The infrastructure of aesthetes in Tonicella marmorea (Polyplacophora; Ischnochitonina) and a new functional hypothesis

Abstract

With 8 plates and 1 figure in the text) The valve surface morphology and aesthete ultrastructure of Tonicella marmorea is described. The possible functions of these very numerous organs are considered with particular reference to the ecological needs of the animal and their similarity to other invertebrate organs. A periostracum-secreting function is proposed as an alternative to the more commonly postulated light receptor theory. PLATE 111. (a) Dorsal surface of an apical cap showing the layer of periostracum and the fringe of T-shaped bodies. (b) Section through an apical cap showing the ventral and horizontal channels filled with periostracum-like material. All scales in micrometres. P, periostracum; F, fringe of T-shaped bodies. PLATE IV. (a) Ventral region of an apical cap showing cup-shaped depression filled with microvilli produced by cells seen to have a granular cytoplasm containing small, flattened secretory granules. (b) Transverse section in the distal region of a megalaesthete showing the interrelationship between the apical cap, the microvilli and the cytoplasm of the cells which produce them. Interspersed amongst the microvilli are cilia which show the typical 9+2 structure. All scales in micrometres. Ac, apical cap; c, cilium; gc, granular cytoplasm; mv, microvilli; 2, small, flattened secretory granules. PLATE V. (a) Large central cells in the megalaesthete body packed with large, electron-dense secretory droplets. (b)Central cell in the megalaesthete body containing a large, prominent nucleus, mitochondria, RER and golgi, together with secretory droplets with variable contents. Adjacent to this cell is a neurosecretory-like body and a number of microtubules. All scales in micrometres.

The fine structure of the pigment body complex in the intrapigmented aesthetes of Callochiton achatinus (Mollusca: Polyplacophora)

“…A large proportion of the light striking the dorsal surface of the valve will be reflected or absorbed by the properiostracum and tegmentum and only a limited amount will penetrate the valve layers to reach the megalaesthete. In addition, the hypothesis proposed by Baxter et al (1987) and further promoted by Baxter et al (1990) and Sturrock & Baxter (1993) that the aesthetes may in some way be monitoring and repairing the thickness and integrity of the properiostracum is not unreasonable. Dual function receptors of this type are not unique, and in describing the eyes of Pecten maximus L., Barber, Evans & Land (1967) suggested that the distal cells which have ciliarly derived receptors, and can be presumed analogous to the C. achatinus rhabdomer, function in shadow response, whereas the proximal cells, comparable to the myeloid body, monitor light intensity.…”

“…The debate as to the true function/s of the aesthetes in chitons has been ongoing since Moseley (1885) first described these structures and referred to them as 'eyes'. Baxter et al (1987) questioned whether performing a light receptor role was the primary function of aesthetes, arguing instead that the secretion of properiostracum was their principal function. This hypothesis has been further substantiated by Baxter et al (1990) and Sturrock & Baxter (1993), although in these studies and others (Haas & Kriesten, 1978;Fischer & Renner, 1978;Fischer, 1988) it has become increasingly apparent that some sensory function may also be performed.…”

“…obs. ), Tonicella marmorea (Fabricius) (Baxter et al, 1987) and L. asellus (Sturrock & Baxter, 1993) are each very thin and all have much simplified, sparsely distributed rhabdomer and lamellate whorl structures.…”

“…The fine structure of the rhabdomer and surrounding pigment cup ocellus has been described in some detail (Baxter et al, 1990), but the myeloid body was simply reported as an area of tightly packed, lamellate whorls. Several papers have described the aesthetes of a number of species of chiton (Boyle, 1974;Fischer, , 1979Fischer, , 1988Fischer & Renner, 1978;Haas & Kriesten, 1978;Baxter, Jones & Sturrock, 1987;Baxter et al, 1990;Sturrock & Baxter, 1993), but the function of these unusual organs remains obscure and unresolved. In particular, the role of the sensory cells of the pigment body complex located on the periphery of the intrapigmented aesthetes is not known, although a number of different functions have been proposed.…”

THE CANAL SYSTEM IN SCLERITES OF LOWER CAMBRIANSINOSACHITES(HALKIERIIDAE: SACHITIDA): SIGNIFICANCE FOR THE MOLLUSCAN AFFINITIES OF THE SACHITIDS

“…The function of the micro‐aesthetes remains unresolved. Baxter et al . (1987), studying Callochiton , suggest a secretory function, but the fact that the subsidiary cap is imperforate led Reindl et al.…”