Use of chemical cues by marine isopods Paracerceis sculpta in the location and selection of mates Em

1
Use of chemical cues by marine isopods
(Paracerceis sculpta) in the location and
selection of matesEmily M. Arnold and Stephen M.
ShusterNorthern Arizona University, Flagstaff,
Arizona, 86011
Methods and Materials Isopods were
collected from the spongocoels of Leucetta
losangelensis in the intertidal zone of Station
Beach and Playa Miramar near Puerto Peñasco,
Sonora, México (Figures 4 5). The length,
cuticle pattern, sex, reproductive stage, and
other physical characteristics were documented
for each individual. Only Stage 1 (S1) and Stage
3 (S3) females and a- and ?-males were retained
for experiments all other isopods were returned
to the intertidal zone. Individuals were kept in
250 ml cups containing seawater.
Experiment 1 (E1) Our Y-maze was of standard
design (Fig. 5). Reservoir 1 (R1) contained the
water of one alpha male and three S3 females
combined with seawater to total 1000 ml. As a
control, Reservoir 2 (R2) contained 1000 ml of
clean seawater. Reservoir side was changed every
other trial to account for side preferences.
Each trial began by allowing the water from both
reservoirs into the Y-maze at an approximate rate
of 1 drop/sec. After 7 min (the time necessary to
establish a chemical gradient within the maze),
an S1 female was released into Zone 0 of the
Y-maze and observed for 20 min. Time (in sec)
spent in each of three zones was recorded. Two
trials were run for each set of reservoir
contents (Ntrials 34) 3 trials were run with a
single reservoir set (Ntrials 3) NTotal trials
37) Experiment 2 (E2) The water of one
a-male was combined with clean seawater to total
1000 ml in R1, and the water of one ?-male was
similarly combined in R2. The experiment
continued as above (NTotal trials 12).
Experiment 3 (E3) R1 contained 1000 ml of
seawater and a Leucetta losangelensis sponge. R2
contained 1000 ml of clean seawater and served as
a control. An a-male was released into Zone 0
and observed for 15 min. Time (in sec) spent in
each of three zones was recorded as above. Each
set of reservoir was used once for each trial
(NTotal trials 20). Data from E1-2 were
analyzed using a 2-way and 1-way ANOVA to examine
the effects of reservoir set, treatment and the
interaction of these effects on time spent in
each zone of the Y-maze. Data from E3 were were
analyzed using one-way ANOVA to test for effect
of treatment on time in each Y-maze zone.
Results Our 2-way ANOVA for E1 (breeding
aggregation vs. control) was significant overall
(F5,493.60, P.007) with significant effects
of reservoir set (F1,494.50, p.04), and
treatment (F2,494.21, P.02) but no
significant interaction between these factors
(F2,491.91, P.16). We removed the interaction
from the model (F3,514.58, P.007) and found a
significant effect of treatment (F1,514.72,
P.013), but no significant effect of reservoir
set (F2,512.01, P.16). We then used 1-way
ANOVA to compare the effect of treatment alone on
time in each zone, and again found a significant
treatment effect (F2,525,75, P.006) (Figure
7). However, this effect appeared to be caused by
the large amount of time female spent in zone 0.
There was no significant difference between times
spent in Zone 1 (avg.95CI 528.9245.3) and
Zone 2 (avg.95CI 626.7233.9), indicating
that S1 females showed no preference for chemical
cues associated with breeding aggregations.
Our 2-way ANOVA for E2 (a-male vs. ?-male) was
non-significant overall (F5,140.25, P.94)
with non-significant effects of reservoir set
(F1,14.27, p.61), treatment (F2,14.23,
P.80) and interaction effects (F2,14.06,
P.94) (Fig. 8), indicating that S1 females
showed no preference for chemical cues associated
with a-males over cues associated with ?-males.
Our 1-way ANOVA for E3 (sponge vs. control)
was significant (F2,5714.8, Plt.0001) (Fig. 9),
and while a-males spent most of their time in
Zone 0 of the Y-maze (avg.95CI 542.5128.3
sec), they spent significantly less time in Zone
2 (control avg.95CI 39.6128.3 sec) than
they spent in Zone 1(sponge avg.95CI
317.5128.3 sec). This result indicates that
a-males are attracted to chemical cues associated
with L. losangelensis sponges.

• Abstract
• Chemical cues are used by many animals to
  locate breeding habitats and mates. Semelparous
  species face time and energy constraints that
  require rapid location of suitable breeding
  habitats, and in aquatic habitats, individuals in
  such species may use chemical cues to locate
  their breeding habitat from a distance. We used 3
  Y-maze experiments to test for sensitivity to
  chemical cues in a population of the marine
  isopod Paracerceis sculpta. This population
  contains three distinct male morphs, the dominant
  of which forms breeding aggregations in an
  intertidal sponge. Females in this population
  are semelparous and face extremely short
  receptivity times. In choice tests, females
  showed no sensitivity to chemical cues from
  breeding aggregations and no preference between
  cues from two different male morphs. However,
  males were attracted to chemical cues from
  sponges (F14.81, Plt0.0001). Our results indicate
  that males locate breeding sites using chemical
  cues from sponges, and that chemical cues from
  males and from breeding aggregations may be
  unimportant in female mate choice.

Figure 7 Experiment 1. Effect of treatment
(1alpha male 3 Stage 3 females, 2control,
3neutral) on time spent in area of Y-maze
Figure 8 Experiment 2. Effect of treatment
(1gamma, 2alpha, 3neutral) on time spent in
area of Y-maze
Figure 4 Collecting isopods in Puerto Penasco
Figure 5 Leucetta losangelensis
Introduction Three discrete male morphs
coexist in Paracerceis sculpta, a Gulf of
California isopod (Shuster 1987 Shuster and Wade
1991). The 2 most common morphs, a and ? (Figures
1 2) make up 82 and 14 of the male population
respectively. Alpha males defend territories in
intertidal sponges (Leucetta losangelensis),
while ? males invade breeding territories by
stealth (Shuster 1990). Females in this species
(Fig. 3) are semelparous, with sexual
receptivities lasting 24-48 hours (Shuster 1989
1990 Arnold, pers. obs.). Such species clearly
face time and energy constraints in breeding
habitat and mate selection (Andersson 1994).
Female P. sculpta are known to prefer harems to
unpaired a-males, and tend to breed within the
first spongocoel they encounter that contains an
a-male (Shuster 1990). However, the cues used by
females to make such choices remain uncertain
(Shuster 1986). Males appear to use chemical
cues to locate breeding sites, but the importance
of tactile and visual cues in site selection have
not been eliminated (Shuster 1992). We
conducted Y-maze experiments to investigate the
use of chemical cues by males and females in
breeding site location and in mate
discrimination. It was hypothesized that males
would be attracted to chemical cues from their
typical breeding habitat. We also hypothesized
that females use chemical cues to locate
potential mates and could discriminate between
cues from males of different morphotypes.
Figure 9 Experiment 3. Effect of treatment
(1sponge, 2control, 3neutral) on time spent in
area of Y-maze
Discussion Our results indicate that a-males
use chemical cues from sponges to locate
potential breeding habitats (Figure 9).
Alpha-males were significantly more attracted to
water containing a sponge than to control water.
Contrary to our expectations, S1 females were not
significantly attracted to chemical cues
associated with breeding aggregations or male
phenotype (Figs. 7 8). Attraction to control
water was no different from that containing a
typical harem, and was not significantly
different between water of an a-male and a
?-male. For a semelparous female, viability
selection and mate choice could have a large
effect on reproductive effort, but the extremely
short duration of receptivity in this species
appears to drive the effort toward reproductive
habitat. Further studies will investigate the
chemical attractivity of water containing a
sponge and a male versus that of a sponge alone,
as the presence of a male may magnify the
chemical signal of a sponge. Additional
experiments will test the difference between the
water of a male versus that of a sponge. Also,
the potential for selection between mates using
other signal modes and characteristic use in mate
discrimination will be investigated. Other
possible explanations for the absence of chemical
cues use in mate evaluation are that females may
require multiple senses in determining mate
quality, as in the crayfish Austropotamobius
pallipes (Acquistapace et al, 2002). It is also
possible that females use sound as an attractant
in male choice a-males move their pleotelsons
when females approach them and may transmit
vibrations through water.

Figure 6 Schematic diagram of the Y-maze
References Acquistapace, P.A. 2002. Multimodal
communication in crayfish sex recognition during
mate search by male
Austropotamobus pallipes. Can. J. Zool/Rev. Can.
80 (11) 2041-2045. Andersson, M. 1994. Sexual
Selection. Princeton University Press.
Princeton, New Jersey. Shuster, S. M. 1992. The
reproductive behaviour of a, ß-, and ?-males in
Paracerceis sculpta, a marine isopod crustacean.
Behaviour 121 231-258. Shuster, S. M. M. J.
Wade. 1991. Equal mating success among male
reproductive strategies in a marine isopod.
Nature 350 606-610. Shuster, S. M. 1990.
Courtship and female mate selection in a
semelparous isopod crustacean (Paracerceis
sculpta). Anim. Behav. 40 390-399. Shuster, S.
M. 1989. Female sexual receptivity associated
with molting and differences in copulatory
behavior among the three male morphs in
Paracerceis sculpta, (Crustacea Isopoda). Biol.
Bull. 177 331-337. Shuster, S. M. 1987.
Alternative reproductive behaviors Three
discrete male morphs in Paracerceis sculpta, an
intertidal isopod from the northern Gulf of
California. J. Crust. Biol. 7 318-327. Shuster,
S.M. 1986. Reproductive biology of Paracerceis
sculpta (Crustacea Isopoda). Doctoral
dissertation.
Acknowledgements

Research
Experience for Undergraduates NSF Grant NSF grant
DBI-0243914, the Shuster Lab, REU Summer 2004
members for help in collection of specimens