Ontogeny of shell morphology and shell strength of the marine snails Littorina obtusata and Littorina mariae: different defence strategies in a pair of sympatric, sibling species

Abstract

The sibling marine snails Littorina obtusata (L.) and Littorina mariae Sacchi & Rastelli are sympatrically distributed and the shells of both species are subject to similar breaking forces by predatory crabs. Nevertheless, the two species exhibit rather different growth and defence strategies. To determine growth patterns, we measured changes in five morphological variables with increasing shell length: body whorl thickness at the point of crushing force application, shell height (related to globosity), shell mass, body mass, and apertural lip thickness. We also measured ontogenetic changes in the ability to withstand shell crushing. For most morphological variables, L. mariae showed uniformly allometric growth of juveniles into adults. In contrast, L. obtusata usually exhibited a distinct change in growth pattern upon reaching maturity. As adults, L. mariae showed a more sustained increase in overall shell mass and in body whorl thickness (defence against crushing attacks) and also had proportionally thicker apertural lips (defence against peeling attacks). Littorina obtusata, however, grew to a larger size and their shells could accommodate larger bodies at all sizes. Furthermore, the strength of L. obtusata shells increased faster than could be accounted for by either overall shell mass or thickness at the point of force application, suggesting strengthening by other means such as changes in shell microstructure or shape (other than globosity). These results illustrate the viability of two contrasting antipredator strategies, despite a highly similar phylogenetic history and selective regime.

Correlates of snail shell variation along a unidirectional freshwater gradient in Lithasia geniculata (Haldeman, 1840) (Caenogastropoda: Pleuroceridae) from the Duck River, Tennessee, USA

“…Additionally, shell shape, mass, and crushing force in L. geniculata increased with discharge rate. Increased shell mass correlates with shell thickness, and both are defences against shell-crushing predators (seely 1986, lowell et al 1994, edGell & neuFeld 2008) which tend to occupy deeper and faster-flowing river reaches. Resistance to crushing in L. geniculata was significantly correlated to shell mass and shape, but not size.…”

Quantifying phenotypic gradients in freshwater snails: a case study in Lithasia (Gastropoda: Pleuroceridae)

“…Similar responses due to shape have been suggested in fish body morphology (Vogel, 1994), marine gastropod shells (Trussell et al, 1993), and other freshwater snails (Vermeij & Covich, 1978), where individuals possessing lower drag profiles occur more frequently in deeper and/or faster flowing water. Sculpture has also been shown to reduce predation by increasing handling time by predators and providing physical defense (Bertness & Cunningham, 1981;Lowell et al, 1994;Trussell & Smith, 2000). Since predator diversity may increase with distance from the headwaters (e.g., fishes, Harrell et al, 1967;Penczak & Mann, 1990;Kouamélan et al, 2003), sculptured shells may provide a single defense against numerous predators.…”

Ontogeny of a symbiont-produced chemical defense in Bugula neritina (Bryozoa)

“…Lagodon rhomboides), the simultaneous increase in palatability and reduction in bryostatin concentration supports the hypothesis that juvenile and adult B. neritina relies less on chemical defense, which appears to be the only defensive strategy at the larval stage. Ontogenetic changes in defense, both morphological and chemical, have been demonstrated in other marine invertebrates (Lowell et al 1994, Cowart et al 2002, and may be a mechanism to reduce the cost of defense when it is not needed.…”