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

Growth of the Shell

Growth of the Shell.

Graphs for two areas (Te Kaminaru and Island Bay) covering a total of 825 shells were made in order to find growth peaks. There was a difference of approximately one mm in shells from these two areas and this may be due to earlier settling time for larvae in one of the areas. The Te Kaminaru shells were lmm shorter in longest diameter at each age peak, but for all practical purposes the two areas can be counted as one and when graphed in this manner produce four definite peaks as shown in text figure 1.

Text-figure 1

Text-figure 1

These four peaks came approximately 1cm apart, the first being at 1.9cm mark, the second 2.9cm, the third 3.8cm, and the fourth at 4.7cm. The peaks after the highest at the 4.7cm mark are indistinct but appear to become closer together.

Crofts (1929) considers specimens from 30 to 40mm in spring and summer are probably two years old and that growth gets progressively slower after the first few months. In this she disagrees with Stephenson (1924) who considers specimens from 20 to 40mm in summer as one year old. If the distance between troughs on the present graph is taken as indicating a year's growth, then H. iris agrees with H. tuberculata (Crofts, 1929) in being approximately 3cm in length when 2 years old. Because of this agreement in age and length of shell with the figures given by Crofts and also on account of the quite distinct peaks (approxi-page 8mately 1cm apart) in the present specimens, it seems reasonable to consider that the distance between one trough and another represents a year's growth. Shells that are 1.9 to 2.9cm, and 2.9 to 3.8cm in length are in their second and third years of growth respectively.

Plate 2, Fig. 3 shows an example of a small shell 4.7cm in longest diameter with well defined intervals of growth showing on the outside of the shell. These demarcations are approximately 1cm apart and would appear to lend further support to the hypothesis that 1cm growth (at least over the first three years) represents a year's time interval.

It would appear from Text Fig. 1 that growth becomes progressively slower after the 4.7 peak. These intervals on the graph decrease from 1cm to approximately 0.5cm until the shells are 8cm in length. However, the peaks of shells greater than 4.7cm in length could not be determined accurately from this graph because as difference in growth each year becomes less, the number of shells necessary to give a definite peak is increased and sufficient quantities were not available within this size range to give a clear indication. Figures for the graph in Text Fig. 1 cover a range of shell from 1 to 8cm in length. All these shells were collected in the littoral area and the graph shows that the greatest number of shells inhabiting this area fall in the size range between 4.5 and 5.5cm in length.

Crofts (1929) collected H. tuberculata as small as 2mm in length. No specimens as small as this were found in the littoral zone in the parts of Cook Strait area covered by the present paper. Crofts (1929) reports that the smallest specimens are found at very low tides and this may also be the case with H. iris. The smallest H. iris shell found by the writer was 1cm in length and this was considered to be under one year in age, Text Fig. 1.

The estimated age of shells with a size range between 1 and 5cm was calculated from the growth peaks as stated above and the occasionally well defined intervals of growth visible on the external surface of some small shells. An attempt was made to calculate the age of shells from 5 to 17cm in length by means of the number of conchiolin growth lines laid down in the shell. These growth lines are usually well defined as shown in Fig. 4. Approximately 300 shells sectioned through the longitudinal axis were examined. The lines on each shell were counted from the nucleus to the lower edge of the columella plate and again from the nucleus to the anterior margin of the shell. The former count generally gave two more lines than the latter. It was seen that shells with a size range between 4 and 5cm in length show no distinct growth lines. A definite growth line first appears in shells between 5 and 6cm in length and in this group four out of eight shells had no lines showing at all. The average number of growth lines for each centimetre group increases as the size of the shell increases. There is, however, no regular increase between the averages, for example, the average number of lines for shells from 9 to 10cm in length is 5.0; 10 to 11cm is 9.0 and 11 to 12cm is 9.9. In addition, there is a great range in the number of growth lines within any centimetre group, e.g. in the 11 to 12cm group 1 shell has only five lines while another within the same group possesses 27 lines.

It was noticed on many occasions that the growth lines were laid down in pairs. This gave rise to the idea that the growth between one line and another may represent six months' time interval. The nacreous material between the pairs is greater than between the two lines of a pair and it was thought that this greater deposition of nacre alternating with a lesser period possibly represented summer and winter seasons of growth.

The occurrence of pairs of lines is not, however, a constant feature and from examination of the data it seems unlikely that a shell 14 to 15cm in length could be 13 years old. This would be the case if half the average number of page break
Fig. 1.—H. iris shell, photographed through the axis in order to calculate the constant angle of the shell.

Fig. 1.—H. iris shell, photographed through the axis in order to calculate the constant angle of the shell.

Fig. 2.—Photograph of ground section of shell through the longitudinal axis and nucleus to show the conchiolin lines. A, growth lines. B, nucleus.

Fig. 2.—Photograph of ground section of shell through the longitudinal axis and nucleus to show the conchiolin lines. A, growth lines. B, nucleus.

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Fig. 3.—Photograph to show the well defined intervals of growth visible on the external surface of some small shells. 1, 2, 3, 4, external demarcations of growth.

Fig. 3.—Photograph to show the well defined intervals of growth visible on the external surface of some small shells. 1, 2, 3, 4, external demarcations of growth.

Fig. 4.—Photograph to show encasement of conical caecum by the shell. A, caecum encasement.

Fig. 4.—Photograph to show encasement of conical caecum by the shell. A, caecum encasement.

Fig. 5.—Photograph of posterior region of a diseased shell to show the gross distortion ol the shell and the numerous openings of worm tubes. A, gross distortions of the shell. B, openings of worm tubes.

Fig. 5.—Photograph of posterior region of a diseased shell to show the gross distortion ol the shell and the numerous openings of worm tubes. A, gross distortions of the shell. B, openings of worm tubes.

Fig. 6.—Photograph of the feeding tracks of two small Haliotis iris. A, tracks of H. iris, approximately 3cm in lengths. B, tracks of H. iris, approximately 2cm in length.

Fig. 6.—Photograph of the feeding tracks of two small Haliotis iris. A, tracks of H. iris, approximately 3cm in lengths. B, tracks of H. iris, approximately 2cm in length.

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Fig. 7.—T.S. through a diseased shell to show tubercles and distortion of the conchiolin growth lines. A, tubercles. B, distortion of growth lines. C, position of insertion of shell muscle.

Fig. 7.—T.S. through a diseased shell to show tubercles and distortion of the conchiolin growth lines. A, tubercles. B, distortion of growth lines. C, position of insertion of shell muscle.

Fig. 8.—A polished specimen of H. iris to show the appearance of conchiolin growth lines on the outside of the shell. 1, 2, 3, growth lines.

Fig. 8.—A polished specimen of H. iris to show the appearance of conchiolin growth lines on the outside of the shell. 1, 2, 3, growth lines.

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Fig. 9.—L.S. of diseased shell to show cavity from which one annelid worm was removed. Several small pearl-like formations also appear. A, cavity from which worm was removed. B, small pearl-like formations.

Fig. 9.—L.S. of diseased shell to show cavity from which one annelid worm was removed. Several small pearl-like formations also appear. A, cavity from which worm was removed. B, small pearl-like formations.

page 9 lines shown in Table I is taken, plus 4 for the four years' growth which occurs prior to the formation of conchiolin. From Text Fig. 1 it was concluded that in the first few years a shell grows approximately 1cm a year. It would be unusual if this growth rate did not decrease over a number of years. Yet a shell 14cm in length which is 13 years old would necessitate a growth of about 1cm a year for 13 years.

On the other hand the majority of the shells had the lines appearing singly and sometimes in groups of three as in Fig. 8. Therefore, it seems more probable that each growth line represents a year's growth. In this case a shell 14 to 15cm in length could be considered approximately 22 years old, i.e., the average number of growth lines plus 4. It must be remembered that the average for a group such as the 14 to 15cm group was obtained from a wide range in the number of growth lines. If a specific instance within a group is taken the age would be calculated from the number of lines actually present and not from the average for the group. For example, some old polished shells and pieces of shell have been examined with over 36 lines visible. This would mean that in some cases a shell was over forty years old. In contrast, however, some fairly large shells on the Table show very few growth lines, e.g., in the 12 to 13cm group two shells have only five lines. Therefore, their estimated age would be 9 years. This age is well below the estimated average age for the group and is open to the same objection as raised above where it was pointed out that a shell is not likely to continue growing at the same rate over a large number of years. As the rate of growth does not exceed 1cm a year the first 4 years it does not seem feasible to consider a shell 12 to 13cm in length as only 9 years old.

The only conclusion which can be drawn at present regarding the conchiolin growth lines is that these lines are put down regularly by the animal in response to some physiological need. It is not possible to state whether these lines are annual or biannual (i.e., twice yearly), or indeed give any indication of the growth-size ratio without further work being carried out.