| 1. |
- Jonsson, Lisbeth G., 1955, et al.
(författare)
-
Symbiotic associations between anthozoans and crustaceans in a temperate coastal area
- 2001
-
Ingår i: Marine Ecology Progress Series. - 0171-8630. ; 209, s. 189-195
-
Tidskriftsartikel (refereegranskat)abstract
- While symbiotic associations between anemones and crustaceans appear to be common in tropical waters, few such associations are known from temperate waters, except for the symbiosis between hermit crabs and anemones. In this study, observations made with ROVs (remotely operated vehicles) suggested that certain shrimps (Pandalus borealis, P. propinquus, P. montagui and in particular Spirontocaris liljeborgii and Lebbeus polaris) associate with the anemone Bolocera tuediae and the cerianthid Pachycerianthus multiplicatus by aggregating beneath their tentacles. The lithodid crab Lithodes maja was also observed associating with B. tuediae. Laboratory experiments suggested that female crabs have a stronger association than males. The associations are presumably facultative commensalistic, as the species of crustaceans live as non-symbionts on the sea floor as well, and the 2 anthozoans do not seem to gain any benefits from the associations. In the field, S. liljeborgii had the closest association with both species of anthozoans, while B. tuediae was the preferred host of all associating species. The main benefit for the crustaceans to be associated with the anthozoans is protection against predators. In the case of the shrimps, access to a food source is probably also important.
|
|
| 2. |
- Granovitch, A., et al.
(författare)
-
Digenetic trematodes in four species of Littorina from the west coast of Sweden
- 2000
-
Ingår i: Ophelia. - 0078-5326. ; 53:1, s. 55-65
-
Tidskriftsartikel (refereegranskat)abstract
- Four species of Littorina (L. littorea, L. obtusata, L. fabalis and L. saxatilis) from the Swedish west coast were examined fur infections with digenetic trematodes. Four species of the genus Microphallus, Podocyte atomon, Renicola roscovita, Himasthla elongata, Cryptocotyle lingua, and Paramonostomum chabaudi were found, in addition to encysted metacercariae of the two genera Himasthla and Renicola. The prevalence of sporocysts and rediae varied substantially among sampled populations (0-44%), as did the prevalence of metacercarial infections (3-100%), and this variation was due both to different microhabitats and to different host species. In L. saxatilis exposed rocky shore populations had lower prevalence than sheltered boulder shore populations. Within the same shores, L. littorea was the trust species with highest prevalence of digeneans. The general pattern of distribution of the nine trematode species ol er host species was similar to what have been reported from other North Atlantic areas. However, prevalence and intensity differed both at a geographic scale (in comparison with other studies) and at a local scale. The temporal variation over years was less significant.
|
|
| 3. |
- Johannesson, Kerstin, 1955
(författare)
-
Evolution in Littorina: ecology matters
- 2003
-
Ingår i: Journal of Sea Research. - 1385-1101. ; 49:2, s. 107-117
-
Tidskriftsartikel (refereegranskat)abstract
- Organisms of marine rocky shores are exposed to physical stress from abiotic factors, such as temperature, salinity and wave action. These factors vary over compressed temporal and spatial scales, producing an exceedingly heterogeneous habitat with steep gradients of selection, and it seems likely that this has a strong influence on the evolution of populations of rocky shore organisms. With the periwinkles (genus Littorina) as a model group, I review strategies for coping with small-scale heterogeneous environments and what implications these strategies have on the evolution of these species. Some species of Littorina have long-lived pelagic larvae and sites of various habitats are thus recruited from a common gene pool. This largely prevents local adaptation but minor adjustments are possible through a plastic phenotype. Other species of the genus are directly developing with no larval dispersal and among these there is evidence of strong local adaptation forming distinct ecotypes in contrasting habitats by parallel evolution. In at least one of the directly developing species (L. saxatilis) divergent selection among ecotypes has resulted in partial reproductive barriers that further impede gene flow among ecotypes. Furthermore, convergent evolution among species has produced superficially similar morphs in different habitats. Ecotype formation, ecological reproductive barriers and convergence among species all indicate that ecological processes are critical for evolution of Littorina species. (C) 2002 Elsevier Science B.V. All rights reserved.
|
|
| 4. |
|
|
| 5. |
- Johannesson, Kerstin, 1955, et al.
(författare)
-
Hybrid fitness seems not to be an explanation for the partial reproductive isolation between ecotypes of Galician Littorina saxatilis
- 2000
-
Ingår i: Journal of Molluscan Studies. - 0260-1230. ; 66, s. 149-156
-
Tidskriftsartikel (refereegranskat)abstract
- Partial reproductive isolation between closely related groups of organisms is suggested to be of central importance during speciation. Galician populations of Littorina saxatilis are phenotypically differentiated into an upper-shore and a lower-shore morph. These mate assortatively in the mid-shore: zone of overlap, and genetic assessment shows an impeded gene flow between the parental morphs. The traditional explanation as to why assortative mating occurs is that reproductive isolation is reinforced due to hybrid unfitness. Earlier studies have, however, not found hybrids to be less viable. Likewise, growth and migratory behaviours are merely intermediate between those of the parental morphs. In the present study we compared male and female fertility components of the parental morphs and the hybrids to test hypotheses of decreased hybrid fertility. The results showed that hybrid males were as fertile as other males, and hybrid females did not produce fewer embryos, nor aborted embryos at a higher rate, than the parental morphs.
|
|
| 6. |
|
|
| 7. |
- Johannesson, Kerstin, 1955
(författare)
-
Parallel speciation: a key to sympatric divergence
- 2001
-
Ingår i: Trends in Ecology & Evolution. - 0169-5347. ; 16:3, s. 148-153
-
Tidskriftsartikel (refereegranskat)abstract
- Until recently our view of speciation was that reproductive isolation evolves during long periods of allopatry through the accumulation of genetic differences that result in genetic incompatibility. This view now contrasts with new findings that characters affecting reproductive isolation can diverge rapidly in sympatry as a result of natural selection. Recent studies combining research on phylogeny and ecology of natural populations cast new light on patterns, timescales and mechanisms, and emphasize the role of ecological factors in speciation, Studies of parallel speciation provide a strong case for sympatric speciation and for natural selection generating reproductive barriers.
|
|
| 8. |
|
|
| 9. |
- Johannesson, Kerstin, 1955, et al.
(författare)
-
Selective predation favouring cryptic individuals of marine snails (Littorina)
- 2002
-
Ingår i: Biological Journal of the Linnean Society. - 0024-4066. ; 76:1, s. 137-144
-
Tidskriftsartikel (refereegranskat)abstract
- Many gastropods have inherited conspicuous shell colour polymorphisms. A challenging question is, are colour frequencies under selection or is polymorphism owing to random evolutionary processes? The intertidal species Littorina saxatilis (a rock-dweller) and L. obtusata (confined to macroalgae) both have genetically determined shell colour variation. In Iceland, Littorina obtusata are mostly cryptic on brown macroalgae by having brown or yellow shells (similar to95% of the snails), while Littorina saxatilis often appears conspicuous to the background of dark rocks owing to non-cryptic colours (15-20%). This difference may be due to selective elimination of conspicuously coloured L. obtusata by visual predators, while L. saxatilis, largely living in another habitat, is not under a similarly intense colour selection. To test this hypothesis we increased the frequencies of conspicuous L. saxatilis in experimental populations (from <12 to 55%) and placed these in the seaweed zone, the main habitat of L. obtusata. Fifteen populations were released on isolated spots of seaweed and three of these were covered by net cages to exclude bird predators. One month later, yellow snails had increased in frequency within the patches, and to our surprise the result did not differ between bare and caged patches. This suggests selection favouring a colour that matches the background of fucoid seaweeds by visual predators able to enter the cages. Birds acted as important predators by picking 16% of the experimental snails in the uncaged spots, but were unable to enter the caged spots. However, the bird predation was non-selective with respect to snail colour. For various reasons the most likely predators able to enter the cages were intertidal fish, these were thus responsible for the selection of non-cryptic snails. (C) 2002 The Linnean Society of London.
|
|
| 10. |
- Panova, Marina, 1973, et al.
(författare)
-
Microscale variation in Aat (aspartate aminotransferase) is supported by activity differences between upper and lower shore allozymes of Littorina saxatilis
- 2004
-
Ingår i: Marine Biology. - : Springer Science and Business Media LLC. - 0025-3162 .- 1432-1793. ; 144:6, s. 1157-1164
-
Tidskriftsartikel (refereegranskat)abstract
- The role of natural selection in maintaining inherited variation in morphological characters is indisputable. In contrast, genetic variation in allozyme loci is mostly assumed neutral or under weak selection, although enzyme functions are critical to central metabolic processes. A locus coding for aspartate aminotransferase (AAT) is polymorphic with two alleles in the rocky shore gastropod Littorina saxatilis. One allele (Aat(120)) is most frequent (0.8-0.9) in upper shore subpopulations, while the other (Aat(100)) dominates (0.7-0.8) lower shore subpopulations only meters below. Earlier studies show this variation to be maintained by divergent selection, but the mechanism is unknown. We measured enzyme activities in AAT extracts from individuals of known Aat genotype in populations from two islands and at three different temperatures. In all treatments lower shore homozygotes (Aat(100/100)) had higher enzyme activity than upper shore homozygotes (Aat(120/120)), with heterozygotes having intermediate levels of activity. AAT has a central role in anaerobic energy production in marine mollusks during low-tide periods of emergence, and it might appear surprising that upper shore snails, which are most likely to be emerged, have lower enzyme activity. However, upper shore snails are forced to survive long periods of desiccation, and a low metabolic rate will help to conserve limited resources of energy and nutrients and reduce accumulation of waste products.
|
|
