Growth line patterns in whelk shells

Yesterday I talked about the basic structure of a whelk shell and described a few of its characteristic parts. Today, the photographs illustrate those points in a number of different whelk shells.

The textures and patterns of gastropod mollusc shells, like those belonging to the Common Whelk (Buccinum undatum L.), are very different from those found in bivalved shells described in earlier posts. The pictures in this post show the variations and intricate details of the ridges, furrows, and grooves of the striations – crossed by diagonal broad ribs and fine growth lines in whelk shells – resulting in a slightly chequered or reticulated pattern on the smaller scale as well as the more obvious one typified by parallel ridges. There is evidence for breakages and repair in some specimens. Older, larger shells have seemingly experienced minor damage to the outer lip and mantle edge that has contributed to thick ridges parallel to the aperture.

You can also see the remains of a selection of the marine organisms that commonly attach themselves to the outer surface of the shells. The encrustations include the sinuous calcareous tubes of marine polychaete worms such as Pomatoceros triqueter; lace-like layers of seamat or Bryozoa; and empty acorn barnacle shells. The brown, hairy, paper-like coating on some specimens is the outermost layer of the shell – the periostracum. This is present on younger, smaller shells but soon wears away when the mollusc reaches maturity.

The most distinct sculpturing or ornamentation on the shells is seen in the young specimens, especially where the protective periostracum has just peeled away. Older, more worn shells, tend to lose the crisp, glossy outer surface and look slightly furry in close up – due no doubt to the disintegration of the shell crystal structure.