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Volume 72, 1942-43
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Discussion.

The regularity with which spawning periods coincide with the appearance of full and new moon is so marked in the years 1938 and 1940 that it would appear necessary to find some reason to account for it and for the irregularity in the year 1939.

In the first place, however, the question of what induces spawning must be discussed. Individuals of Cryptoconchus porosus show no tendency to accumulate in groups before or during spawning, nor do isolated individuals of either sex fail to produce sperms or eggs at the same time as the others. Added to that, Table I shows that the presence of sperms is not necessary to stimulate the female or vice versa. From these facts it would appear that either the internal metabolic rate itself is responsible for the regular intervals between spawning, or some purely external factor is responsible.

The length of time needed for the ripening of the sex cells in the gonads will depend entirely on the metabolic rate of the animal and should be approximately the same for animals living under the same conditions. Under normal conditions the length of time needed for the ripening of the eggs and sperms should remain fairly constant as it did in 1938 and 1940, and thus a periodicity would arise—ripening of gonads—spawning—ripening of gonads—spawning, etc. Unfavourable conditions, however, would upset this rhythm, but if conditions became normal again the rhythm should be restored. This seems to be the case in 1939, when the 2nd spawning was delayed by 4 days, the 3rd by 6, the 4th by 7 and the 5th by 7. Table II gives a comparison of the average 9 a.m. temperatures recorded at Dunedin for the Meteorological Department between new and full moon for the years 1939 and 1940.

Table II.
Average Air Temperatures. 1939. 1940.
Av. Temp. for 15 days before 1st spawning 46.8° F. 44.7° F.
Av. Temp. between 1st and 2nd spawnings * 39.5° F. 42.8° F.
Av. Temp. between 2nd and 3rd spawnings 34.6° F. 40.9° F.
Av. Temp. between 3rd and 4th spawnings *39.9° F. 43.1° F.
Av. Temp. between 4th and 5th spawnings 41.7° F. 45.3° F.

Average Temperature for the period before each Spawning in 1939 and 1940.

[Footnote] * Represents a heavy snowfall.

[Footnote] † An extremely heavy snowfall.

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The snow, which fell before the second spawning period, was probably responsible for delaying it. This delay was accentuated by the extremely heavy snowfall before the third period, and still further by the lighter fall before the 4th. Then improvement in conditions restored the usual time for ripening of the gonads and normal rhythm was resumed. Arey and Crozier [1] found, in their experiments in the Bermudas, in Chiton tuberculatus normally adapted to a temperature of 26°–27° C., that below 15° C. an anaesthetised condition was arrived at. Cryptoconchus porosus is adapted to a temperature of 15° C. so that the temperature below which it would become anaesthetised would be much lower. During the very cold periods of 1939, specimens kept in a comparatively shallow aquarium would naturally be subjected to falling temperatures and along the shore the melting snow would lower the temperature of the water slightly.

Although the internal metabolic rate may account for periodicity in spawning, it will not account for the fact that each year spawning started at full moon. The coincidence of full moon and spawning has been noted for many marine animals, but no thoroughly satisfactory reason for it has been propounded. Fox [6] suggests that bilunar periodicity may be due to polarised light. As the polarisation of the light of the moon is greatest at the first and third quarters, it seems unlikely that seven days would elapse between the stimulus of poralised light and the actual spawning. Fox's other suggestion [6], that the moon may perhaps cause a lunar cycle in reproduction by the additional total number of hours of illumination per 24 hours at full moon over and above a threshold light value, may account for the coincidence of full moon and the first appearance of eggs and sperms, in that spawning commences in the middle of the winter, when the number of hours of daylight is at a minimum. Under these conditions the extra number of hours of illumination would be a sufficient stimulus to initiate spawning.

The full tides which accompany full and new moon are probably useful in that the greater ebb and flow at these times helps to distribute eggs and sperms and thus ensure a greater percentage of fertilisation. Two objections can be raised to tidal rhythm as a causal factor in the spawning of Cryptoconchus porosus. One objection is that no matter at what time of the day the tide is full, spawning begins with great regularity between 10 a.m. and 12 noon. The increased illumination is probably the cause of the regularity of the actual spawning time, although no retardation occurred on dull days. On three of the four occasions on which spawning commenced in the afternoon, other individuals on the same day spawned at the normal time, indicating that delay was not due to external factors of temperature, light intensity, etc., but to the condition of the actual individual. The other objection is that, unlike Chiton tuberculatus which lives on rocks left exposed at low tides and which spawns upon submergence by the rising tide (Arey and Crozier [1]), Cryptoconchus porosus lives beneath the low-water mark, and is therefore less influenced by the rising tide. A theory which would account for a tidal rhythm, however, is that the ancestral chitons became adapted to life near the high-tide mark, where spawning was only possible at times of spring tide. A tidal rhythm was thus imposed and has not been lost by species like Cryptoconchus porosus which may be supposed to have taken secondarily to a submerged life.

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From the above statements the following summary may be made to account for the rhythmical periodicity of spawning. The original stimulus is the increased number of hours of illumination per 24 hours at the time of full moon, and the rhythmic sequence of breeding periods is due to the internal metabolic rate of the individual. Under normal conditions the breeding periods coincide with new and full moon and the accompanying spring tides, but under the effect of low temperatures the metabolic rhythm is upset and the coincidence of spawning with the phase of the moon is also upset. A tidal rhythm may have been acquired from ancestral types which lived higher up the shore, where spawning was possible only at spring tides. The regularity with which individuals start to deposit eggs or sperms at a certain time of the day may be due to the increased illumination during daylight.