Abstract

1. Figure 2: Gamma male Figure 1: Alpha male Figure 3: Female 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 α- and γ-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 α-male was combined with clean seawater to total 1000 ml in R1, and the water of one γ-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 α-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 (F[5,49]=3.60, P=.007) with significant effects of reservoir set (F[1,49]=4.50, p=.04), and treatment (F[2,49]=4.21, P=.02) but no significant interaction between these factors (F[2,49]=1.91, P=.16). We removed the interaction from the model (F[3,51]=4.58, P=.007) and found a significant effect of treatment (F[1,51]=4.72, P=.013), but no significant effect of reservoir set (F[2,51]=2.01, 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 (F[2,52]=5,75, 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.±95%CI = 528.9±245.3) and Zone 2 (avg.±95%CI = 626.7±233.9), indicating that S1 females showed no preference for chemical cues associated with breeding aggregations. Our 2-way ANOVA for E2 (α-male vs. γ-male) was non-significant overall (F[5,14]=0.25, P=.94) with non-significant effects of reservoir set (F[1,14]=.27, p=.61), treatment (F[2,14]=.23, P=.80) and interaction effects (F[2,14]=.06, P=.94) (Fig. 8), indicating that S1 females showed no preference for chemical cues associated with α-males over cues associated with γ-males. Our 1-way ANOVA for E3 (sponge vs. control) was significant (F[2,57]=14.8, P<.0001) (Fig. 9), and while α-males spent most of their time in Zone 0 of the Y-maze (avg.±95%CI = 542.5±128.3 sec), they spent significantly less time in Zone 2 (control; avg.±95%CI = 39.6±128.3 sec) than they spent in Zone 1(sponge; avg.±95%CI = 317.5±128.3 sec). This result indicates that α-males are attracted to chemical cues associated with L. losangelensis sponges. 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 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 (F=14.81, P<0.0001). 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 (1=alpha male + 3 Stage 3 females, 2=control, 3=neutral) on time spent in area of Y-maze Figure 8: Experiment 2. Effect of treatment (1=gamma, 2=alpha, 3=neutral) 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, α and γ (Figures 1 & 2) make up 82% and 14% of the male population respectively. Alpha males defend territories in intertidal sponges (Leucetta losangelensis), while γ –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 α-males, and tend to breed within the first spongocoel they encounter that contains an α-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 (1=sponge, 2=control, 3=neutral) on time spent in area of Y-maze Discussion Our results indicate that α-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 α-male and a γ-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 Austropotamobiuspallipes (Acquistapace et al, 2002). It is also possible that females use sound as an attractant in male choice; α-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 α, β-, and γ-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