Masting,seed dispersal and seed predation in the cycad Macrozamia communis

Summary Little is known about the adaptive value of mast seeding, a common phenomenon in temperate trees and shrubs. Masting is likely to affect both seed dispersal and seed predation. In systems where similar taxa of animals are involved in these two processes, the consequences of mast seeding are likely to be particularly complicated. This study examined the effects of mast seeding in a cycad, Macrozamia communis, on the dispersal of seeds, the pattern of dispersion of seeds and post-dispersal predation on seeds. Dispersal of seeds by possums was poorer from source plants in a masting population than from source plants in an adjacent, non-masting population. This resulted in fewer seeds per seeding female plant in the masting plot being dispersed to favourable sites. We conclude that this is caused by the feeding behaviour and movements of possums in the masting site. The abundance of seeds in this site did not satiate the post-dispersal predators, native rats. In fact, more seeds in this site were eaten than in the nonmasting site. We suggest that the mast seeding observed in M. communis may not be adaptive, but is more likely a consequence of other factors which synchromize flowering within local populations.     

Satiation of predispersal seed predators: the importance of considering both plant and seed levels

Plants can reduce the fitness costs of granivory by satiating seed predators. The most common satiation mechanism is the production of large crops, which ensures that a proportion of the seeds survive predation. Nevertheless, satiation of small granivores at the seed level may also exist. Larger seeds would satiate more efficiently, enhancing the probability of seed survival after having been attacked. However, a larger seed size could compromise the efficiency of satiation by means of large crops if there were a negative relationship between seed size and the number of seeds produced by an individual plant. We analyze both types of satiation in the interaction between the holm oak Quercus ilex and the chestnut weevil Curculio elephas. Both crop size and acorn size differed strongly in a sample of 32 trees. Larger crop sizes satiated weevils, and higher proportions of the seeds were not attacked as crop size increased. Larger seeds also satiated weevil larvae, as a larger acorn size increased the likelihood of embryo survival. Seedling size was strongly related to acorn size and was reduced by weevil attack, but seedlings coming from large weeviled acorns were still larger. The number and the size of the acorns produced by individual trees were negatively related. Larger proportions of the crop were infested in oaks producing less numerous crops of larger acorns. However, contrary to expectations, these trees did not satiate more effectively at the seed level either. Effective satiation by larger acorns was precluded by larger multi-infestation rates associated to smaller seed crops, in such a way that the proportion of attacked seeds that survived did not vary among trees with different acorn sizes. These results highlight the need of considering satiation by means of large crops and large seeds in studies of predispersal seed predation. Long-term monitoring on individual oaks will help to assess whether there is a trade-off between the number and the size of the acorns and, if it existed, how it could condition the fitness consequences of both types of satiation.     

Arthropods in tropical oaks: differences in their spatial distributions within tree crowns

The arthropod activity in the upper and the lower zone of nine emergent oak trees (genus Quercus) of a submontane forest in Sabah, Borneo, Malaysia, was investigated by using flight interception traps, yellow colour traps, and arboreal pitfall traps. Number of arthropods did not differ significantly between the upper and the lower region of the tree crowns. Nevertheless, the results reveal significant differences in the abundances of several arthropod groups between the two zones. In the lower part of the oak crowns Homoptera and ants (mostly alates) were sampled in higher numbers, whereas the upper zone was dominated by Diptera and Hymenoptera (ants excluded). Beetles and most other groups showed no significant differences in abundance between the lower and upper zone of the oak trees. Thus, the arthropods of the investigated community are not evenly distributed. The arthropod community in tree crowns, which often has been treated as a homogenous unit, is in fact divided into subunits formed by differently composed arthropod assemblages.     

Effect of beetle size on predation of cabbage root fly eggs by ground beetles

Laboratory studies were done to determine the numbers of cabbage root fly (Delia radicum L.) (Diptera: Anthomyiidae) eggs eaten by sixty species of ground beetles (Coleoptera: Carabidae) collected from a range of habitats. For 35 species that were between 2.7 mm and 10 mm long, there was a linear relationship between the numbers of eggs eaten and beetle length. For each 1 mm increase in length above 2.7 mm, an additional 18 eggs were eaten/beetle/day. Eight further species within the above size range did not eat any fly eggs, as these species are known to be either phytophagous or to feed on moving prey. Large numbers of eggs were eaten by beetles in 13 of the 25 genera tested. Based on size, maximum numbers of eggs were eaten by 7 species of Agonum Bonelli, 5 species of Amara Bonelli and seven species of Bembidion Latreille. Only one of the 6 species of Pterostichus Bonelli tested, P. strenuus (Panz.), ate the expected numbers of eggs, the other species being too large to feed from such small prey items as cabbage root fly eggs. The numbers of beetles larger than 10 mm that ate eggs was highly variable and so 10 mm was considered the upper size limit of carabid predators of cabbage root fly eggs.     

Pollen grain morphology supports the taxonomical discrimination of Mediterranean oaks (Quercus,Fagaceae)

The exine architecture and some quantitative parameters (polar and equatorial axes and their ratio) of pollen grains allow a clear discrimination between groups of Mediterranean oak species. For the differentiation of species within groups only the quoted parameters prove useful.     

Effects of seed mass on seedling height and competition in European white oaks

The relationship between seed size and fitness in plants may depend on offspring density, especially in cases where seed size affects the outcome of competition. We investigated the relationship between seed mass, germination, intraspecific competition and seedling height in a glasshouse experiment on three European white oak species (Quercus robur, Q. petraea, Q. pubescens). Within offspring families, seed mass showed a moderate, but statistically significant effect on seedling height, i.e. seedlings from heavier seeds were slightly taller. In contrast, competition caused pronounced inequality in seedling height in pairs of competing seedlings, but in only 55.2% of all pairs the dominant competitor arose from the heavier seed. It is thus possible that a positive effect of seed mass on seedling growth can be mediated through the density of conspecific seedlings and that heterogeneity in offspring density will contribute to the maintenance of seed mass variation in oak populations.     

Acorn mast drives long-term dynamics of rodent and songbird populations

Resource pulses can have cascading effects on the dynamics of multiple trophic levels. Acorn mast is a pulsed resource in oak-dominated forests that has significant direct effects on acorn predators and indirect effects on their predators, prey, and pathogens. We evaluated changes in acorn mast, rodent abundance, raptor abundance, and reproductive success of a ground-nesting songbird over a 24-year period (1980–2004) in the southern Appalachian Mountains in an effort to determine the relationships among the four trophic levels. In particular, we examined the following: acorn mast from red oaks (Quercus rubra) and white oaks (Q. alba), abundance of white-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus), population estimates of seven raptor species from three feeding guilds, and nest failure and number of juveniles of dark-eyed juncos (Junco hyemalis). Finally, we recorded seasonal temperature and precipitation to determine the effects of weather on each trophic level. We found that weather patterns had delayed effects of up to 3 years on these trophic interactions. Variation in acorn mast, the keystone resource in this community, was explained by weather conditions as far back as 2 years before the mast event. Acorn mast, in turn, was a strongly positive predictor of rodent abundance the following year, whereas spring and summer temperature and raptor abundance negatively affected rodent abundance. Dark-eyed junco nests were more likely to fail in years in which there were more rodents and raptors. Nest failure rate was a strong predictor of the number of juvenile juncos caught at the end of the summer. Our results improve our understanding of the complex ecological interactions in oak-dominated forests by illustrating the importance of abiotic and biotic factors at different trophic levels. Ethan D. Clotfelter and Amy B. Pedersen contributed equally to the writing of this paper.     

Present and future extension of the Iberian submediterranean territories as determined from the distribution of marcescent oaks

The present work proposes new boundaries for the current submediterranean territories of the Iberian Peninsula, defining them at the smallest scale attempted to date. The boundaries proposed are not sharp divisions but somewhat ‘gradual’, reflecting the transitional nature of the territories they encompass. Climate change predictions were used to estimate how the distribution of these submediterranean regions might change in the near future. The maps constructed are based on the distribution of marcescent Quercus species—trees that characterise the submediterranean plant landscape where they form the main forest communities. To determine their climatic range, the distribution of different types of Iberian oak forest was represented in ‘climate diagrams’ (ordination diagrams derived from principal components analysis), both in terms of individual species and groups of species based on leaf ecophysiological type, i.e. marcescent (Submediterranean), sclerophyllous (Mediterranean), semideciduous (Mediterranean) and deciduous (Eurosiberian). The climate range of each type of forest was determined, and the means of representative climate variables are analysed by one way ANOVA. The variables differentiating the forest groups were also examined by discriminant analysis. The range of the climate variables found to be associated with the majority of marcescent forests was used to determine the distribution of territories throughout the Peninsula with the same conditions (i.e. whether marcescent forests were present or not), thus providing a map of the Iberian submediterranean territories. Predictions of climate change were used to investigate possible climate-induced modifications in the boundaries of these territories in the near future. The patterns obtained show dramatic reductions in the extension of the Iberian submediterranean environment. Submediterranean conditions will probably disappear from the areas where they currently reign, and it seems unlikely that any new, large submediterranean areas will form by displacement towards higher altitudes. The outlook for the unique submediterranean vegetation of the Iberian Peninsula is gloomy.

内蒙古典型草原区狭颅田鼠集群数量与被捕食风险的关系

2005年9月15日—30日,在内蒙古锡林郭勒盟西乌珠穆沁旗的典型草原上研究了狭颅田鼠(Microtus gregalis)冬季集群与来自艾虎(Mustela eversmanni)的捕食风险。采用鼠洞口数量作为狭颅田鼠集群大小的指标,分析了艾虎对不同大小狭颅田鼠集群的捕食优先选择偏好。对实验样地内狭颅田鼠的洞群洞口数进行了调查统计(样地内总共涉及102个狭颅田鼠有效洞群),并计数此期间的艾虎粪便堆数以及攻击掘开狭颅田鼠洞群的情况。运用非参数的Mann-Whitney U检验法进行数据分析,结果表明:从艾虎遗留的粪便痕迹来看,没有被艾虎访问过的田鼠洞群与被艾虎访问过的洞群之间差异达到极显著水平;另外,从狭颅田鼠洞口被艾虎掘开的痕迹来分析,没有被艾虎挖掘的洞群与被艾虎挖掘的洞群之间的差异也达到极显著的水平;另外,随着田鼠洞群洞口数量的增加,出现艾虎粪便和掘洞的频次和概率就越高。表明艾虎对狭颅田鼠集群洞口数的选择差异性非常显著,明显倾向于选择在洞口数量多的狭颅田鼠洞群停留,同时也更倾向于掘开洞口数量较高的狭颅田鼠洞群作为捕食对象。     

Density and size distribution of Daphnia populations at different fish predation levels

Density and size structure changes of natural daphniids populations were studied in enclosures with a different level of fish predation. Daphnia pulicaria was totaly removed in high predation variants, and the differences of the mean body length both in adults and juveniles are apparent between low predation enclosure and enclosure without fish. Daphnia galeata was replaced by D. magna and D. pulicaria in the enclosure without fish. The decline of densities and the substantial (30–50%) and fast (during 12 days) shift of mean body length both in adults and juveniles of D. galeata was induced by the fish (carp fry) introduction to the high predation enclosures.     

Hierarchical pre-dispersal fitness assessment in a Mediterranean shrub plant

Under the concept of modularity, it is possible to recognise how seed production, as well as any other process affecting it, are hierarchically structured within fruits, within individual plants and within populations. In this work, we analysed the effects of pre-dispersal seed predation by insects upon a set of hierarchical levels in a population of the Mediterranean shrub plant Cistus ladanifer (“rock rose”) throughout a complete fruit-producing season (which takes place during the summer months). Almost all individual plants were predated, which implies that the effects of predation at the population level (regardless of the extent of predation within each individual) were virtually uniform. Within the individuals, however, the predation rate was close to a proportion of 0.5 (half of the fruits of each individual were predated), which indicates that this hierarchical level is likely to be subjected to a differential action of selection. Predation rates within the fruits showed an intermediate value (lower than that observed at the population level but higher than that at the individual level). According to these results, the pressure of phenotypic selection may therefore give rise to greater variation among fruits of the same individual than among seeds of the same fruit. In terms of the temporal patterns observed there was a large variation in the increments of predation along the fruiting season, which implies a high degree of heterogeneity in the temporal distribution of the effects of predation pressure on fitness. Besides its use in the specific example of the plant species studied in this work, the methodological procedure presented in this paper (integration of the temporal changes of different hierarchical levels) might be foreseen, in fact, as a useful tool for analysing the hierarchical structuring of fitness in modular organisms in general. This procedure allows to discriminate and integrate selection pressures and their effects across different phenotypic levels, from the infra-individual ones up to the population level.     

Anatomical changes and evolutionary trends in the foliar epidermis of extant and fossil Euro-Mediterranean oaks (Fagaceae)

The differentiation of foliar epidermis structures, cuticles, stomata, and indumentum of most of the extant Euro-Mediterranean taxa ofQuercus have been studied by light and scanning electron microscopy, and compared with other genera and fossil material. Results allow the recognition of the subgg.Sclerophyllodrys, Cerris andQuercus, and of various sections and even individual species. The cyclocytic stomata type, regarded as plesiomorphic, is found in the geologically oldest representatives of the familiy (Dryophyllum, resp.Eotrigonobalanus) and in the relatively old, evergreen to semi-evergreen species ofQuercus subgg.Sclerophyllodrys andCerris. These taxa also have relatively thick and smooth cuticles. Anomocytic stomata, thinner cuticles and the presence of crystalline wax flakes are regarded as apomorphic features and characterize the deciduous taxa of subg.Quercus. These changes are interpreted as ecological adaptations to the changing climates from the Tertiary to the present. An analysis of the various character combinations found in macrofossils and the often poor species separation inQuercus suggest that hybridization has played a major role in the evolution of the Euro-Mediterranean oaks.     

Early season cuticular conductance and gas exchange in two oaks near the western edge of their range

Seasonal changes in minimum leaf conductance to water vapor (g_min), an estimate of cuticular conductance, and photosynthetic gas exchange in two co-occurring oak species in north-east Kansas (USA) were examined to determine if leaf gas exchange characteristics correlated with differences in tree distribution. Bur oak (Quercus macrocarpa Michx.) is more abundant in mesic gallery forest sites, whereas chinquapin oak (Quercus muehlenbergii Englm.) is more abundant in xeric sites. Early, during leaf expansion, g_min was significantly lower in chinquapin oak than in bur oak, though midday water potentials were similar. After leaves had fully expanded, g_min decreased to seasonal minimum values of 4.57 (±0.274) mmol m^-2 s^-1 in bur oak, and 2.66 (±0.156) mmol m^-2 s^-1 in chinquapin oak. Water potentials at these times were significantly higher in chinquapin oak. As leaves were expanding, photosynthesis (A_net) was significantly higher in chinquapin oak than in bur oak. Later in the growing season, A_net and gleaf increased dramatically in both species, and were significantly higher in bur oak relative to chinquapin oak. We concluded that bur and chinquapin oak have a number of leaf gas exchange characteristics that minimize seasonal water loss. These characteristics are distinct from trees from more mesic sites, and are consistent with the distribution patterns of these trees in tall-grass prairie gallery forests.     

Clonal and genetic structure of two Mexican oaks: Quercus eduardii and Quercus potosina (Fagaceae)

Quercus eduardii and Q. potosina are dominant oak species in Sierra Fría, Aguascalientes, Mexico. These species have been exploited for multiple purposes since the 16th century. Both species produce clonal offspring through root suckering and acorns through sexual reproduction. To understand clonality for the implementation of the most adequate actions for the conservation of these species, we addressed the following questions: (a) what is the spatial clonal structure of both species? (b) How much clonal and genetic diversity is maintained in these species? Random Amplified Polymorphic DNAs (RAPDs) were used as molecular markers for these analyses. Genets of both species have few ramets and these grow close the parent tree. Autocorrelation analyses at the ramet level showed an aggregated distribution at short distances and a random spatial distribution at larger distances. Also, at the genet level the autocorrelation analyses showed a random distribution. Clonal diversity was high in both species (Q. eduardii: D=0.963, G/N=0.60; Q. potosina: D=0.985, G/N=0.65). Genetic diversity was high within populations (Q. eduardii: H _e =0.33±0.11; Q. potosina: H _e =0.35±0.11). Low levels of genetic differentiation among populations were observed (Q. eduardii ? _st =0.19, P < 0.002; Q. potosina ? _st =0.13, P < 0.002). Both species maintain high levels of clonal and genetic diversity, probably due to successful sexual reproduction, which allows gene flow among populations. Conservation and/or reforestation programs must include seed collections and germplasm banks. Due to the small genet size and the high clonal diversity of these species, seeds can be collected in any place in Sierra Fría, Aguascalientes.     

Cucurbitacins and predation of the spotted cucumber beetle, Diabrotica undecimpunctata howardi

Two hypotheses about the ralationship of diabroticina beetles and plants in the family Cucurbitaceae are tested: (1) evolution of sensory receptors for cucurbitacins by diabroticina beetles was in part due to the predator protection offered by ingestion of these compounds, and (2) commercial varieties of cucurbitacin-producing cucumber offer Diabrotica undecimpunctata howardi Barber chemical protection from some potential sources of natural controlSpotted cucumber beetles fed either Marketmore 70 cucumber which contains cucurbitacin-C or Marketmore 72 which totally lacks cucurbitacin were presented to four species of vertebrate predators that commonly occur in the summer and/or winter habitats of D. u. howardi: Bufo americanus, B. fowleri, Peromyscus maniculatus, Colinus virginianus. None of the four species were significantly deterred from preying on beetles that had eaten Marketmore 70 cucumber. These results do not support either of the two hypotheses. The limitations of these negative results as evidence for refutation of the first hypothesis are discussed.

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Résumé Deux hypothèses concernant les relations entre les Diabroticina et les Cucurbitaceae ont été examinées: (1) l'évolution chez les Diabroticina des récepteurs pour les Cucurbitacines est due en partie à la protection contre les prédateurs apportée par la consommation de ces produits (2) les variétés modernes de concombres produisant de la cucurbitacine fournissent à D. undecimpunctata howardi Barber une protection chimique contre quelques sources potentielles de lutte biologique.Des D. undecimpunctata howardi, ayant consommé soit des concombres Marketmore 70 contenant de la cucurbitacine C, soit des concombres Marketmore 72 qui ont totalement perdu leur cucurbitacine, ont été présentés à quatre espèces de vertébrés prédateurs, qui s'observent fréquemment dans les habitats d'été ou d'hiver de D. undecimpunctata howardi soient Bufo americanus, B. fowleri, Peromyscus maniculatus, Colinus virginianus. Aucune de ces quatre espèces n'est significativement dissuadée d'attaquer les coléoptères qui on consommé les concombres Marketmore 70. Ces résultats n'étayent aucune des deux hypothèses. La discussion porte sur les limites de ces résultats négatifs comme preuve de la réfutation de l'hypothèse 1.

     

Allometric allocation in fruit and seed packaging conditions the conflict among selective pressures on seed size

Selective pressures on seed size could vary among the different stages of plant life cycles, so no simple relation could explain a priori its evolution. Here, we determined the relationships between seed size and two fitness components—seed dispersal and survival from predation—in a bird-dispersed tree, Crataegus monogyna. We interpret these relationships in relation to the patterns of mass allocation to fruit and seed components. Selection patterns were assessed at two levels (1) selection pressures on the parent tree; comparing seed dispersal efficiency among individual plants and (2) selection pressures at the individual seed level; comparing seed size variation (i) before and after dispersal, and (ii) before and after postdispersal seed predation. Dispersal efficiency (percentage of seed crop dispersed) was positively correlated with fruit mass and fruit width. Differences in crop size did not offset this effect, and larger seeds were overrepresented in the seed rain relative to the seed pool before dispersal. However, the advantage of larger seeds during the dispersal stage was cancelled later by an opposite selection pressure exerted by seed predators. As a result, smaller seeds had a higher probability of surviving postdispersal seed predation, establishing an evolutionary conflict imposed by the need for dispersal and the danger of being predated. Birds and rodents preferentially selected highly profitable fruits and seeds in terms of the relative proportion of their components. Larger fruits had a higher pulp to seed proportion than smaller ones, and all seeds had the same proportion of coat relative to the embryo-plus-endosperm fraction. Hence, although predator pressures were stronger than disperser ones, larger seeds invested proportionally less in structural defense than in dispersal.     

Early seedling growth of pine (Pinus densiflora) and oaks (Quercus serrata,Q. mongolica,Q. variabilis) in response to light intensity and soil moisture

In climatic chambers seed germination and seedling growth of Pinus densiflora Sieb. et Zucc., Quercus serrata Thunb., Quercus mongolica Fisch. ex Turcz. and Quercus variablilis Bl. were investigated as functions of light intensity and soil moisture. In Korea these tree species occur widely and form mixed forests with different distributions. Species clearly differed in the pattern of germination and early seedling growth between light and soil treatments. The germination of pine did not differ between the experimental treatments until the breaking of the primary buds. After that, light intensity was the deciding factor for further development. In the most moist treatment (approx. field capacity) growth of the pine seedlings was strongly inhibited. For the three oak species, differences between experimental treatments first occurred after complete formation of primary leaves. Seed development strongly correlated with the weight of the acorn. The large seeded Q. variabilis (acorns with mean weight of 4.7 g) developed faster and reached larger dimensions towards the end of the experiment than Q. mongolica (2.8 g per acorn) and Q. serrata (0.9 g per acorn). Regarding height and biomass growth, the oak species showed a higher shade tolerance than pine. The proleptic shoot growth was clearly influenced by the light intensity. Root formation was favoured by a high exposure to light. In case of the oak species reduction of soil moisture increased the length of primary roots and the number of secondary roots.     

Evolutionary ecology of mast-seeding in temperate and tropical oaks (Quercus spp.)

Mast-seeding is the synchronous production of large seed crops within a population or community of species every two or more years. This paper addresses three non-mutually exclusive hypotheses explaining the evolution of mast-seeding in temperate tree species, especially the genus Quercus: (1) mast-seeding is a consequence of mast-flowering which evolves to increased pollination efficiency in mast-flowering years; (2) mast-seeding has evolved as an anti-predator adaptation by which large seed crops during mast years satiate the seed predators and allow survival of some of the seeds; (3) selection on seed size by habitat can indirectly affect the evolution of masting if trees with large seeds require more time to accumulate reserves to mature those seeds. I find support for the pollination hypothesis in several wind-pollinated temperate tree species but not oaks. However, oaks show evidence favoring the predation and seed size hypotheses. I then develop a model to illustrate the relationships among the three hypotheses in their effects on the evolution of masting. Finally, using data from herbaria and Floras, the influence of selection via flowering, fruiting, and seed size in the evolution of masting in tropical oaks is discussed. I conclude that the need for a supra-annual cue to synchronize flowering and fruiting as well as the larger seed size found in many tropical oak species should contribute to the evolution of masting to a greater extent than seen among temperate oaks.     

Spatial and temporal patterns of granivorous ant seed predation in patchy cereal crop areas of central Spain

Summary Patterns of granivorous ant seed predation in extensive cereal croplands of central Spain were investigated by measuring seed removal rates on artificial seed patches. Sampling was designed to cope with the seasonal and daily foraging cycle of ant colonies. Simultaneously with removal rates, I measured seed availability, habitat physiognomy at two spatial scales (landscape and microhabitat), weather variables (temperature and rainfall), and distance to the nearest ant nest. Ant seed predation was concentrated on shrublands, and associated with places with high covers of shrubs, chamaephytes and stones. These results were in close agreement with those obtained by analyzing the spatial distribution of granivorous ant nests (Díaz 1991). Moreover, there was a close relationship between seed removal rates and distance to the nearest ant nest, that fitted the predictions of the optimal foraging model developed by Reyes-López (1987). Seasonal and daily patterns of ant foraging activity seemed to depend more on endogenous factors than on environmental variation. I conclude that ants were not able to track the spatial and temporal variation of their food resources in these man-modified habitats, so that their potential to interact with other members of the granivore system is greatly reduced by human activities.     

Geographical variation in the breeding cycles and recruitment of Patella spp.

The effect of geographical differences in breeding cycles on the recruitment variation of the limpets — Patella vulgata L., P. aspera Röding and (to a limited extent) P. depressa Pennant — has been studied over much of their range in Britain and NW. Europe. In spite of considerable annual and local variation in recruitment success, broader patterns can be distinguished, which can be linked to spawning times and factors affecting the survival of newly-settled spat. The breeding cycles of P. vulgata and P. aspera differ across their ranges in that, in both species, spawning begins, and gametogenesis ends, earlier in the north and east than in the south and west. The cause of these differences can be correlated with geographical and annual differences in sea temperature over the potential breeding periods, and can be related to the regional incidence of conditions found experimentally to be necessary for successful settlement and survival of spat during a critical stage of their growth. The significance of this temperature window in determining the littoral and geographical distribution of the species is discussed.