Johan Erlandsson

The maintenance of a cline in the marine snail Littorina saxatilis: The role of home site advantage and hybrid fitness

Steep clinal transitions in one or several inherited characters between genetically distinct populations are usually referred to as hybrid zones. Essentially two different mechanisms may maintain steep genetic clines. Either selection acts against hybrids that are unfit over the entire zone due to their mixed genetic origin (endogenous selection), or hybrids and parental types attain different fitness values in different parts of the cline (exogenous selection). Survival rate estimates of hybrids and parental forms in different regions of the cline may be used to distinguish between these models to assess how the cline is maintained. We used reciprocal transplants to test the relative survival rates of two parental ecotypes and their hybrids over microscale hybrid zones in the direct‐developing marine snail Littorina saxatilis (Olivi) on the rocky shores of Galicia, Spain. One of the parental forms occupies upper and the other lower shores, and the hybrids are found at various proportions (1–38%) along with both parental forms in a midshore zone a few meters wide. The survival rate over one month was 39‐52% of the native ecotype on upper shores, but only 2‐8% for the lower‐shore ecotype. In contrast, only 4‐8% of the upper‐shore ecotype but 53% of large (> 6 mm) and 8% of small (3‐6 mm) native lower‐shore ecotype survived in the lower shores. In the midshores, both the two parental ecotypes and the hybrids survived about equally well. Thus there is a considerable advantage for the native ecotypes in the upper and lower shores, while in the hybrid zone none of the morphs, hybrids included, are favored. This indicates that the dimorphism of L. saxatilis is maintained by steep cross‐shore selection gradients, thus supporting the selection‐gradient model of hybrid zones. We performed field and laboratory experiments that suggest physical factors and predation as important selective agents. Earlier studies indicate assortative mating between the two ecotypes in the midshore. This is unexpected in a hybrid zone maintained by selection gradients, and it seems as if the reproductive barrier compresses the hybrid zone considerably.

Trail following, speed and fractal dimension of movement in a marine prosobranch, Littorina littorea, during a mating and a non-mating season

We quantitatively studied movement behaviour in the gastropod Littorina littorea in laboratory experiments during periods of their non-mating season (November 1992) and their mating season (April–May 1993). Snails were collected in 1992 and 1993 from one boulder shore on the north west coast of Sweden. In a comparison between the two seasons (one non-mating and one mating) we measured trail complexity of unsexed snails using fractal dimesion, the degree of mucus trail following (coincidence index of marker and tracker trails) and average movement speed of marker and tracker snails. We found no differences in fractal dimension and coincidence index of trails between the two seasons. Tracker snails moved, however, significantly faster than marker snails during both seasons. This could not be explained by trackers, when locomoting, using the mucus trail deposited by the marker to increase their speed, since there was no correlation between coincidence index and tracker speed. During the mating season we also conducted trail complexity, trail following and speed experiments comparing the behaviour of males and females. There was no difference between males and females in the fractal dimension of movement, nor was there any difference between the mean speed of male and female snails, although male marker snails tended to move faster than female marker snails. Males tracking other males, females tracking other females and females tracking males followed trails about equally long distances (i.e., coincidence indices did not differ). In contrast, males following female mucus trails showed a significantly higher degree of trail following than the other sex combinations. This new finding may suggest that females of L. littorea release pheromones in their mucus trails and that males are able to identify them.

Mechanisms of incomplete prezygotic reproductive isolation in an intertidal snail: testing behavioural models in wild populations

Two morphs (ecotypes) of the marine snail Littorina saxatilis coexist along Galician exposed rocky shores. They hybridize, but gene flow is impeded by a partial prezygotic reproductive barrier, and we have earlier suggested that this is a case of incipient sympatric speciation. To assess the mechanisms of prezygotic reproductive isolation, we estimated deviations from random mating (sexual selection and sexual isolation) of sympatric snails in 13 localities on the shore, and performed mate choice experiments in the laboratory. We also investigated the microdistribution of both morphs over patches of barnacles and blue mussels in the hybridization zone. We used computer simulations to separate the mechanisms contributing to reproductive isolation.

Sexual selection on female size in a marine snail, Littorina littorea (L.)

Abstract The mating pattern of the common shore-dwelling gastropod Littorina littorea was studied in five boulder shore populations on the Swedish west coast, to examine the occurrence of size assortative mating and sexual selection on size. We looked for correlations between sizes of mates in copulating male-female pairs in three of the populations to test the possibility of assortative mating. Furthermore, we compared the distribution of small and large individuals among copulating and non-copulating snails in all five populations to reveal components of sexual selection among males and females. Size assortative mating was found in one of three populations. In three of five populations, sexual selection on size among females were found, large females being favoured as mating partners over small ones. There was a trend towards this pattern in a fourth population. In no population did we find any indications of sexual selection on male size. Deviation from random mating and/or observation of sexual selection at a particular time may be effects of a non-random size distribution of snails, a pre-copulatory choice and/or different copulation durations among pairs of different composition. We performed laboratory experiments to test these possibilities. Although encounter rates may have been the same, large females were mounted at a higher rate and initiated in copulations more often than small females, and large females copulated for longer than did small females. These observations supported our field data in that large females were favoured as mates by most males. The shell length of females of Littorina littorea is positively correlated with their fecundity, and thus, males seem to prefer to mate with the more profitable large females.

Migratory differences between ecotypes of the snail Littorina saxatilis on Galician rocky shores

Local migration patterns may be crucial to gene flow in species of marine gastropods which do not broadcast pelagic larvae. In some species, dispersal over distances of a few metres may influence micro-scale population structures. We investigated the migration pattern in Galician populations of the snail Littorina saxatilis in which populations of contrasting morphologies occupy different tidal levels of the same rocky shore. Two distinct morphs, one at the upper and one at the lower shore, overlap in distribution in a small mid-shore region where hybrids are produced. We documented the distances and directions of migration of both parental morphs and hybrids 1 month after they had been marked and released at different shore levels. When placed at their native shore level, snails migrated less than about 2m and usually in independent directions. This supports the suggestion of a low local gene flow. At an alien shore level, however, the morphs often moved further and more directionally compared with native morphs. These differences may help to keep the two morphs separated at different shore levels. As fitness of an individual is highest in its native habitat, this seems to be an adaptive strategy.

Microdistribution of the polymorphic snail Littorina saxatilis (Olivi) in a patchy rocky shore habitat

Abstract Galician populations of the marine snail Littorina saxatilis have two main morphs on exposed rocky shores. The ridged and banded morph inhabits the barnacle (Chthamalus stellatus) dominated upper shore, while the smooth and unbanded morph is mainly found in the lower-shore zone of blue mussels (Mytilus galloprovincialis). The distribution of the two morphs overlaps in the mid shore where hybrids are also present. The mid-shore habitat is a mixture of barnacle and mussel patches. We hypothesised that the different architectural complexity of barnacles and mussels would affect the distribution of the two morphs. Fractal dimensions were used to describe the substratum complexity of 96 small patches from three sites with different proportions of barnacles and mussels. Increased proportions of mussels in a patch increased the fractal dimension, and thus surface complexity. Snail abundance decreased with increased substratum complexity in the smooth and unbanded morph and in the hybrids but not in the ...

A field technique for estimating the influence of surface complexity on movement tortuosity in the tropical limpet cellana grata gould

Most studies of the movement patterns of intertidal herbivores assess overall displacement in an assumed homogeneous environment. In the present study, the impact of substrate complexity on spatial...

Sexual selection and assortative mating by size and their roles in the maintenance of a polymorphism in Swedish Littorina saxatilis populations

Two independent components of mating behaviour, sexual selection and assortative mating, were studied in two allopatric morphs, one sheltered boulder shore form (S-morph) and one exposed cliff shore form (E-morph), of Littorina saxatilis from the west coast of Sweden. Sexual selection was studied by comparing the sizes of copulating and non-copulating snails in the field. Size assortative mating was studied by collecting copulating pairs in the field, while assortative mating between morphs was investigated by bringing the pure morphs together in intermediary habitats and then noting the matings. The S-morph mated randomly in relation to size in two of the studied populations and exhibited a trend towards size assortative mating in a third, while the E-morph showed size assortative mating in both studied populations. The microdistribution of sizes of snails on the shores could not explain all the size assortative mating found, and instead it is argued that a size-based mate rejection behaviour also contributes to the assortative mating in at least some of these populations. There was sexual selection on size in both males and females in the S-morph, with large individuals being favoured as mates. In contrast, copulating snails of the E-morph were smaller than non-copulating ones. The significantly different sexual selection intensities between the two morphs may help to explain the size differences between them. There was random mating between the E- and the S-morphs of L. saxatilis, which suggests no incipient reproductive isolation between morphs on Swedish rocky shores. This is in agreement with earlier studies of Swedish populations, but is in contrast to the situation found in other geographical areas.

Mate search and aggregation behaviour in the Galician hybrid zone of Littorina saxatilis

In Galician rocky shores two ecotypes of the snail L. saxatilis can be found in sympatry. A ridged and banded ecotype (RB‐morph) and a smooth and unbanded ecotype (SU‐morph) overlap in midshore with the production of some hybrids. The distinct morphs mate assortatively and there is evidence of a partial reproductive barrier between them. This sexual isolation is caused by a nonrandom microdistribution and mate choice behaviour. Mucus trail‐following, movement rate and aggregation behaviour were studied to determine their roles in the mating behaviour and sexual isolation of this species. Morph‐specific mucus trail‐following could not, in our experiments, explain either of these two processes. The reasons for the aggregation of morphs were investigated by Monte Carlo simulations of data from natural populations, which showed that size aggregation (refuge sizes fit different sized morphs differently) could explain only about 36% of the morph aggregation in adult snails. In the laboratory, morph aggregation was still present, and simulations suggested that size aggregation was the possible explanation. Thus, morph aggregation in Galician L. saxatilis has to be explained also by other causes in addition to size aggregation. These may be a combination of contrasting preferences for barnacle and mussel patches in the two morphs, and possibly longer copulation and pair formation time with similar sized snails of the same morph. Thus aggregation behaviour, but not trail‐following, contributes to incipient reproductive isolation and perhaps sympatric speciation in Galician L. saxatilis populations.

Intertidal zone management in the Western Indian Ocean: assessing current status and future possibilities using expert opinions.

This expert opinion study examined the current status of the intertidal zone in the Western Indian Ocean (WIO) and ranked and discussed future management approaches. Information was gathered from scientists, practitioners, and managers active in the WIO region through a questionnaire and a workshop. The experts stated that the productive intertidal environment is highly valuable for reasons such as recreation, erosion protection, and provision of edible invertebrates and fish. Several anthropogenic pressures were identified, including pollution, harbor activities, overexploitation, and climate change. The experts considered the WIO intertidal zone as generally understudied, undermanaged, and with poor or no monitoring. The most important management strategies according to the expert opinions are to develop and involve local people in integrated coastal zone management (ICZM), to increase knowledge on species–environment relationships, and to develop awareness campaigns and education programs. To improve coastal environmental management and conservation, we argue that the intertidal zone should be treated as one organizational management unit within the larger framework of ICZM.