..Three types of secretory cells are also observed in the sole foot epithelium. One of them is similar to the type B described in the side foot epithelium (Figure 4(h)). The new second one is more abundant and presents a secretory product similar to that of type B cell, but it is concentrated in denser granules. We identify this new secretory cell as type E (Figures 4(g) and 4(h)). The third type (type F) possesses small and dense grains (Figure 4(h)), with higher electron density in the center than in the periphery.In addition to these three types of epithelial secretory cells, clusters of secretory cells localized in a subepithelial position form multicellular glands on the sole foot (Figure 5(e)).

Under TEM microscopy, these multicellular glands contain very dense granular material, which could be discharged on the sole via neck openings located between epithelial cells (Figures 1(b) and 4(i)). These secretory cells are characterized by a very well-developed Golgi complex arranged in a circular manner (Figures 5(e) and 5(f)). Occasionally, subepithelial unicellular secretory glands have been found with granules that resemble those found in the epithelial secretory type B cells (Figure 5(g)).4. Discussion4.1. Epithelial CellsThe foot epithelium of Haliotis tuberculata presents unique features in both the side and the sole foot epithelia. The side epithelial cells are highly pigmented with melanin and phycobilin granules and possess a prominent microvillus border. A similar epithelium has been described in the side foot of other gastropod, Patella vulgata [14], but, in this case, only melanin granules were observed.

Pigmented cells containing mature and nonmature melanosomes, similar to those observed in the epithelium of the abalone (present results), have been reported in Sepia officinalis [34]. However, in the case of similar pigmented cells which provided a red-purple color to the skin of Aplysia californica [35], no melanin granules were described. Moreover a high abundance of bluish-green pigment was observed in the side epithelial cells located on the crests of the folds, which has the characteristics of the phycobiliprotein previously detected by using a fluorescence microscopy [29]. This type of pigmented granules content electron-lucent finely granular material, that has not been described before in any other gastropod.

In a review on the biochromy of the Mollusca, Fox [36] described in Haliotis a bilichrome pigment known as haliotisrubin, which is accumulated from the consumption of red algae. Therefore, a dietary origin is also plausible for the pigment we detected in the epithelium of Haliotis tuberculata.Moreover, the AV-951 side foot of Haliotis tuberculata has been also characterized by the presence of toxins in a type of secretory cell [29].