The aim of this study was to analyse the tolerance of the planktonic stages of Psammechinus miliaris to hydrostatic pressure and temperature. Embryos of Psammechinus miliaris were subjected to different combinations of pressure and temperature for 3, 6 and 12 h. The percentage of embryos at each stage and the percentage of embryos developing abnormally were measured. Larvae at the gastrula and prism stages were subjected to pressure and temperature combinations for 24 h and the larval survival was calculated measuring the percentage of swimming larvae. Both embryos and larvae could survive at greater pressures than the known adult depth limits. Larvae showed a much greater potential than embryos for surviving deeper, with approximately 100% of both gastrulae and prisms surviving up to 200 atm at 5°C. These results are similar to other shallow-water species of Echinoida. Thus larval tolerance of high pressure and low temperature may have been important for the success of this group in colonizing the deep-sea.
Estuaries often show clearly recognizable changes in the distribution of organisms along environmental gradients from riverine to fully marine conditions. Surveys performed along the horizontal axis of the Plym and the Yealm Estuaries identified patterns of distribution and abundance of intertidal barnacles and provided a new assessment on the dominance exhibited by the non-native species Elminius modestus in these estuaries. Elminius modestus occurred furthest up in estuaries and was dominant along most of their length, with the exception of few sites closest to the sea; Chthamalus montagui had the most restricted degree of penetration up-estuary; and Semibalanus balanoides occurred at low abundances, with limits of penetration located between those of C. montagui and E. modestus. At many sites, E. modestus was the only barnacle species found. There were changes in the relative abundances of these three species in several particular locations within the Plym and the Yealm in comparison to previous accounts made in the last decades, which, in most cases led to increased dominance of E. modestus. This was mainly due to reductions in the abundances of S. balanoides. Physico-chemical conditions experienced after settlement, especially deposition of silt, exposure and salinity regime contribute to the patterns described here.
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