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.

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

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. 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. 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. 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. 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.

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.

Shell Perforations

Boutan (1886) states that the shell of Haliotis commences as in Fissurella and Pleurotomari a without slit or perforations and Crofts (1929) found a distinct protoconch in her smallest specimens. In the smallest specimens examined by the writer for H. iris, H. virginea and H. australis the protoconch was plainly visible and approximately the same size (1.5mm long) in all three species. As soon as growth begins perforations are formed and the difference in number of perforations for approximately the same length of shell is considerable in the three species.

Only single specimens were obtained in the size range shown above. The difference in number of perforations per unit length between the three species is due to the distinctive nature of the logarithmic spiral in each species, i.e., it follows that the constant angle of the logarithmic spiral in H. virginea will fall probably between that of the other two species.

There is a close correlation between the number of perforations present in tho shell and the length of the shell. This can most easily be ascertained in the page 10smaller shells. It is very difficult to obtain an accurate count of perforations in the older shells because of the heavy encrustation of coralline algae, tube worms, etc. In a count of fifty shells taken from different areas ranging from one to seven centimetres in longest diameter the number of perforations increased regularly with the length except in one case. The average number of open perforations was five but ranged in number from 4 to 7. Suter (1913) states that open perforations in H. iris range from five to seven. In a count of approximately 340 shells ranging between 12 and 16cm the following estimate of shell perforations was obtained.

It can be said that the range for open perforations for this species is from 0 to 7 while the average is from 3 to 5 rather than between 5 and 7 as stated by Suter (1913).

Crofts (1929) found in a count of 194 specimens of H. tuberculata of marketable size that 101 had 6 perforations and she states that the number of perforations in H. californica vary from 5 to 9 in young animals and from 2 to 3 in the adult. These latter figures resemble those given for H. iris in having fewer perforations in older shells. Crofts found only one shell imperforate and this had closed holes in the older part of the shell.

Pelseneer (1920) has described abnormalities of the shell in Haliotis as instances of continuous and discontinuous variation and cited the variations in the number of perforations in H. tuberculata and H. californica.

Growth Relation

Sasaki (1926) recorded the growth relation between the shorter and longer diameter in H. gigantea and in two varieties of H. gigantea from places varying in temperature in Japan. He took the ratio at Omoi as the mean (71.29) and he read his results to show that high temperature probably produced narrower shells. Crofts (1929) found the ratio for H. tuberculata was 68.7 in the Channel Islands.

In H. iris from the Runaround and Chaffer's Passage, Wellington, the ratio is 75.7 for 120 specimens varying from 9 to 17cm in longest diameter. They, therefore, are wider than H. tuberculata at Brecqhoua (Channel Islands) or any H. gigantea specimens mentioned by Sasaki. Ten specimens from Kaikoura gave a ratio of 78.1 which is wider again than any of the specimens mentioned above. In small H. iris varying from 5 to 50mm the shells are narrower not wider as Crofts (1929) found in H. tuberculata but the ratio is not constant. H. australis specimens give a ratio very close to that of H. iris, namely 70.0.

Sasaki found that the growth relation between shorter and longer diameters in H. gigantea could be expressed by the equation S = kL^x where S is the shorter diameter; L the longer diameter; k the local constant and x the specific exponent. The probable specific exponent for H. gigantea is 0.85 for mature specimens and for immature specimens 0.97 showing that the larger shell is the narrower. In H. iris the following values for the specific exponent were obtained; 0.98 for mature specimens and 0.96 for immature specimens. These values for the specific exponent indicate that in H. iris the larger shells are wider. No record of temperature range for the places in New Zealand from which the calculations on H. iris and H. australis were made were available to the writer. Consequently no correlation between the growth relation figures and temperature could be made as in the paper by Sasaki (1926).

Determination of Logarithmic Spiral

The large shell of H. iris has a very small apical spire and an extremely large last whorl which is very depressed with a relatively enormous aperture.

Crofts (1929) suggests that the flattened shell of Haliotis has been evolved from a shell with a taller spiral because of the habit of squeezing into confined spaces between rocks. The shell of Haliotis grows in the form of a logarithmic spiral. The form of a single curve following a logarithmic spiral is given by the expression where r is the radius of the shell from centre to circumference; θ is the angle of revolution which the spiral has described and α is the angle between the tangent of the curve and the radius vector of this curve, which remains constant. This is known as the constant angle of the curve and has been determined in many species of Haliotis. D'Arcy Thompson (1942) states that in Haliotis the constant angle (α) varies from about 70 degrees to 75 degrees while in the majority of gastropods it lies between 80 degrees to 85 degrees or even more.

To determine the constant angle of H. iris a photograph was taken through the axis of a shell 9.9cm in largest diameter. The curve made by the line of holes in the shell was taken as the logarithmic spiral. Plate I shows a photograph and a diagram can be drawn from it. The following expression of the formula given above was used in the actual determination of the angle.

The result obtained gives α = 53 degrees 46′ which as far as the writer can ascertain is smaller than any other value of α obtained for a Haliotis. This means then, other things being equal, that H. iris has fewer whorls per unit height of shell than other species of Haliotis.

Moore (1936) found that the value of α in Purpura lapillus varied during the lifetime of the shell. The value of α calculated from a H. iris specimen 3.72cm in largest diameter was considerably higher in degree than in the case of the specimen 9.9cm in largest diameter. Therefore it appears probable that the value of α decreases as the shell grows.

The constant angle of the spiral of H. australis in the above table shows a large increase over the values given for H. iris and is higher than some other species of Haliotis, e.g. H. tuberculata where α = 69 degrees 48′ (Moore, 1936).

Crofts (1929) states that the shell of H. tuberculata is so flattened that the animal is unable to retract completely into the shell. From the value of the constant angle in H. iris given above it follows that the shell of H. iris is lower in relative height; but in contrast to H. tuberculata, H. iris can retract completely within the shell when disturbed.