No abstract

The coiled shell of gastropods begins as a cap-shaped lens of organic and calcined material that covers the posterior dorsal side of the larva. During development the cap enlarges to cover the larval visceral mass. Marginal growth then produces the characteristic coiled shell. One model of the initiation of shell coiling in "archaeogastropods" requires that the shell remains flexible and uncalcined until after torsion, and that muscle contraction during torsion deforms the shell. We describe early shell calcification and tested this requirement of the model for the patellogastropod limpets Tectura scutum and Lottia digitalis, the trochids Calliostoma ligatum and Margarites pupillus and the abalone Haliotis kamtschatkana. We determined the stage of initial calcification by staining larvae with the fluorescent calcium marker calcein and observing them with bright field, crossed polarizing filter, and fluorescence microscopy. In T. scutum the earliest observable shell was calcified and calcium was sometimes detected even before the initial shell was visible. Larvae of the other species deposited a noncalcified matrix that was subsequently calcified, and in C. ligatum and M. pupillus this initial calcification was distinctly spotty. Shells of both patellogastropods and the abalone were demonstrably rigid prior to torsion while the shells of the trochids were not. These results suggest that shell coiling in patellogastropods and abalone is not initiated by contraction of the larval retractor muscle during torsion; in trochids this mechanism is possible. However, analysis of camera lucida drawings of pre-and post-torsional shells of T. scutum and C. ligatum did not detect shell shape changes during torsion.

Some terrestrial gastropods are able to move using two gaits: adhesive crawling, where the entire foot is separated from the substrate only by a thin layer of mucus and the snail leaves a continuous mucus trail; and loping, where regions of the foot arch above the substrate and the snail leaves a discontinuous mucus trail. Loping has been interpreted as a means of rapidly escaping predators. We found that the pulmonate Cornu aspersum moved using adhesive crawling on dry acrylic or glass substrates, but loped on dry concrete or wood. Loping snails did not move more rapidly than snails using adhesive crawling. Snails moving on concrete secreted a greater volume of pedal mucus per area of trail than those moving on acrylic; locomotion on concrete thus requires greater expenditure of mucus than does locomotion on acrylic. Because loping snails deposit a smaller area of mucus per distance traveled than do snails using adhesive crawling, loping may conserve mucus when moving on porous, absorbent substrates. Members of several other terrestrial pulmonate taxa can also lope on concrete, suggesting that this plasticity in gait is widespread among terrestrial snails.

Thigmomorphogenesis includes the effects of mechanical perturbation on plant growth. To test whether thigmomorphogenesis is evident at different scales within plants, we investigated the effect of wind on allometric relationships between specific plant parts. We chose two species from the elfin cloud forest of Puerto Rico that have contrasting growth habits, the shrub Clibadiun erosum (Asteraceae) and the palm Prestoea acuminata var. montana (Arecaceae), and subjected them to barrier-protected and wind-exposed treatments. For C. erosum, we compared the allometry of stems and branches against three allometric models that predict that plant height or branch length increases at the 1, 2/3, and 1/2 power of stem diameter. Only the geometric similarity model (scaling exponent of 1) seemed to hold when plants were exposed to the wind. We found relatively fewer leaves per number of branches produced and fewer leaves per increment of branch diameter in the plants of C. erosum exposed to the wind. Mean petiole length ratios (petiole length/basal radius) of P. acuminata were higher on leaves of barrier-protected plants for both simple and compound leaves, indicating that petioles were stouter and mechanically safer in the wind-exposed plants. We suggest that alteration of the allometric relationships of plant parts, organs, or plant modules (stems and branches of C. erosum and leaves of P. acuminata) and alteration of the number of plant parts (leaves and branches of C. erosum) are adaptive responses resulting from the mechanical perturbation induced by wind in the elfin forest.Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp.

How important is ecotypic differentiation along elevational gradients in the tropics? Reciprocal transplants of two shrubs, Clibadium erosum (Asteraceae) and Psychotria berteriana (Rubiaceae), and a palm, Prestoea acuminata var. montana (Palmaceae), were used to test for the effect of environment and population origin on growth and physiology in the Luquillo Experimental Forest of Puerto Rico. Two sites were used, one at Pic0 del Este (1000 m in cloud forest) and one at El Verde (350 m in lower montane rain forest). At the cloud forest site, plastic barriers were erected around a subset of the plants to examine if protection from wind affected survival or biomass accumulation. Survival of C. erosum and I? berteriana was not affected by site, population origin, or the presence of barriers. For I? acuminata var. montana, survival was higher for plants with barriers, but not affected by site and population origin. Plants of C. eiosum and I? berteriana at El Verde grew larger than at Pic0 del Este, but there was no effect of population origin or barrier treatment on biomass accumulation for these species. For I? acuminata var. montana, there was no effect of environment, population origin, or barrier treatment on biomass accumulation. Light-saturated photosynthetic rate (Amax) of C. erosum, I? berteriana, and I? acurninata var. montana, as well as leaf anatomical characteristics of C. erosum, were unaffected by environment, population origin, and barrier treatment. O n balance, there seems to be little evidence of ecotypic differentiation in these species along the gradient.