were analyzed in 2009 (one larva/enclosure): the percentage of time a larva was hiding, the time used by the larva to reach the substrate, the number of times a larva left the substrate or the shelter, and the duration of tail flicks (proxy used a stress measure). The following dependent variables were analyzed in 2010 (five larvae/enclosure): the time used by the first three larvae (out of the five) to reach the substrate and the duration of tail flicks. All analyses were done using three-way analysis of variance (ANOVA) with larval batch, acclimation temperature, and temperature exposure as independent variables. All data were nonparametric rank transformed to meet the assumptions of parametric analyses (Conover & Iman 1981), which were done using SAS 9.1. (SAS Institute 2003).

RESULTS

Behavioral Observations on Single Larvae

Overall, the time stage IV lobster larvae spent hiding within a 1 h time frame was high for all larval batches (Table 1; Fig. 1). Larvae spent at least 60% of their time hiding except in a few scenarios (acclimation at 20[degrees]C and exposed to 10[degrees]C in batches 1 and 2, acclimation at 15[degrees]C and exposed to 20[degrees]C in batch 5). Exploration was the second most important behavior displayed by larvae (Table 1; Fig. 1). Larvae from all treatments and larval batches spent less than 10% of their time swimming or surfacing. Hiding behavior was investigated in further detail as it was the most important behavior displayed by stage IV lobster larvae in terms of occurrence as well as for its effect on survival (hidden larvae will be less vulnerable to predators).

The percentage of time larvae spent hiding varied among larval batches and with acclimation and exposure temperatures (Fig. 2). For instance, the percentage of time spent hiding was often greater than 80% but dropped to less than 60% in two particular larval batches when larvae were acclimated at 20[degrees]C and exposed to 10[degrees]C. The time larvae spent hiding varied with acclimation temperature and was also a function of the interactions between acclimation temperature and both larval batches and temperature exposure (Table 2).

The time that larvae took to reach the substrate was usually less than 300 sec except for certain larval batches for which greater than 1,000 sec were needed (Fig. 3). Time to reach the substrate varied significantly with temperature exposure and as a function of the interaction between acclimation and exposure temperatures (Table 3). The number of times larvae left the substrate was generally higher when individuals were acclimated at 15[degrees]C compared with individuals acclimated at 20[degrees] C (Fig. 4). The number of times larvae left the substrate varied with acclimation temperature and as a function of the interactions between both larval batches and temperature exposure (Table 4).

[FIGURE 3 OMITTED]

Almost no tail flicks were observed for larvae acclimated at 15[degrees]C for all temperature exposures (Fig. 5). Larvae displayed tail flicks very often when acclimated at 20[degrees]C and exposed to 10[degrees]C. Duration of tail flicks (multiple succeeding occurrences) varied between 1 and 10 sec as a function of both acclimation and exposure temperatures and as a function of the interaction between these two variables (Table 5).

Behavioral Observations on Larvae Within Groups of Larvae

As observed for single larvae, the time budget for stage IV lobster larvae within a group of five individuals showed that larvae spent most of their time hiding (Fig. 6). The percentage of larvae hiding reached almost 80% within the first 5 min for larvae acclimated at 15[degrees]C for all temperature treatments. The percentage of larvae hiding was over 80% after 5 min of observation for larvae acclimated at 20[degrees]C, except in treatments where larvae were exposed to 10[degrees]C. Under that temperature treatment, it took 30 min for 80% of larvae to be hidden. A relatively stable time budget was observed after this with a constant proportion of time devoted to surfacing, swimming, or exploring under all treatments. Overall, the percentage of larvae hiding was low when temperature exposures were lower than the acclimation temperature (acclimation at 15[degrees]C and exposed to 10[degrees]C, acclimation at 20[degrees]C and exposed to 10[degrees]C and 15[degrees]C). These data could not be examined statistically.

The time taken by the first three larvae to reach the substrate was generally fast (Fig. 7) but increased when the temperature exposure was lower than the acclimation temperature (acclimation at 15[degrees]C and exposed to 10[degrees]C, acclimation at 20[degrees]C and exposed to 10[degrees]C or 15[degrees]C). This period of time also varied among larval batches and as a function of the acclimation temperature by temperature exposure interaction (Table 6).

As for the single larva experiments, the tail-flick behavior was not common for larvae acclimated at 15[degrees]C (Fig. 8). A few tail flicks were observed when individuals were exposed to 10[degrees]C. The tail-flick duration in these particular treatments was less than 5 sec. Larvae displayed tail flicks often when acclimated at 20[degrees]C and exposed to 10[degrees]C. The tail-flick duration varied between 10 and 40 sec within that treatment. The duration of tail flicks varied significantly with the acclimation temperature by temperature exposure interaction (Table 7).

DISCUSSION

The time budget of stage IV lobster (Homarus americanus) larvae revealed that hiding was the most commonly displayed behavior by individuals during the transition phase between the pelagic and benthic stages. The hiding behavior was common for all treatments and larval batches, confirming that stage IV larvae are competent and that they will rapidly seek a refuge to decrease their vulnerability to predators (Stein & Magnuson 1976, Barshaw & Rich 1997, Chau et al. 2009, Jones & Shanks 2009). Exploration was the second most important behavior. This is consistent with observations made by Karnofsky et al. (1989) on American lobsters. Lobster larvae start to explore the substratum for an appropriate shelter as soon as they reach stage IV. The exploration behavior, however, was quite variable in this study and tended to be related, although not tested statistically, with the larval batches. Swimming and surfacing were not important in terms of absolute time allocated to these behaviors. Transitory locomotory behaviors were displayed to get from the top of the water to the bottom (swimming) or from the bottom to the surface (surfacing). Transitory movements are generally observed when bottom environmental conditions are not optimal (e.g., absence of shelters, soft-bottom habitats) (Boudreau et al. 1990, 1993).

[FIGURE 4 OMITTED]

The hiding behavior showed a relatively high variability in all treatments, as indicated by the significant three-factor interaction. The variability within a larval batch was greater for larvae acclimated at 15[degrees]C than those acclimated at 20[degrees]C. Larvae were probably less stressed to explore and swim in a warm environment after being acclimated at a low temperature. The variability associated with larval batches could be related to female sizes, as larvae from early summer come from larger females than those obtained late in the summer. The size shift and physiological state of females throughout the season may affect eggs and larval development. Sibert et al. (2004) showed that there are seasonal variations in the lipid contents of American lobsters eggs. These observations were also made yearly by Wickins et al. (1995) for the European lobster (Homarus gammarus). The amount of lipids, such as triglycerides, will play an important role in the energy content of larvae and ultimately their activity level (Theriault & Pernet 2007). Stage IV larvae with high lipid contents may then get to the bottom and explore for shelters more rapidly than larvae with low lipid contents. This may also help larvae to better compete with conspecifics (or individuals from other species) for shelter space and diminish their vulnerability to predators.

The percentage of time allocated to hiding was high and varied least among larvae acclimated at 20[degrees]C (except for two particular batches of larvae exposed to 10[degrees]C). A larva developing at 20[degrees]C may age physiologically more rapidly than a larva developing at 15[degrees]C even though they have the same age in terms of the time passed since hatching. Larvae acclimated at 20[degrees]C may thus be more physiologically advanced in their development and show a higher level of competency than larvae acclimated at 15[degrees]C. A similar relationship was observed in the laboratory and field by McCormick and Molony (1995) in the tropical goatfish Upeneus tragula. These authors suggested that small changes in water temperature (30[degrees]C versus 25[degrees]C) may have the potential to influence the developmental rate to metamorphosis of tropical reef fishes and subsequently their settlement behavior.

Results from the video observations on larvae within a group of larvae showed similar responses as those from single larvae in relation to temperature exposures and acclimation. Overall, a reduction in the time allocated to hiding was observed when the exposure water was warmer than the acclimation temperature. Eighty percent of larvae acclimated at 15[degrees]C and exposed to 10[degrees]C took about 5 min to hide compared with only 2 min when larvae were acclimated at 15[degrees]C and exposed to 15[degrees]C and 20[degrees]C. The period of time to reach the same hiding level (80% of larvae) increased when larvae were acclimated at 20[degrees]C and exposed to 10[degrees]C or 15[degrees]C. This period decreased for larvae acclimated at 20[degrees]C and exposed to 20[degrees]C. These laboratory results suggest that lobster larvae exposed to large temperature differences (5[degrees]C and 10[degrees]C) may allocate more time for other behaviors (e.g., swimming, tail flicks) and potentially increasing their vulnerability to predators under natural settings.

Stage IV lobster larvae generally took little time to reach the substrate, regardless of the acclimation and exposure temperatures and larval batch. Patterns were generally the same for larvae acclimated at 15[degrees]C except for larval batches 6 and 7. Larvae acclimated at 20[degrees]C showed that larvae may have suffered a thermal shock when exposed to 10[degrees]C. Low temperatures may stress larvae (display of tail flicks) and increase their swimming or surfacing time. Surprisingly, this was observed in only two of the seven larval batches (first two batches). As discussed above, increase in swimming time will increase the vulnerability of larvae to predators. Van der Meeren (2000), for instance, found in the field that over 10% of reared larvae were caught by predators within the hour following their introduction. Larvae must reach the bottom quickly to maximize survival.

[FIGURE 5 OMITTED]

The number of times larvae left the substrate or shelter was generally low for those acclimated at 20[degrees]C relative to those acclimated at 15[degrees]C. This was also observed for larvae acclimated at 20[degrees]C and exposed to 10[degrees]C. Larvae acclimated at 20[degrees]C may have developed greater competency than those acclimated at 15[degrees]C. Larvae with high competency may display more stable behaviors than do larvae with low competency and stay in their shelter more effectively or dig into the gravel. This particular behavior may, however, also be related to larvae being less active at low temperatures, which may lead them to restrict their movement. Van der Meeren (2000), for instance, showed that low temperatures tended to reduce the aggressivity in European lobsters, which led individuals to stay in their shelter for a longer period.

The display of tail flicks was greatest when the difference between acclimated and exposure temperatures was high. This behavior was particularly obvious when larvae were acclimated at 20[degrees]C and exposed to 10[degrees]C. This stress response was likely induced by a thermal shock. Larvae within larval groups from all acclimation temperatures displayed great tail-flick behavior when the exposure temperature was 10[degrees]C. Tail-flick duration and the number of larvae that displayed this behavior was, however, greater for larvae acclimated at 20[degrees]C than those acclimated at 15[degrees]C. In most cases, the tail-flick behavior was displayed within a 5 sec time frame for larvae acclimated at 15[degrees]C but was almost 40 sec for larvae acclimated at 20[degrees]C. Observations made in this study during the 10[degrees]C exposure treatments tend to confirm findings by various authors. Larval development is known to be inhibited below 10[degrees]C (Waddy & Aiken 1998, Wahle & Fogarty 2006). MacKenzie (1988) showed that stage IV larvae will not reach stage V at temperatures less than 12[degrees]C. A temperature of 12[degrees]C may be the minimum temperature at which larvae remain viable (MacKenzie 1988). Similarly, Annis (2005) found that stage IV larvae remained in surface waters above 12[degrees]C and rarely descended toward bottom waters below this temperature. This temperature appears to be a threshold temperature for many biological features and support the observations made in this study on stressed individuals. Van der Meeren (1991) also observed this particular behavior in her work. Larvae tend to respond to inadequate conditions by displaying tail flicks and swimming high in the water column. This may increase their vulnerability to predators because tail flicks may alert and attract predators.

[FIGURE 6 OMITTED]

CONCLUSION

Overall, the results presented in this study tend to support the initial working hypotheses and predictions. Figure 9 summarizes the main observations. Water temperature affects the behavior of stage IV American lobster larvae. Acclimation to cold water temperatures decreases the effect of a thermal shock suffered by larvae. Stage IV lobster larvae hide faster in warm water than in cold water. Results also show that lobster larvae may reach the bottom quickly in a cold environment once acclimated at low temperatures. Acclimated lobster larvae, however, may leave the substrate more often. This may increase their vulnerability to predators.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

This study provides interesting results to better understand the ecology of the American lobster in a context where larval information is being used to model dispersal and to better understand potential climate change effects on settlement behavior. The results of this study should also be of interest for the lobster industry. Acclimation could improve enhancement in cold habitats and help increase coastal lobster stocks. These results must, however, be validated by field studies that more closely simulate the methodology and the natural settings in which enhancement programs are carried out. Enhancement techniques would be improved if they decrease the amount of time larvae spent in the water column and increase their time in natural shelters after seeding. This must be done by reducing larval stress. Tail flicks will increase the time spent in the water column and ultimately contacts with predators.

[FIGURE 9 OMITTED]

ACKNOWLEDGMENTS

We thank Remy Hache and his team from CZRI for their work during the production of lobster larvae in 2009 and 2010. We also thank Yves Hebert for his technical contributions. Michel Comeau and Stephan Reebs were instrumental in providing equipment and advices during the project. Additional thanks to Fabrice Pernet and Chris McKindsey for reviewing the manuscript. Funding for this research has been provided by grants from Homarus Inc., Fisheries and Oceans Canada, Canadian Foundation for Innovation, Canadian Capture Fisheries Research Network (a strategic network funded by NSERC) and Universite de Moncton.

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MELANIE CHIASSON, (1) GILLES MIRON, (1) * DOUNIA DAOUD (2) AND MARTIN D. MALLET (2)

(1) Departement de Biologie, Universite de Moncton, Moncton, Nouveau-Brunswick, Canada El A 3E9; (2) Homarus Inc., 408 rue Main, Shediac, Nouveau-Brunswick, Canada E4P 2G1

DOI: 10.2983/035.034.0239
TABLE 1.
Description of behavioral categories observed in the time
budget of stage IV American lobster (Homurus americanus)
larvae.

Behavior                           Description

Hiding      in the shelter, the larva is hiding at the entrance of the
              shelter or under the shelter:
            under the substrate, the larva is hiding between rocks or
              below them
Exploring   the substrate, the larva walks on the gravel or digs in it
            the walls of the aquarium, the larva walks on the glass or
              plastic panel of the walls of the aquarium
Surfacing   The larva swims directly along the surface without going
              to the substrate
Swimming    Active: the larva moves to a specific location or does
              tail flicks;
            Passive: the larva drift with the current (water movement
              induced by filter)

TABLE 2.
Summary of results from the three-way ANOVA on the
percentage of time stage IV American lobster (Homarus
americamts) larvae are hidden (2009 experiment).

                                 F     df   P level

Larval batch (LB)               1.39    6   0.230
Acclimation temperature (AT)    5.93    1   0.017 *
Temperature exposure (TE)       2.67    2   0.075
AT x LB                         2.30    6   0.042 *
TE x LB                         1.77   12   0.067
AT x TE                         4.38    2   0.016 *
AT x TE x LB                    0.62   12   0.820

* Significant P value.

TABLE 3.
Summary of results from the three-way ANOVA on the time
that stage IV American lobster (Homarus americanus) larvae
took to reach the substrate (2009 experiment).

                                F       df   P level

Larval batch (LB)                0.84    6    0.540
Acclimation temperature (AT)     1.91    1    0.170
Temperature exposure (TE)       10.59    2   <0.01 *
AT x LB                          1.80    6    0.110
TE x LB                          0.50   12    0.910
AT x TE                          4.20    2    0.018 *
AT x TE x LB                     1.83   12    0.056

* Significant P value.

TABLE 4.
Summary of results from the three-way ANOVA on the
number of times that stage IV American lobster (Homarus
americanus) larvae left the substrate (2009 experiment).

                                  F     df   P level

Larval batch (LB)               0.650    6    0.690
Acclimation temperature (AT)    24.04    1   <0.01 *
Temperature exposure (TE)        1.39    2    0.250
AT x LB                          3.38    6   <0.01 *
TE x LB                          1.39   12    0.190
AT x TE                          3.19    2    0.046 *
AT x TE x LB                     1.07   12    0.39

* Significant P value.

TABLE 5.
Summary of results from the three-way ANOVA on the
duration of tail flicks displayed by stage IV American lobster
(Homarus americanus) larvae (2009 experiment).

                                  F     df   P level

Larval batch (LB)                1.00    6    0.430
Acclimation temperature (AT)    15.34    1   <0.01 *
Temperature exposure (TE)       23.36    2   <0.01 *
AT x LB                          2.12    6    0.059
TE x LB                          0.53   12    0.89
AT x TE                         11.70    2   <0.01 *
AT x TE x LB                     1.42   12    0.170

* Significant P value.

TABLE 6.
Summary of results from the three-way ANOVA on the time
that the first three stage IV American lobster (Homarus
americanus) larvae (out of five individuals) took to reach the
substrate (2010 experiment).

                                  F      df   P level

Larval batch (LB)                 3.38    6   <0.01 *
Acclimation temperature (AT)     10.28    1   <0.01 *
Temperature exposure (TE)       100.60    2   <0.01 *
AT x LB                           1.88    6    0.09
TE x LB                           1.49   12    0.14
AT x TE                           6.54    2   <0.01 *
AT x TE x LB                      1.19   12    0.30

* Significant P value.

TABLE 7.
Summary of results from the three-way ANOVA on the
duration of tail flicks by stage IV American lobster (Homarus
americanus) larvae (2010 experiment).

                                  F      df   P level

Larval batch (LB)                 1.64    6    0.150
Acclimation temperature (AT)     45.60    1   <0.01 *
Temperature exposure (TE)       188.13    2   <0.01 *
AT x LB                           0.70    6    0.650
TE x LB                           1.09   12    0.38
AT x TE                         39.14     2   <0.01 *
AT x TE x LB                      1.01   12    0.45

* Significant P value.
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Author:Chiasson, Melanie; Miron, Gilles; Daoud, Dounia; Mallet, Martin D.
Publication:Journal of Shellfish Research
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Date:Aug 1, 2015
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