Littorina mariae — a factor structuring low shore communities?

The role of littorinids in structuring communities is discussed. On hard substrates preferential grazing of the dominant algae often enables competitively inferior species to utilize the rock substrate. Investigations of these systems has led to the proposal of a number of factors that should be analyzed in order to assess the effect of herbivores on community structure. Some of these factors have been investigated for Littorina mariae Sacchi & Rastelli. L. mariae is a micro-epiphytic grazer browsing the surface covering of epiphytes off the alga Fucus serratus (L.). The relationship between the host alga, F. serratus, and L. mariae is far more intricate than that between winkles and hard substrates as the alga itself is a dynamic resource. L. mariae is spatially and temporally linked to F. serratus. L. mariae is found almost exclusively on the alga and is positively attracted by extracts of the alga. The life history of the winkle is closely synchronized with that of the alga (the winter decrease of L. mariae populations is associated with the seasonal die-back of the host alga). On analysis of the factors considered important to assess herbivory, and examples of the effects of other epiphytic herbivores on algal success, it is suggested that L. mariae could potentially play an important role in structuring the community of it's host fucoid and that this may influence larger scale community structure. Experimental manipulations are proposed to evaluate these hypotheses.     

Über das Vorkommen von Sexualhormonen bei der Meeresschnecke Littorina littorea L.

Zusammenfassung Es wurde versucht, ein wirksames Prinzip als Ursache für den sexuellen Jahresrhythmus bei Littorina littorea L. zu finden. Die Tiere wurden auf den Gehalt an androgenen und oestrogenen Stoffen untersucht. Die Prüfung auf Extrakte aus Gonaden, Mitteldarmdrüsen und den Rest-Tieren ergab: Littorina littorea enthält keine im Test am Kückenkamm, an der männlichen, kastrierten Maus und papierchromatographisch nachweisbaren Mengen an androgenen Stoffen.Im Allen-Doisy-Test wirksame Substanzen konnten aus den Ovarien extrahiert werden. Aus der Dosiswirkungskurve ergibt sich ein Gehalt von etwa 3 mg Oestradiolbenzoat-Äquivalenten/kg Frischgewebe.Das Vorkommen von oestrogenen Stoffen ist auf die Ovarien beschränkt. Männliche Tiere sowie Mitteldarmdrüsen und andere Gewebe der Weibchen enthalten keine erfaßbaren Mengen an Oestrogenen.Es herrscht Parallelität zwischen der extrahierbaren Oestrogenmenge und dem Ausbildungsgrad der Keimdrüsen.Im Vergleich mit den Sexualhormonen der Wirbeltiere mittels Papierchromatographie zeigt sich, daß das wirksame Agens nicht identisch mit Oestradiol-(3,17) oder Oestron ist.Durch Abkühlungsversuche konnte ein Einfluß der Temperatur auch für die Aufbauphase im Sexualzyklus bei L. littorea wahrscheinlich gemacht werden.Die Injektion arteigener Extrakte bei den Schnecken führte bisher — wegen der Empfindlichkeit der Tiere — zu keinem Ergebnis.Für die mir immer gewährte freundliche Unterstützung bei der Durchführung der Arbeit sage ich meinem verehrten Lehrer, Herrn Professor Dr. H. Giersberg, meinen herzlichen Dank.Für die freundliche Überlassung von Hormonpräparaten und Testsubstanzen bin ich Herrn Prof. Dr. J. Schmidt-Thomé, Frankfurt a. M.-Hoechst, Herrn Dr. J. Hübener, Institut für vegetative Physiologie, Frankfurt a. M., und der Schering AG zu besonderem Dank verpflichtet.Herrn C. Lüders, Seewasseraquarium Wilhelmshaven, danke ich verbindlichst für die Beschaffung der Littorinen.     

High levels of multiple paternity in Littorina saxatilis: hedging the bets?

The mating system of a species can have great effects on its genetic structure and evolution. We studied the extent of multiple paternity in a gastropod with internal fertilization, the intertidal snail Littorina saxatilis. Paternal genotype reconstruction based on microsatellite markers was performed on the offspring of wild, naturally fertilized females from 2 populations. The numbers of males contributing to the offspring per female were among the highest detected in invertebrates so far, with the exception of social insects. No reproductive skew in favor of males that were genetically more distant from the females was detected, and the pattern of fertilization appeared random. The result fits a hypothesis of indiscriminate mating, with genetic bet hedging as the most likely explanation. Bet hedging may have evolved as a form of inbreeding avoidance, if the snails are not able to recognize relatives. However, nutritional benefits from sperm or sexual conflict with males are additional possibilities that remain to be assessed in this species. Whatever the causes, such high levels of multiple paternity are remarkable and are likely to have a large impact on population structure and dynamics in a species in which migration between populations is spurious.     

Micro- and macrogeographic allozyme variation in Littorina fabalis; do sheltered and exposed forms hybridize?

Earlier studies of Swedish populations of the marine snail Littorina fabalis show that snails from different microhabitats (with a greater and lesser exposure to wave action) have almost diagnostic differences in one allozyme locus (arginine kinase, Ark ), and differ in adult size. Snails with 'sheltered' and 'exposed' Ark genotypes occur in sympatry in intermediary exposed sites and here adult sizes remain distinct. Approaching the microgeographic differentiation we studied the parts of two populations where the frequency of Ark changes dramatically over zones 50–120 m wide. The aim was to test if the transitional zones are best described as areas of mixing of two genetically separate populations, or if hybridization between the exposed and sheltered groups occurs. Heterozygotes were in deficiency along both clines but were still roughly twice as common as expected from a pure mixing of 'sheltered' and 'exposed' groups suggesting hybridization. Hybridization was also supported by the observation that snails homozygous for sheltered and exposed alleles mated at random with each other in both populations. On the macrogeographic scale, we found populations from exposed and sheltered sites in France and Wales being fixed for the same exposed and sheltered Ark alleles as found in Sweden. However, variation in three other highly polymorphic loci indicated geographic affinity rather than habitat similarity being the main factor of genetic coherence. These observations support a hypothesis of gene flow between exposed and sheltered populations of L. fabalis . Two Spanish populations were remarkably different with unique alleles at high frequencies in three of four strongly polymorphic loci.     

Microgeographic variation in shell characters of Littorina saxatilis Olivi—a question mainly of size?

The question of whether there are shape differences between populations of Littorina saxatilis living in different environments is examined by multivariate analyses of 13 morphological characters. Principal component analysis reveals that morphologic differences between populations from habitats with contrasting degrees of wave exposure are mainly due to a general size factor, including shell thickness. Utilizing the group structure among the snails, canonical variate analysis discloses that the main character excluding size that influences subpopulation differentiation is pointedness.     

Testing alternative models for sexual isolation in natural populations of Littorina saxatilis: indirect support for by‐product ecological speciation?

Two ecotypes of the rough periwinkle Littorina saxatilis occur at different shore levels, showing assortative mating for size and partial reproductive isolation when they meet at the mid-shore. This system represents a putative case of incomplete speciation in sympatry. Two processes contribute to the assortative mating: morph-specific microhabitat aggregation and mate choice. The estimation of mate choice coefficients in nature and a simulation of the aggregation effects on sexual isolation were used to disentangle these processes as well as to test alternative mechanisms of mate choice. Mate choice significantly increased the frequency of within-morph pairs and significantly decreased the frequency of between-morph pairs, whereas those pairs including at least one hybrid morph mated randomly. These results allow us to reject a discriminant mate choice and support a model of evolution of sexual isolation as a side-effect of size-assortative mating in a context of divergent natural selection for size in the population. This mechanism is more compatible with a model of incomplete by-product ecological speciation, as suggested by previous evidence.     

Aerobic glucose metabolism in the digestive gland of Littorina saxatilis rudis (Maton) and in the daughter sporocysts of Microphallus similis (J?g.).

Experiments, some in vitro and some in vivo, with glucose-U-14C, acetate-U-14C, succinate-1,4-14C, pyruvate-U-14C and citrate-1,5-14C indicate that the digestive gland of Littorina saxatilis rudis and the daughter sporocysts of Microphallus similis aerobically catabolise glucose via an EMP pathway, a TCA cycle, phosphoenolpyruvate carboxylation and malate decarboxylation. The host and parasite have similar transamination and synthetic reactions and alanine as a major carbohydrate excretory product. Oxidative metabolism appears to be less efficient in the parasite than in the host.