Pathological Shells

From over a thousand Haliotis examined in a period of two years no specimen was observed with macroscopic disease of the soft parts, although a number had the dorsal region in various positions affected by contact with a diseased shell. Crofts (1929) states that she found no record of parasites occurring in Haliotis and in her examination of approximately four hundred specimens found only page 12two diseased. On the other hand, in H. iris the crab Elamena producta has been found on a large number of occasions apparently living in association with the New Zealand species. On removal of the shell from H. iris the crab is seen to be situated between the foot and the. mantle. Crofts found H. tuberculata shells frequently damaged by parasites, namely boring bivalves such as Lithodomus and various Pholadidea, but she does not mention tube worms. In the case of H. iris the shell is often heavily infested with these worms. Shells up to about 10cm in length are relatively clean on the surface and dark brown in colour. Above this size, however, out of a count of approximately 1,000 large shells all carried a heavy calcareous incrustation formed by algae covering the shell and in every case harbouring various species of annelid worm. Up to a hundred worms could be removed from one shell. These worms burrow through the periostracum of the shell and enter the nacreous portion ruining the shell for commercial use.

The most common worm found in the shell is Polydora sp. a member of the Spionidae. This is apparently P. monilaris. Polydora contains well known shell boring tube worms and has been described as causing a great destruction of oyster shells (Haswell, 1885). In some H. iris heavily infested with Polydora the calcareous crust of the shell will lift off in large pieces exposing a network of intertwining worms and their tubes. It is not known for certain how the worms burrow in the shell but it is probably by chemical means as the shell is much too hard for any mechanical means. The next most common worm found in the shell is the Terebellid Polycirrus. This dark brown worm has tubes very similar to those formed by Polydora. The tubes are not formed by many small particles adhering together but appear as thin chitin-like envelopes surrounding the worms. Nereis species are commonly found on H. iris shells although they probably do not destroy the shell as much as either Polydora or Polycirrus. The Nereis were identified as Nereis kerguelensis. Others were present. A number of Serpulids were found and one was identified as Hydroides.

In the shell shown in Plate 4 (Figure 9) the whole posterior dorsal region was completely deformed by worms. A large specimen of Thelepus was situated in the deep indentation shown in the photograph. Although the shell in this instance may have been slightly crushed to allow entry of the worms in the first place the major destruction of the shell appeared to be due to the worms. On the ventral side of the shell the indentation was covered by nacreous material except anteriorly along the edge of the muscle attachment where a great many small round protuberances on the shell were actually embedded in the muscle itself. In addition, some of these rounded projections had become detached from the shell and were lying in the muscle. It seems that the deformation made by the worm infestation caused the shell to proliferate these small round bodies which are not nacreous but rather resemble the periostracum structure of the shell. In section they show concentric rings. Although Polydora was seen on deformed shells it could not be said to cause the deformation as many shells perfectly normal on the inner surface had numbers of Polydora living on the outer surface. Only Thelepus and Nereis were found in direct connection with malformed shells. Plate 2 (Figure 5) shows a broken shell which was picked up from the beach near Island Bay. The whole outer surface of this shell is covered with worm holes and the severe distortion of the shell is without doubt due to heavy infestation of worms.

Plate 2 (Figure 4) shows a shell with a peculiar growth almost entirely enclosing the conical caecum of the visceral hump which contains a core of liver surrounded by gonad. This portion of the shell encasing the liver caecum appears to be made of nacreous layers in a way similar to the rest of the shell although it has not the lustre of the inner surface of a normal shell. In a normal shell the nacreous layers are put down at the posterior region of the shell by the mantle page 13covering the visceral hump but the major part of the caecum which extends forwards passes under a sheet of mantle stretching from the edge of the muscle attachment to the edge of the mantle plate, forming as it were a pocket in which the caecum is lodged. Thus the anterior part of the caecum has no contact with the shell either dorsally or ventrally. There is a distinct line of demarcation between the portion of shell laid down by the mantle of the posterior region of the visceral hump and that laid down by the mantle covering the anterior prolongation of the liver caecum.

In Fig. 4, however, there is no marked distinction on the inner surface of the shell showing where the mantle on the visceral hump ceases deposition of nacre and deposition of nacre by the mantle of the caecum pocket begins. In this case, therefore, it appears that for some reason the conical caecum has not been enclosed in the mantle pocket but has lain in direct contact with the shell and begun to secrete shell layers. Apparently the whole surface of the lobe has taken on a secretory function as shell has been formed enclosing the whole caecum. The ventral side of this cone of shell may have been formed as protection but it is not known whether the usual ventral covering of the visceral mass was present or not. Out of many thousands of specimens, Mr Taylor of Island Bay has found not more than 12 specimens of H. iris showing this peculiar formation of the shell. In some cases the cone of shell was not as complete as that shown in Fig. 4.

In one case in which the writer was able to see the animal that had been removed from the shell the caecum was twisted dorsally and was not covered by mantle except ventrally. The whole caecum had secreted shell dorsally but not ventrally where it was covered by the ventral portion of the mantle.

The most common type of diseased shell found was that depicted in Fig. 7. This formation of protuberances or tubercles on the inner surface of the shell was found frequently in some areas. In a section of the shell as shown in Fig. 7 these tubercles can be seen to have a core surrounded by numerous layers of nacreous material alternating with the dark conchiolin lines. This abnormal growth of the shell was probably due to irritation caused by sand particles becoming lodged between the mantle and the shell or, in some cases such as in Fig. 5, to the irritation caused by worms boring in the shell. Boutan (1923) succeeded in producing pearls in Haliotis by introducing foreign bodies in a suitable manner. Pearls have been found in H. iris in close proximity with such malformations as described above. These pearls were similar in structure to those tubercles seen in section but were always very small. Small shells are not often found with any tubercle formation on the shell and it is characteristic of this abnormal formation that it always occurs near or posterior to the muscle attachment region of the shell. Perhaps this is because only in this region does the animal find it impossible to rid itself of foreign bodies which have penetrated beneath the mantle. In the few small shells that were found with the tubercles just beginning to form the shell had a rough irregular inner surface apparently due to particles of sand which had been covered over by a dark conchiolin layer.

About 200 "frosted" shells were examined. These shells completely lacked the bright lustre of the inner surface of a normal shell. All the examples seen were large shells up to 17cm in longest diameter. The frosted appearance was due to some abnormality of the nacreous material laid down. Often this nacreous layer was flecked by conchiolin unlike a normal shell where the conchiolin layers are laid down in a fairly regular pattern. In addition, the shells were thick, the "frosted" nacreous layer being much thicker than a normal nacreous layer. The shell mentioned previously with the encased caecum (Fig. 4) is an example of a "frosted" shell. These shells are of no use commercially because they will not take a polish.