Figure 1.

Major transitions during shell development of the tropical abalone Haliotis asinina. (A) A newly hatched trochophore larva 9 h post fertilisation (hpf) during the initial stages of biomineralisation; the shell field (sf) is evident. (B) The calcified protoconch (pc) is present by 11 hpf. (C) The completed larval shell displays fine sculpturing and markings indicative of a high degree of control over the biomineralisation process from an early age. (D) Newly settled postlarvae on coralline algal surface. An abrupt transition in shell morphology accompanies metamorphosis (white arrows). This initial postlarval shell is unpigmented, and displays a rippled texture (inset). (E) 1–2 month old juveniles have developed a pigmented shell that is initially a uniform maroon, but soon develops a series of blue and orange dots and cream and maroon fields. (F) Animals with a shell size of approximately 1 – 10 mm maintain this complexity in pigmentation which follows a simple set of rules: blue dots occur against a maroon background, orange dots against a cream background. Tremata (t) and ridges (arrowheads) have also developed. (G) Animals with a shell size larger than approximately 10 mm gradually loose the underlying swaths of maroon and cream pigmentation, but maintain the expression of blue and orange dots on the now prominent ridges (arrowheads). (H) The shells of sexually mature animals no longer possess blue and orange dots and the ridges present in smaller shells are less prominent. A pattern of tan – brown triangles of varying intensity now patterns the shell. (I) A cross section from the shell of a 1 mm juvenile. The ordered aragonitic tablets characteristic of later stages are not present, but a transition in crystal morphologies is evident (arrow). (J). Cross section through the shell of a 5 mm juvenile. A relatively thin layer of nacre (n) composed of ordered aragonitic tablets is overlaid by the prismatic layer (p). (K) A cross section through the shell of a 20 mm animal reveals a new structural layer of CaCO₃ that has been added to the ventral most region of the shell (yellow bracket). (L) The thickness of this ventral most layer continues to increase and in 100 mm animals is approximately 20 μm thick.