Control of shell pigmentation by secretory tubules in the abalone mantle

Background: Molluscan shells exhibit a wide diversity of pigmentation patterns and are often used as models for understanding the mechanisms underlying biological pattern formation. Numerous mathematical models have been put forward to describe these patterns, but all rely on assumptions regarding the nature of the pigments and the cells from which they are secreted. We investigated pigmentation and cellular morphology of the mantle (shell-secreting organ) of the tropical abalone, Haliotis asinina, as a crucial step towards understanding the molecular mechanisms of shell patterning in this gastropod mollusc. Results: Accumulation and release of pigmented products occurs within secretory tubules in the prismatic zone of the juvenile H. asinina mantle. The colour observed within these tubules closely matches the colour deposited most recently within the shell lying directly above. The pigments are autofluorescent, and confocal microscopy reveals that multiple pigments can be present within a single tubule at any one time. Examination of mantle morphology in other gastropods reveals that mantle tubules are not restricted to abalone, but are not universal. Conclusions: The presence of a tubule-based secretory system within the abalone mantle demonstrates that pigmentation is not controlled by a simple line of cells. Instead, co-ordination of patterning events is likely to be modular, with signals received by individual cells being transmitted throughout the entire tubule to synchronise the accumulation and secretion of pigmented material.