Comparative Analysis of Escape Behavior in Male, and Gravid and Non-Gravid, Female Lobsters

9 Pages·1998·3.7 MB·English

by  S I Cromarty

Reference: Biol. Bull. 194: 63-71. (February. 199S)
Comparative Analysis of Escape Behavior in Male,
and Gravid and Non-Gravid, Female Lobsters
S. I. CROMARTY*, J. MELLO, AND G. KASS-SIMONt
Biological Scieiices Department, University ofRhode Island. 100 Flagg Rd. Kingston.
Rhode Island 02881-0816.
Abstract. Few studies exist in which the parameters of males but higher frequency tailflips in females results in
a single behavior have been quantitatively compared for the same chances of survival for the sexes.
male and female lobsters. Here, we have examined the
effects of sex and gravidity on the parameters of the es- Introduction
cape behavior of the American lobster, Homarus ameri-
canus, elicited by a visual threat. There are very few quantitative studies comparing sex-
Both non-gravid females and male lobsters readily tail- ual differences in agonistic and avoidance behavior
flipped in response to the stimulus, but gravid females among lobsters (Scrivener, 1971; Stein, 1976; Atema et
failed, with one exception, to initiate a swim, even when al. 1979; Waddy and Aiken. 1991). Anecdotal evidence
stimulus strength was increased. and references in the literature suggest that gravid female
Although the total distance swum by males and non- lobsters are much more aggressive than either males or
gravid females was not statistically different, males cov- non-ovigerous females. Gravid females are described as
ered more ground in the initial power swim and during being asocial, secretive, and defensive; when approached,
the subsequent swims than did non-gravid females. Males they are reported to backup and raise their wide-open
swam for a longer time, performing more tailflips, than claws (Atema and Voigt. 1995).
females. Relative to their length and weight, males swam Sexual dimoiphism ofmature individuals is well docu-
a greater distance at each stroke during the initial power mented. Males develop larger claws, and females have
swim and the subsequent swims, although, females might broaderabdomens (Templeman, 1935; McLeese and Wil-
have compensated by swimming at a higher frequency. der, 1964; Lang et al., 1977; Waddy and Aiken, 1991).
There were no significant differences in swimming veloc- Other physiological differences of yet-undetermined sig-
ity or acceleration, nor in the calculated force or work nificance also exist. Males have higher hemolymph con-
performed by the two sex classes (male and non-gravid centrations of octopamine, serotonin, tryptophan, and an
females). unidentified compound known as compound S than do
Therefore, apart from egg-bearing, which severely in- females (Fadool et al.. 1989).
hibits the escape response, it remains to be seen whether The sexual anatomical dimorphism has led to the idea
the subtle physiological and anatomical sexual dimor- that there were sexual differences in aggressive behavior
phism that produces longer and more swim strokes in and in the establishment of dominance. The suggestion
has been made that the larger claws of males were im-
Received 14 May 1997; accepted 24 October 1997. portant in establishing dominance over other males or
* Present address; Department of Neurobiology. Harvard Medical females, because lobsters with larger claws usually win
School, 220 Longwood Ave.. Boston, MA 02115. E-mail; scromart@ in a fight (Scrivener, 1971). Alternatively, larger claws
warren.med.harvard.edu among males could be used as a sexual signal (Atema
tTo whom correspondence should be addressed. E-mail: avfl01@ and Voigt, 1995), as is the case in crayfish where females
uriAabcbcr.euvrii.aetdiuons: FEP, Fisher exact probability test; SSI, subsequent selectmales with largerclaws (Stein, 1976). Inthe female,
swims offirst half; SS2, subsequent swims ofsecond half. on the otherhand, sexual dimorphism has been associated
63
64 S. I. CROMARTY ET At.
—
with reproduction the wider female abdomen increases changes in the physiology and morphology of crayfish
egg-carrying capacity (Templeman. 1935; McLeese and nerve terminals have been documented (Lnenicka and
Wilder, 1964; Waddy and Aiken. 1991). Zhao. 1991 ). The protocol for inducing escape responses
As part of a larger study to determine whether and in lobsters is described in detail elsewhere (Cromarty et
to what extent male and female lobsters differ in their cil.. 1991). In summary, experiments were run between
nonsexual behaviors, we have quantitatively compared 1200 and 1700 h in an aquarium filled with filtered recir-
the escape response in males, gravid females, and non- culated seawater from Narragansett Bay. Salinity was
gravid females. Lobsters ofthe three "sex classes'" were measured before each experiment and ranged between
presented with a threatening visual stimulus. We recorded 297(f and MVk. Waterwas replaced or added as necessary
theescape behaviorsequencesandpost-stimuli behaviors, to maintain salinities within this range. The experimental
which included whether animals re-approached and at- tank was kept between 18° and 20°C by a Frigid Units
taafavtcoekireddtehdethtiehneistsitatilimmurulelususpson(asanfedt.ercWotneatiilnnfouliewpdpiptnroge)sbeaoncrtkewavhwieadtyehnecfrerottmhheaiytt eoAfmEe-ar2ns3eo4dpeAinGn-t-eo6n0ad2eldacrhgtiealrlnekmr.a(iT1.nh0etmaenxkLpe(rx2i.m20e.mn3tamLl sWx'et0u.Xp75c0o.mn3smiWstHeX)d
amongthese threeclasses oflobsters, in addition todiffer- 0.91 m H). The layout was designed so that a threatening
ences in the propensity to flee between gravid and non- stimuluscould he intri)duced at theopen endoftheexperi-
gravid animals, there are significant differences in the mental tank. A weighted wooden partition with a pulley
parameters of the escape response between males and acted as a blind (and a separation from the main tank) at
non-gravid females. the open end (Fig. lA). To ensure that lobsters were
initially at theclosed, non-stimulusend. alightwasplaced
Materials and Methods at the open end. The partition was raised once the lobster
had reached the closed end. while the light was moved
Animals
to the closed, non-stimulus end. This served to "push" the
Adult American lobsters (carapace length 78-85 mm) animal back towards the open (stimulus) end. A piece of
were obtained through the Rhode Island Department of PVC tubing (0.15 m L X 0.10 m W) weighted with peb-
Environmental Managementfromanearshore lobsterves- bles to a total weight of 1.45 kg served as the threatening
sel fishing in Narragansett Bay. Rhode Island. Animals visual stimulus. The stimulus was raised above the open
were housed at the Narragansett Bay Campus ofthe Uni- end of the tank as depicted in Figure IA. The stimulus
versity of Rhode Island, in separate but connecting tanks was released into the water at a preset distance of 10 cm
in a free-flow seawater system, at ambient temperatures measured from the open edge of the tank to the lobster)
(
ranging from 16° to 23°C and under an illumination cycle whenever a lobster approached the open end. In a second
of 14h light to lOh dark. set of experiments, a larger stimulus of PVC tubing
Animals were fed three times per week on a mixed (0.30 m L X 0.20 m W. weighing 2.45 kg) was used on
diet of squid, crab, and fish, but they were not fed for an additional set of gravid lobsters.
48 h prior to an experiment. Four to six hours before Cameras were placed in two positions (a Sony cam-
an experiment, the animals were moved to the Kingston corder above the tank and a Panasonic WV-CD20 camera
campus ofthe university, where they were placed in sepa- to the side) and experiments were recorded (Panasonic
rate holding tanks (30 cm'). and visually isolated from AG-6010 and Panasonic NV-8950) simultaneously from
one another. Each tank was supplied with its own air the horizontal and vertical perspectives. Video recordings
supply, and water was obtained from the same source that of each lobster were analyzed frame-by-frame. For mea-
supplied the tanks at the Bay Campus. Individuals were surements of distance traveled, a metric grid divided into
kept at the same temperature (20°C) as the animals held 0.5-cm units was paintedonto the sideoftheexperimental
at the Bay Campus. Three hours prior to an experiment, tank. Transparent overlays on the video monitor were
lobsters whose claws and antennae were intact were later used to record escape swimming distance of each
placed in the experimental tank and left undisturbed. One animal. Distance traveled along the length of the tank
hourbefore theexperiment, the physical condition ofeach was measured by using the position of the tip ot the
animal was checked. Animals were used only once and lobster's rostrum as a guide. The number of tailflips was
only ifthey moved around the tank orexhibited antennule counted. Time was automatically recorded on the video-
flicking. tape. An independent observer inspected all recordings
and rejected runs in which the experimental parameters
Experiments were not strictly adheredto: e.g.. cases in which the stimu-
Experiments were conducted from April to October to lus was released closer than 10cm to the experimental
avoid possible seasonal differences in behavior. Seasonal animal.
SEXUAL DIFFERENCES IN ESCAPE BEHAVIOR 65
(total II = 30) were chosen for stimulus trials. Only one
gravid lobster escaped in the first set of experiments, so
an additional 10 gravid females were used in a second
set of experiments with a larger stimulus. Abdominal
width was not measured in the original experiments, but
this measurement was later made on an additional 16
animals of each sex whose weights were equivalent to
those of the original specimens.
Analysis of the escape response follows the methods
used in ourearlieranalysisofescapebehaviorinjuveniles
(Cromarty et—al.. 1991). The response was broken into
two elements the initial tailflip, henceforth designated
as the "power swim," and the many subsequent tailflips,
designated as "subsequent swims." The number of sub-
sequent swims in this study ranged from one to three. A
tailflip, orswim, isdefinedas beginning immediately after
6
the start of abdoininal flexion and ending at abdominal
extension (Fig. IB). The following characteristics of the
escape response were analyzed for each lobster: distance
traveled (centimeters), number oftailflips, duration ofthe
response (seconds), frequency of tailflips (tailflips/sec-
ond), velocity (meters/second), acceleration (meters per
second squared), force (mass X acceleration), work (force
X distance), distance traveled per unit of weight per tail-
flip (meters/kilogram/number of tailflips), distance trav-
eled per unit of weight (meters/kilogram), and distance
per lobster body length (dimensionless). The latter two
parameters werecalculatedtodetermine whethervariabil-
ity in the weight and size of individual lobsters altered
Figure 1. (A) Diagram ofthe experimental tank (E). The stimulus the relative significance of a parameter, even though the
was apieceofPVC tubingweightedwithpebbles (A); the partition (B) mean values were not significantly different among all
was lifted while the lobster(F) was at the opposite end ofthe tank. A three classes of experimental animals.
laingdhttsheervsetidmauslutshewamsecrhealneaissemd twohe"nputshhe"lotbhsetearniwmaasl twoitthhieno1p0encmendo.f In evaluating acceleration, the added-mass forces
the edge of the open end. The escape reaction of each lobster was (Batchelor, 1967) that act on accelerating bodies in fluids
photographedagainstagrid(G)withdivisionsof0.5cm.Therecording were ignoredsincethese are amultiple ofmass andwould
system consisted of two cameras, one vertical (C) and one horizontal act equally on all animals of the same weight. Another
(H); monitor(I); and time-lapse VCR (J). (B) Schematic breakdown of variable which we did not include in ourcalculations was
a single tailflip as it was seen in the video analysis {original drawings body rotation. Body rotation during swimming was slight
done by K. Davignon, Graphics specialist, URI).
in our experiments and not measurable by our methods.
Animals whose gross swimming pattern deviated from a
rectilinear motion were not included in the analysis. (The
After each experiment, the animal"s molt stage was analysis ofthe escape response is meant to reflect relative
determined by examining cuticular changes and setal de- changes in lobsterescape behaviorand not kinematicrela-
velopment in the pleopods (Aiken, 1973, 1980). Only tionships such as those investigated by other researchers
stage C (intermolt) animals were used, since the probabil- [Batchelor, 1967; Daniel and Meyhofer, 1989; Nauen and
ity of escape depends on the molt stage of the lobster Shadwick 1992].)
(Cromarty et al.. 1991; Cromarty et al.. 1995). Measure- Each ofthe escape parameters was analyzed for(1) the
ments of carapace length, cutter length, lobster weight entire escape response, (2) the initial power swim, (3) the
and volume, temperature, and salinity were recorded at subsequent swims over the entire subsequent swimming
the end of each experimental trial. Gravid lobsters were distance, and (4) the subsequent swims in each half of
difficult to obtain; thus, to ensure that equal numbers of that distance, since earlier experiments showed that there
We
animals were tested in each sex class (males, non-gravid were differences in the distance traveled by lobsters.
females, gravid females), only 10 lobsters in each class therefore divided the distance traveled in the subsequent
—
66 S. I. CROMARTY ET AL
swims by half and analyzed each half (Cromarty et al.. 0.19. Furthermore, there were no dif—ferences in carapace
1991). Because the distance was divided equally in half length(mean ± SEM. in millimeters) male orfemale
for each escape sequence for each animal, no data were between the two groups. Females: measured (79.6 ± 0.5;
available to compare distance traveled between the two ;; = 16) versus experimental (79.7 ± 0.6; n = 20); AN-
halves of the subsequent swims for male and non-gravid OVA. F( 1. 34) = 0.003, P = 0.95. Male: measured (77.9
lobsters. ± 1.3; n = 16) versus experimental (82.0 ± 2.0; /; = 10);
ANOVA, F(l, 24) = 2.72. P = O.W. Therefore, abdomi-
Statistical analysis nal widths from this data set were analyzed and are as-
sumed to reflect the abdominal widths ofthe animals that
Differences in weight, carapace length, and cutter
were used in the experiments on escape behavior.
length among the three sex classes were determined by
parametric analysis of variance (ANOVA). In the addi- In this data set, significant differences in abdominal
width were found between male (42.93 ± 0.52) and fe-
tional set of measured lobsters (16 ofeach sex), abdomi- male (53.38 ± 0.97) lobsters (ANOVA. F(l. 30) = 89.33.
nwailthwipdatrha,mectarircapaAcNeOlVeAng.th,ThaendFiwsehiegrhtexawcetreproabnaabliylzietdy P = 0.0001), suggesting that a similar sexual dimorphism
also existed in the experimental escape animals.
test (FEP) was used to determine differences in the proba-
bilities of escape between the three sex classes.
A separate Kruskal-Wallis test (KW) was run for each
Escape behavior in response to a threatening stimulus
escape parameter(except the subsequent swims). The first
and second halves of the subsequent swims were com-
pared with an ANOVA and a one-way repeated measures The probability ofresponding to a threatwith anescape
follow-up test (Keppel, 1982). ANOVAs were run on response was unequal among the three sex classes (Table
Gravid female lobsters were significantly less likely
SPSS software (SPSS Inc., Chicago). Values wereconsid- 1 ).
ered significant at P < 0.05 for all the statistical tests. FtoEPe,scaPpe<tha0n.0e0i0t4h)eromralneosn-(gFriashveirdefxaecmtalpersoba(bFiElPi.tyPtes<t,
0.0001) when presented with a stimulus (0.30 m L X
Results
0.15 m W) weighing 1.45 kg. No gravid lobsters escaped
Weight (in grams), carapace length (in millimeters), when presented with this stimulus. Eight often males and
cutter length (in millimeters) five often non-gravid females tailflipped when presented
There were no significant differences in the weights with this stimulus. The difference was not significantly
(mean ± SEM for all) ofgravid (464 ± 17.3), non-gravid different (FEP. F = 0.17).
(F(425,2 2±7)9.=3),0.a1n5d, mPal=e (0.48664).±N3o0.s1i)gnliofbisctaenrtsd(ifAfNerOeVncAe.s andWhheeanvi1e0r gsrtaivmiudlufsem(0a.l3e0smweLrexpr0e.s1e5ntmedWw)i.thweaiglahrignegr
were found in the carapace lengths of gravid (79 ± 0.6). 2.45 kg, only one gravid animal tailflipped. This female
non-gravid (80 ± I), and male (82 ± 2) lobsters (AN- traveled approximately 80 cm, tailflipping six times with
OVA, F(2, 27) = 0.80, P = 0.45), and similarly there a large loss ofeggs. The largerstimulus was not presented
were no significant differences in the cutter lengths of to males or non-gravid females since the smaller stimulus
gravid (109 ± 3). non-gravid (110 ± 1), and male (113 was sufficient to initiate their escape behavior.
± 3) lobsters (ANOVA, F(2. 27) = 0.92. P = 0.41).
Abdominal width (in nulVuuvters) Table 1
In the set of data collected to determine if there were Escape andpost-threat behaviorofeach sexclass afterthe small
differences in abdominal width between the sexes, we escape stiimdus waspresented(ten lobsters in each class were tested)
firstneeded todemonstratethat these lobstersweresimilar
in weight and size (carapace length) to those used in the
escape behavior experiments.
No differences were found between the weights (mean
± SEM, in grams) of the measured females (461 ± 9.4;
n = 16) and the experimental females (460 ± 9.5; /; =
20); ANOVA. F(l, 34) = 0.1 1. F = 0.92. Similarly, no
differences were found in the weights of the measured
males (421 ± 18.9; n = 16) and the experimental males
(464 ± 30.1; n = 10); ANOVA, F(l, 24) = 1.81, P =
SEXUAL DIFFERENCES IN ESCAPE BEHAVIOR 67
Post-stimulus behavior differences were found in post-threat behavior among the
three sex classes. All probability values were greater than
After the stimulus was presented, lobsters would either P = 0.15. regardless of whether the comparisons were
(1) tailflip away from the stimulus, (2) tailflip first and between gravid and non-gravid females, gravid females
then continue backing slowly away, (3) tailflip first and and males, or non-gravid female and males.
then approach, (4) tailflip, and then approach and attack.
"Approach" is distinguished from "approach and at- Escape parametersfor male and non-graviil adult
tack" by the absence of any physical contact on the part
lobsters
of the lobster and might just as well be due to curiosity
as to agonistic causes, unlike "approach and attack," Only non-gravid females and males were compared
which clearly suggests agonistic responses. For example, because only one gravid female could be induced to tail-
four lobsters in each of the three sex classes (that had flip.
originally tailflipped). immediately returned, approached Total escape response (initial power swim plus subse-
and attacked the stimulus (Table I). quent swims). Total distance traveled during an escape.
Post-threat behaviors were evaluated by Fisher exact number of tailflips. swimming velocity, acceleration,
probabilitytests (2x2contingencytables). No significant force, work, distance traveled per kilogram of weight.
68 S. I. CROMARTY ET AL.
and distance traveled per lobster body length were not greater for males (KWs, P = 0.03 and 0.05 respectively).
significantly different (KW. P > 0.05; see Fig. 2A and Althoughthemean values forpowerswim velocity, accel-
2D for examples). The total time spent escaping was. eration, force, and work all were higher for male lobsters,
however, significantly greater for male kibsters (KW. P the differences were not significant, because of the large
= 0.02, Fig. 2B). Distancetraveledperkilogramofweight variance within each class.
pertailflip was not significantly different (KW. P = 0.40). Tolal subsequent swims. The time spent in subse-
Although the difference was not significant at the 0.05 quent swims and the numberoftailflips were significantly
level, non-gravid females may perform more tailflips per higher for male lobsters than for non-gravid females
second than male lobsters (KW. P = 0.06, Fig. 2C). (KWs. P = 0.04 and 0.03. Fig. 3C and D respectively).
Initial power swim. Distance traveled was signifi- Distance swum per body length and distance swum per
cantly higher for males than for non-gravid females (KW. kilogram of weight for the subsequent swims were also
p = 0.04; Fig. 3A). Although males tended to have longer statistically higher for males (KWs. P = 0.01 and 0.02
power swim durations, the difference was not signifi- respectively). Nootherparameterswerefoundtobe statis-
cantly different at the P = 0.05 level (KW. P = 0.08. tically different for the total subsequent swims.
Fig. 3B). Distance swum per body length and distance Comparison of SSI and SS2 between and within se.x
swum per kilogram ofweight were also both significantly classes. In the first halfofthe subsequent swims (SS1).
SEXUAL DIFFERENCES IN ESCAPE BEHAVIOR 69
male lobsters spent more time escaping (duration) and and escape behavior: age, size, molt-stage, and sex. Thus,
produced more tailflips (KWs; P = 0.05 and 0.03, Fig. large adults are more likely to win in aggressive encoun-
4A and C respectively). Similar differences were found ters (Scrivener, 1971 ) and aredominantoversmallerjuve-
in the second half of the subsequent swims (SS2), with niles (Atema and Cobb, 1980); and soft-shelled premolt
malesagain spending more timeescaping andtaking more animals win over soft-shelled postmolt animals (Tamm
tailflips (KWs. P = 0.04 and 0.04, Fig. 4B and D respec- and Cobb. 1978). In avoidance or escape behavior, small
tively). When SSI and SS2 were compared to each other, juveniles swim faster and farther than large adults, which
for each sex, no significant differences were observed. tend not to respond to a threat with escape swimming
There was no reduction in any ofthe parameters between (Cromarty etai, 1995). In addition, soft-shelled juveniles
the first and second halves of the subsequent swims for swim faster and farther than hard-shelledjuveniles (Cro-
either males or females. No other parameters, either of marty et al.. 1991), but hard-shelled adults swim farther
total subsequent swims orwithin SSI or SS2, were statis- than soft-shelled ones (Cromarty et ai, 1995).
tically different. In this study we have shown that males and non-gravid
females are equally likely to initiate escape swimming
Discussion when threatened. The probability of eliciting escape be-
havior in gravid females is, however, significantly lower.
Among crustaceans, there are four major morphologi- These animals, likelarge adultmales (>600 g), areessen-
cal and physiological determinants that affect agonistic tially inhibited from swimming, even when presented
70 S. I. CROMARTY ET AL.
with larger stimuli (see Cromarty etal. 1997). The physi- since males have higher circulating concentrations ofpu-
ological bases for these inhibitions are not yet known tative excitatory neurohormones (Fadool et al.. 1989),
and may or may not be due to the same physiological theiractions might enhance central orperipheral synapses
mechanisms; these could involve inhibition of(a) sensory (Kravitz. 1988. for review; Schwanke et al.. 1990). In
input to the central nervous system (Watson. 1992, for a this regard, althoughourexperiments were runon isolated
review) or (b) neurons within the central nervous system individuals, it should be noted that in crayfish (Yeh et
(Beall et al.. 1990). or at peripheral synapses (Schwanke al.. 1996). the lateral giant neuron, which is responsible
et al.. 1990: Cromarty et al.. 1995; Cromarty and Kass- for escape responses elicited by tactile or mechanical de-
Simon. 1996), or in both locations. flection of the telson, is affected by the animal's social
Although the same physiology may ultimately be in- experience, with dominant animals having an enhanced
volved in both cases, the adaptive significance in each response to stimuli.
instance appears to be different. In the case of the adult However, although there may be subtle sex-related dif-
male lobsters, the large size of the claws could make ferences in the characteristics of escape swimming, sig-
escape swimming more difficult (Lang et al.. 1977). but nificantdifferencesbetweenthetwosexes wereabsent not
would increase the odds of a favorable outcome in con- only in the total distance swum, but also in the velocity or
frontations with a predator or conspecific opponent. In acceleration of the swim, and in the calculated force or
the case of the ovigerous females, escape swimming work performed. Our relatively small sample size may
would have a deleterious effect. The single gravid female preclude a definitive conclusion about sexual differences
that responded to a threat with swimming lost many eggs. in escape behavior. A larger sample size, or an analysis
underscoring the idea that in this case, swimming inhibi- of additional escape parameters, might reveal differences
tion evolved to ensure reproductive success. between the sexes, given that there are inherent sexual
Differences in the characteristics of escape behavior abdominal dimorphisms.
between non-gravid females and males are more subtle. It remains tobe seen, therefore, whetherthe anatomical
Although the total distance that each group swam did not and physiological sexual differences that result in more
prove to be different, males covered more ground than and longer tailflips in males, but perhaps a higher fre-
did non-gravid females, both in the single initial power quency of swimming in females, have any effect on the
swim and in the sum of the subsequent swims. This sug- relative chances of the sexes for survival.
gests that, like the molt-related differential effects on the
power swim and subsequent swims found injuvenile lob- Acknowledgments
sters (Cromarty et al.. 1991), sex-related differential ef-
fects on the neuronal pathway appear to be mediating the We thank Kathy Castro of the University of Rhode
two components ofthe escape swimming. It thus appears Island Fisheries Department for her assistance in collect-
that, as in crayfish (Wine and Krasne, 1972), in lobster, ing lobsters and measuring theirabdomens. We alsothank
escape behavior is most likely mediated by both giant Dr. Michael Clancy and Malia Schwartz forcritiquing an
(initial power swim) and non-giant fibers (subsequent earlier draft of the manuscript. This research was sup-
swims). ported by a Whitehall Foundation grant to G.K.S.. and
Male lobsters appeared to outperform females in sev- Sigma Xi and Lerner Gray grants to S.I.C.
eral other parameters. On average, they swam for a sig-
nificantly longer time by performing more tailflips than
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