External and internal shell formation in the ramshorn snail Marisa cornuarietis are extremes in a continuum of gradual variation in development

Abstract

Background: Toxic substances like heavy metals can inhibit and disrupt the normal embryonic development of organisms. Exposure to platinum during embryogenesis has been shown to lead to a "one fell swoop" internalization of the shell in the ramshorn snail Marisa cornuarietis, an event which has been discussed to be possibly indicative of processes in evolution which may result in dramatic changes in body plans.

Results: Whereas at usual cultivation temperature, 26°C, platinum inhibits the growth of both shell gland and mantle edge during embryogenesis leading to an internalization of the mantle and, thus, also of the shell, higher temperatures induce a re-start of the differential growth of the mantle edge and the shell gland after a period of inactivity. Here, developing embryos exhibit a broad spectrum of shell forms: in some individuals only the ventral part of the visceral sac is covered while others develop almost "normal" shells. Histological studies and scanning electron microscopy images revealed platinum to inhibit the differential growth of the shell gland and the mantle edge, and elevated temperature (28 - 30°C) to mitigate this platinum effect with varying efficiency.

Conclusion: We could show that the formation of internal, external, and intermediate shells is realized within the continuum of a developmental gradient defined by the degree of differential growth of the embryonic mantle edge and shell gland. The artificially induced internal and intermediate shells are first external and then partly internalized, similar to internal shells found in other molluscan groups.

Figure 1

Marisa cornuarietis individuals after hatching with different types of shells. Arrows indicate the mantle edge, “partial” shells are encircled in white; A: “sluggish” snail without an external mantle and with an internal shell (about 3 weeks old); B, C: snails with “partial” shells, D: snail with an almost normally developed “partial” shell; E: normally developed snail from the control; H: head, SH: shell, VS: visceral sac.

Figure 2

Histological sections of Pt-exposed embryos and respective sketches explaining derection of tissue growth. Arrows indicate directional growth of a tissue, X indicates stop of growth of the respective tissue; black: mantle tissue, red: shell gland, yellow: shell; A: 3-day-old embryo, transverse section through the visceral sac; B: sketch of Figure 2A, frontal view; C: 7-day-old embryo, sagittal section; D: sketch of Figure 2C, left lateral view; E: 9-day-old embryo, transverse section through the visceral sac; F: sketch of Figure 2E, left lateral view; F: foot; L: lobe; LS: larval stomach; MTE: mantle edge; MTG: mantle gap; SH: shell; SHG: shell gland; VS: visceral sac.

Figure 3

Sagittal sections of adult M. cornuarietis. Asterisks indicate corresponding structures; A: snail from the control, six weeks after oviposition, right lateral view; B: Pt-exposed snail, several months old, left lateral view; A: alimentary tract; CML: columellar muscle; CN: ctenidium; DG: digestive gland; F: foot; K: kidney; MTC: mantle cavity; MTE: mantle edge; MTG: mantle gap; O: odontophor; PG: pedal ganglion; SHG: shell gland.

Figure 4

SEM-images of M. cornuarietis embryos from the same clutch (A-E) exposed to platinum and elevated temperature at different stages of development. A: seven-day-old embryo resembling a stage of development present also in “sluggish” individuals as described in Marschner et al. [3], right lateral view; B: eight-day-old embryo, right lateral view; C: nine-day-old embryo, right lateral view; D: ten-day-old embryo, left lateral view; E: eleven-day-old embryo, right lateral view; F: nine-day-old embryo from a different clutch, left lateral view; CN: ctenidium; F: foot; H: head; MTE: mantle edge; OP: operculum; SH: shell; TN: tentacles; VS: visceral sac.

Figure 5

Images of platinum-plus-heat-exposed embryos. Left lateral views, circles highlight the portion of calcium carbonate that was secreted before the re-start of the differential growth of the shell gland-mantle edge-complex; A: 7-day-old embryo; B: 10-day-old embryo; CN: ctenidium; F: foot; H: head; MTE: mantle edge; SH: shell; VS: visceral sac.

Figure 6

Histological sections of M. cornuarietis embryos. Asterisks indicate corresponding structures; A: Sagittal section of a cephalopodium of a control snail, 12 days after oviposition, left lateral view; B: Sagittal section of a platinum-plus-heat-exposed snail, 12 days after oviposition, right lateral view; C: Sagittal section of a platinum-plus-heat-exposed snail, 17 days after oviposition, right lateral view; D: Horizontal section of a platinum-plus-heat-exposed snail, 17 days after oviposition; E: same individuum as in D, section more dorsal; F: Sagittal section of a platinum-plus-heat-exposed snail, 12 days after oviposition; CML: columellar muscle; CN: ctenidium; DG: digestive gland; E: eye; F: foot; H: head; MTC: mantle cavity; MTE: mantle edge; MTG: mantle gap; O: odontophor; SHG: shell gland.

Figure 7

Sketches of three different developmental stages (aged 3 (frontal view) and 7 days (left lateral view) and after completion of embryonic development (left lateral view)) of normal (top row), “partly-shelled” (middle row) and “sluggish” individuals (bottom row). Black: mantle tissue; red: shell gland; yellow: shell, direction of growth is indicated by an arrow, arrest of growth is indicated by X; F: foot; VS: visceral sac.

Figure 8

Differential growth in “partly-shelled” snails. A: SEM-image of a 9-day-old platinum-plus-heat-exposed animal with a well developed shell whorl (right lateral view); B: sketches of a control animal (left) and a “partly-shelled” animal (right), dorsal view, black: mantle tissue; grey: mantle edge; arrows indicate the direction of differential growth; CN: ctenidium; F: foot; H: head; MTE: mantle edge; SH: shell; VS: visceral sac.