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BIO603 Elin Nilsson VT17.Edited DEPARTMENT OF BIOLOGICAL AND ENVIRONMENTAL SCIENCES THE EFFECT OF SIZE DIFFERENCE AND ECOTYPE ON THE RISK OF LITTORINA SAXATILIS GETTING SWEPT AWAY BY WAVES DURING MATING Elin Nilsson Degree project for Bachelor of Science with a major in biology BIO603 Biologi: Examensarbete 30 hp First cycle Semester/year: Spring 2017 Supervisor: Kerstin Johannesson, Department of Marine Sciences Examiner: Susanna Eriksson, Department of Biological and Environmental Sciences Table of Contents Abstract ............................................................................................................................................ 2 Introduction ...................................................................................................................................... 3 Material and Method ........................................................................................................................ 4 Test organism ........................................................................................................................... 4 Experimental design ................................................................................................................. 4 Statistics .................................................................................................................................... 6 Results .............................................................................................................................................. 7 Discussion ...................................................................................................................................... 10 Acknowledgements ........................................................................................................................ 11 References ...................................................................................................................................... 11 Appendix ....................................................................................................................................... 12 1 Abstract The direct developing periwinkle Littorina saxatilis has the highest mounting probability when the female is roughly double the size of the male. A previous bachelor’s thesis showed that mounted females are more likely to be swept away by running water than other females, and this likelihood was larger in a group of females with males larger than themselves, compared to females mated with males of similar or smaller sizes. This thesis raised the same issue but compared female/male pairs of many different relative sizes and of both exposed wave ecotype and the sheltered crab ecotype of Littorina saxatilis where the wave ecotype is under stronger wave stress in the wild than is the crab ecotype. Females were placed in a high-speed aquarium with shells of males glued to their back in mounting position, and the water speed at which they fell off were noted. Results included 160 female snails from Saltö in Kosterhavet, 88 of the crab ecotype and 72 of the wave ecotype. There was a significant correlation between the water speed at which the females fell off and their relative size to the male. There was a greater difference in mean between when the female was larger and the male was larger for the wave ecotype, but it was not significant. In conclusion, both size and ecotype seem to affect the risk of being swept away, but it’s only significant for size. 2 Introduction Littorina saxatilis (Olivi) is an intertidal rough periwinkle common in the North Atlantic which inhabits rocky and boulder shores (Reid 1996). The snail has ecotypes adapted to different habitats with inherited differences in size and shape (Janson 1982; Johannesson et al. 1993). It has been suggested that these inherited differences are part of an ongoing ecological speciation; there is strong evidence that the divergence of the ecotypes is happening in the presence of gene flow (Butlin et al. 2014), and that the ecotypes are partially reproductively isolated. There are many speculations as to what might be the source of the differences between the ecotypes. The snail is ovoviviparous, lacks pelagic larval dispersal, and has a low adult dispersal (1-4 m/month), leading to low gene flow between different populations (Janson 1983). Another factor might be that despite the small geographical difference, the ecotypes have very different habitats and face different threats. On Saltö, a Swedish island in Kosterhavet, there are two ecotypes of L. saxatilis: the wave ecotype, and the crab ecotype, which occur in repeated intervals of crab ecotype and wave ecotype, with hybrid zones in between (Janson 1982). The crab ecotype is found below or slightly above sea level in boulder bays (Janson 1983), where it’s protected from waves but is exposed to crabs and moving rocks, and is therefore benefited from having a small aperture (opening), a thick shell, and a low spire (Janson 1982). Experiments show big, thick shells with a low spire withstand crab attacks better than small, thin, high-spired shells (Johannesson 1986). The wave ecotype, on the other hand, is usually found on cliffs (Janson 1983), high up in the splash zone where crabs can’t reach it, but where it is exposed to waves (Janson 1982). It is smaller, but with a relatively larger aperture (allowing for a larger area connected to the rock), and a thin shell (Janson 1982). The females of L. saxatilis are larger than the males (Reid 1996), and although there has been some contradicting studies whether or not both males of both ecotypes prefer larger females (Erlandsson and Rolán-Alvarez 1998), recent, extensive experiments in lab suggest that both ecotypes do (unpublished data/data in preparation, see Appendix 1 for a graph showing the relationship between relative size and mounting probability). In a study on Littoraria ardouiniana, Ng and Williams (2014) showed similar results; while small males were unselective, large males preferred to follow mucus trails laid by large females, and both sizes copulated for a longer time with larger females. The appearance of small males being unselective might, however, be an effect of the males preferring females that are either the same size or larger than them (Ng and Williams 2014). Another example comes from Littorina littorea, where males from some (but not all) populations prefer to mate with large females (Erlandsson and Johannesson 1994). There are a number of theories for why this preference for large females exist, the most obvious one being that female fecundity increases with female size in littorinid snails (Janson 1985). Another theory is that when the male is equal to or larger than the female, the risk that they will be swept away by waves during mating increases. Saur observed that small females sometimes lose their grip on the rock surface when large males try to mate them (pers. comm., cited from Erlandsson and Johannesson 1994), and snails that have been swept away in this manner rarely make it back (K. Johannesson, pers. observation). Results from a bachelor’s thesis show that for the crab ecotype, when the male was equal to the female in size, or larger than her, the risk for the female to lose her grip was higher than if not mounted. When the female was almost double the size of the male, there was no difference between mounted and unmounted females (Thorén 2016). This effect of the relative size on the fall-off risk should be even more important to the wave ecotype, given that they are more exposed to waves than the crab ecotype, and the size difference between the sexes seems to be greater within the wave ecotype than within the crab ecotype (Erlandsson and Rolán-Alvarez 1998). This thesis compared the wave and crab ecotypes of Littorina saxatilis and the effect of the relative size between the female and male during mounting when impacted by the force of flowing water. Essentially, testing the female’s ability to resist being swept away at different 3 water speeds. Since the wave ecotype is adapted to waves and has a relatively larger foot, it might be that it is less sensitive to a lower or negative relative size. Another hypothesis is that, because the size difference between the sexes is greater within the wave ecotype, it might be more sensitive to a lower or negative relative size. Material and Method Test organism Both ecotypes (wave and crab) of the species Littorina saxatilis were collected from sites on the island of Saltö (Fig. 1), during May-June. In total, measurements were obtained for 88 of the crab ecotype and 72 of the wave ecotype. Female sizes ranged between 5-12 mm aperture diameter in the crab ecotype, and 3-10 mm aperture diameter in the wave ecotype. The snails were sexed based on the presence or absence of a penis. The snails that were glued onto the females were boiled and the soft parts removed. Since there is no difference in shell shape between females and males, shells of females were also used as "male" shells. The ”males” were glued onto the females in a typical mating position (see Fig. 2). Snails were stored at a temperature of 5°C for a maximum of Y days before used in the experiment. Figure 1: Showing Saltö, outside Tjärnö in Strömstad, Sweden. The localities the snails were sampled are marked; the wave ecotype from point A and the crab ecotype from point B. The maps are screenshots from google maps. Experimental design The longest distance off the opening was measured (see Fig. 3), which was used to calculate the relative size of the female to the male,
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