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Studies on the Paua, Haliotis iris Martyn in the Wellington district, 1945-46

Structure of the Shell

Structure of the Shell.

Examination of a thin longitudinal section of a shell under convergent polarised light showed a crystalline aggregate and consequently the isogyres were blurred. From the characters observed, the shell appeared to be composed of calcite intercepted with numerous conchiolin lines and an external very opaque periostracum the nature of which it was not possible to identify.

The shell of H. iris is unique among Haliotis shells by reason of the remarkable iridescence of the shell when polished. This brilliant lustre is caused in all molluscan shells by the nacreous material on the shell being laid down in very fine lamellae giving rise to blues, greens and reds through diffraction of light. All species of Haliotis show a certain amount of iridescence on the inner surface of the shell. H. iris is outstanding for the iridescence of the shell since the dark conchiolin layers alternate with nacreous material so that nowhere is there a great thickness of nacre.

Young shells of H. iris are spirally lirate like H. virginea with a few oblique rows of nodules (Suter, 1913). They remain lirate up to 2cm in length and then abruptly change the external structure of the shell to rows of low radially arranged nodules while the concentric growth lines are very distinct. Shells up to 6cm in length vary a great deal in colour from all shades of brown to dark olive-green. Sometimes a mottled effect is seen. After a length of nine centimetres has been reached the shells become covered by calcareous algae and the sculpture of the shell is no longer visible. Even before this occurs the lirate structure of the small shell has been worn away from the apical region. The small shell does not have any distinct mark on the inner surface for the muscle attachment. Not until the shell is about 7cm or more in length does die region of the muscle attachment become roughened and distinctly oval in outline. In the larger shells this roughened area is anything up to 6cm in longest diameter and is doubtless developed as a result of the need for a very firm muscle attachment. On the other hand H. australis and H. virginea lack any roughened area for the muscle attachment but have a uniform smoothness on the inner surface of the shell.

As mentioned elsewhere it is not until the shell is about 4–5cm in length that the deposition of conchiolin in definite layers begins to take place. This deposition appears to begin at the margin of the shell and spread irregularly inwards. There is a distinct demarcation line along the inner edge of the columella plate and extending down to the muscle attachment region nearest the apex so that the deposition of conchiolin on the columella plate is not continuous with that of the rest of the shell except at the margin of the plate itself. Also conchiolin is not deposited on the area of the muscle attachment after this area has become roughened. As this area of the muscle attachment increases in size the conchiolin layers terminate at regular intervals nearer the anterior lip of the shell. There is never very much conchiolin exposed on the surface at one time because by the page 7time the conchiolin layer is 1–2cm wide the next nacreous layer is being laid down at the margin of the shell and as the conchiolin spreads inwards so does the nacreous material. It seems, therefore, that although the growth of the shell occurs chiefly at the anterior and right margin the deposition of conchiolin and nacre can be carried out by all parts of the mantle in contact with the shell.

In a few specimens in which the shell had been broken or crushed the repair was chiefly done by deposition of conchiolin. This is also the case in the closing of the branchial apertures.