Yamamori L and Kato M (2018), Morphological and ecological adaptation of limp...

ECB-ART-46554

PLoS One 2018 Aug 01;138:e0197719. doi: 10.1371/journal.pone.0197719.

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Morphological and ecological adaptation of limpet-shaped top shells (Gastropoda: Trochidae: Fossarininae) to wave-swept rock reef habitats.

Yamamori L , Kato M .

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Flattening of coiled shells has occurred in several gastropod lineages, while the evolutionary process of shell flattening is little known. The subfamily Fossarininae of the top shell family (Trochidae) is unique, because it includes four genera at various stages of shell flattening. Broderipia and Roya, have zygomorphic shells that has lost coiling, while the sister genera, Fossarina and Synaptocochlea, have respectively turbiniform and auriform shells. Therefore, comparisons of biology, habitats and detailed morphology among these four genera may help us to detect selection pressure driving shell flattening and loss of coiling. Although Broderipia has recently been identified as living symbiotically in the pits of sea urchins, the habitats and biology of the other three Fossarininae species, especially Roya are poorly known. After an extensive search on rocky shores of the Japanese Archipelago, we found live Roya eximia snails on intertidal/subtidal rock surfaces exposed to strong waves. Roya snails crept on the bare rock surface to graze periphyton at low tide, and fled into vacant barnacle shells at high tide. Comparison of the morphology of soft bodies in Fossarininae revealed that the columellar muscle of flattened species has been drastically elongated and arranged in posterior semi-outer edge of the flattened shell as observed in true limpets. The flattering and loss of coiling of the shell in Roya caused acquisition of a zygomorphic flat body, retraction of coiled visceral mass, and expansion of the foot sole. All of these changes improved tolerance against strong waves and the ability to cling to rock surfaces, and thus enabled a lifestyle utilizing both wave-swept rock surfaces and the inside of vacant barnacle shells.

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Species referenced: Echinodermata
Genes referenced: fhl2 LOC100887844 LOC576114 LOC583082 ROCK snai2

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References [+] :

Palmer, FISH PREDATION AND THE EVOLUTION OF GASTROPOD SHELL SCULPTURE: EXPERIMENTAL AND GEOGRAPHIC EVIDENCE. 1979, Pubmed

	Fig 1. The phylogenetic tree of subfamily Fossarininae (Williams et al. 2010, modified).
	Fig 2. Locations of the study sites.A: Toshima in Shirahama, Wakayama Prefecture, B: Muroto Cape, C: Goshikihama, D: Chihiro Cape, E: Kashiwajima Island in Kochi Prefecture.
	Fig 3. Study sites and habitats of snail species belonging to Fossarininae.a: Exposed reef at site A; arrowhead shows the tidal level inhabited by R. eximia. b: Protected reef at site A. c: Exposed reef at site E; arrowhead shows the rock inhabited by R. eximia. d: Protected reef at site E.
	Fig 4. Mean coverage (cm2) of various algae and sessile organisms in 10 × 10 cm quadrats (n = 10) set on exposed and protected rock surfaces at sites A and E. ER: encrusting Rhodophyta, BR: branching Rhodophyta BO: branching Ochrophyta. S: sessile invertebrates, bare: bare surface.
	Fig 5. Species richness and density of the macrobenthos in 10 × 10 cm quadrats (n = 10) set on exposed and protected rock surfaces at sites A and E.
	Fig 6. Microhabitats of Fossarininae snails.a−c: F. picta in rock crevices of protected (a) and exposed (b) rock reefs, and in vacant shells of the barnacle Megabalanus volcano on exposed reefs (c); sandy sediments are sometimes accumulated in crevices, as shown in a. d−e: S. pulchella in a rock crevice. f−g: B. iridescens in sea urchin pits. h−i: R. eximia found on wave-swept rock surfaces at low tide (h), and in vacant shells of the barnacle M. volcano at high tide (i).
	Fig 7. Diurnal changes of tidal level (wavy line) and observed number of R. eximia snails (solid columns).The habitat of the snail in reference to the tidal level is illustrated as a gray band. Roya snails were observed on wet rock surfaces at low tide, while they were found inside vacant barnacle shells on submerged rock surfaces at high tide.
	Fig 8. Side and ventral views of living snails and shells of four Fossarininae species, and opercula of F picta and S. pulchella.F picta (a, e, i, m, q); S. pulchella (b, f, j, n, r); B. iridescens (c, g, k, o); R. eximia (d, h, l, p). Scale bar = 1 mm.
	Fig 9. Dorsal views and schematic drawings of soft bodies of four Fossarininae species.F. picta (a, e, i); S. pulchella (b, f); B. iridescens (c, g); R. eximia (d, h). Columellar muscles in schematic drawings were colored in yellow. cm columellar muscle, ct cephalic tentacle, ept epipodial tentacle, e eye, ft foot, g gill, h heart, ki kidney, nl neck lobe, ov ovary, sn snout, st stomach. Scale bar = 1 mm.
	Fig 10. Views of the whole radulae (left column) and close-up views of the central and marginal teeth (middle and right columns) of four Fossarininae species. F. picta (a–c); S. pulchella (d–f); B. iridescens (g–i); R. eximia (j–l). Scale bar = 30 m.
	Fig 11. A cladogram of subfamily Fossarininae with the information acquired from this study.