Distribución de los gasterópodos del manglar, Neritina virgínea (Neritidae) y Littoraria angulifera (Littorinidae) en la Ecorregión Darién, Caribe colombiano / Distribution of the mangrove gastropods Neritina virginea (Neritidae) and Littoraria angulifera (Littorinidae) within the Colombian Caribbean Darién Ecoregion

Distribution of the mangrove gastropods Neritina virginea (Neritidae) and Littoraria angulifera (Littorinidae) within the Colombian Caribbean Darién Ecoregion. Gastropods are one of the most abundant groups within the Caribbean mangroves, however, little is known about the distribution of particular species at a regional scale. With this aim, we studied the geographic distribution of Littoraria (Littorinopsis) angulifera and Neritina (Vitta) virginea within the Darién Ecoregion in the Caribbean coast of Colombia, from 77 sampling stations along 609km between the Colombian-Panamá border and Córdoba State, Colombia. The fieldwork was conducted in June-August 2009, and a total of 3 963 individuals of both species were hand-picked from the ground, prop-roots and trunks along 50m transects, and shell sizes were measured. The description of geographic patterns considered surface water salinity, mangrove cover and gastropod distribution within the Gulf of Urabá. In the outer-most part of the Gulf, L. angulifera was present in 84.8% of the stations, while N. virginea was only present in 15.2% of the stations. In this part, mangroves areas were patchily distributed, and the gastropods (mainly L. angulifera) were found on woody debris along the supralittoral zone in sandy shores. In the inner-most part, in contrast, N. virginea occurred in 84.6% of the stations, mostly in estuaries, deltas and river margins, while L. angulifera only appeared in Turbo Bay (15.4%). Mean shell size also exhibited a clear geographic pattern: size range was 6-22mm in L. angulifera, and 6-12mm in N. virginea. L. angulifera was found in open-water stations with water salinities >10PSU, but it was absent in sites with lower salinities like the Atrato River Delta and other small rivers. Its presence on coastal woody debris suggests that despite of the recruitment of small individuals from the nearshore stock of larvae, populations are unable to establish due to the absence of mangroves protection. Oppositely, N. virginea was found under estuarine conditions on mangrove roots and ground. Our results confirm that L. angulifera is an esteno-tolerant marine species, and N. virginea is an eury-tolerant estuarine species, thus their geographic distribution is strongly shaped by the large freshwater discharge of the Atrato River. We hypothesize that absence or limited distribution of gastropods in various areas of the Darién Ecoregion may be further explained by the poor conservation state of mangroves.

Aunque los gasterópodos son uno de los grupos más abundantes en los manglares del Caribe, poco se sabe del ámbito de especies particulares debido al enfoque comunitario de la mayoría de los estudios. El ámbito de las poblaciones de L. angulifera y N. virginea fue estudiado en la Ecorregión Darién, Caribe colombiano. El muestreo se realizó entre junio y agosto 2009, se recolectaron 3 963 individuos de ambas especies, a los cuales se les midió la talla de la concha. La Ecorregión presentó diferencias espaciales en el ámbito de la salinidad del agua a nivel superficial, la cobertura de manglar y el ámbito de las poblaciones. En la parte externa de la Ecorregión, L. angulifera presentó el mayor porcentaje de aparición (84.8%) en comparación a N. virginea (15.2%). En el interior N. virginea presentó el mayor porcentaje (84.6%), mientras L. angulifera apareció solo en bahía Turbo (15.4%). Se encontraron diferencias en el ámbito de tallas promedio de las conchas, donde L. angulifera varió entre 6 y 22mm y N. virginea entre 6 y 12mm dentro del Golfo. Se confirma que L. angulifera es una especie de carácter estenohalina y marina, mientras que N. virginea es eurihalina y estuarina.

Differences in predatory pressure on terrestrial snails by birds and mammals.

The evolution of shell polymorphism in terrestrial snails is a classic textbook example of the effect of natural selection in which avian and mammalian predation represents an important selective force on gene frequency. However, many questions about predation remain unclear, especially in the case of mammals. We collected 2000 specimens from eight terrestrial gastropod species to investigate the predation pressure exerted by birds and mice on snails. We found evidence of avian and mammalian predation in 26.5% and 36.8% of the shells. Both birds and mammals were selective with respect to snail species, size and morphs. Birds preferred the brown-lipped banded snail Cepaea nemoralis (L.) and mice preferred the burgundy snail Helix pomatia L. Mice avoided pink mid-banded C. nemoralis and preferred brown mid-banded morphs, which were neglected by birds. In contrast to mice, birds chose larger individuals. Significant differences in their predatory pressure can influence the evolution and maintenance of shell size and polymorphism of shell colouration in snails.

Characterization of the occupied shells by the hermit crab Clibanarius vittatus (Decapoda, Diogenidae) at Baixio Mirim tideflat, Guaratuba Bay, southern Brazil

A characterization of the occupied shells by the hermit crab Clibanarius vittatus was carried out. Hermit crabs were collected in the intertidal zone, during the low spring tide monthly from April 2005 to March 2006. They were sexed and their cephalothoracic shield length (CL) was measured. Shells were identified, dried, weighed and the aperture length (AL) and width (AW) were measured. 1187 crabs were collected (949 males, 216 females and 22 intersexes), which occupied 12 species of gastropod shells. Stramonita haemastoma, Olivancillaria urceus and Dorsanum moniliferum made up 96.55 percent of the total shell species. Male hermit crabs attained significantly larger sizes than females; therefore, males occupied a wider spectrum of shells in size and weight. A stronger correlation ratio was obtained between CL and AW of S. haemastoma. Last whorl with a rounded shape and a spacious inner area is a common feature of all shell species most frequently occupied by this hermit crab where it occurs. The successful establishment of C. vittatus at Baixio Mirim is mainly due to the appropriately shaped and wide range of size of S. haemastoma shells that were most often occupied by the hermit crabs of the studied population.

A caracterização das conchas ocupadas pelo ermitão Clibanarius vittatus (Bosc, 1802) foi realizada. Os ermitões foram coletados de abril/2005 a março/2006, na zona intertidal, durante a maré baixa de sizígia. Os ermitões tiveram o sexo reconhecido e foram medidos no comprimento do escudo ce-falotorácico. As conchas foram identificadas, pesadas depois de secas e edidas no copriento e na largura da abertura. Foram coletados 1187 ermitões (949 machos, 216 fêmeas e 22 intersexos) que ocuparam 12 espécies de conchas de gastrópodes. Stramonita haemastoma, Olivancillaria urceus e Dorsanum moniliferum perfizeram 96,55 por cento do total das conchas ocupadas. Ermitões machos atingiram tamanhos significativamente maiores do que as fêmeas; entretanto ocuparam um maior espectro de conchas em tamanho e peso. Fortes índices de correlação foram obtidos para a relação entre o escudo cefalotorácico dos ermitões e a largura da abertura de S. haemastoma. A última espira espaçosa e de formato globoso é um aspecto comum das conchas mais frequentemente ocupadas pelo presente ermitão em todos os locais de sua ocorrência. O sucesso no estabelecimento de C. vittaus no Baixio Mirim é devido principalmente ao formato e à amplitude de tamanhos das conchas de S. haemastoma, as quais são mais frequentemente ocupadas pelos ermitões da população estudada.