Introduction

Apart from the work of Lami (refs. in Chapman, 1946) and Grubb and Martin (1937), the amount of information on cave fauna and flora in different parts of the world is rather scarce. The obvious factors regulating growth of organisms which may be classed as cave-dwellers are light and humidity (Chapman, loc. cit., p. 637). Lami (1939, 1940) records maximum and minimum light values, pH and relative humidity for a number of “cavernicole” algae. He distinguishes three groups of algae according to the degree of their light tolerance: firstly those which can grow only under intense illumination; secondly those which have a moderately wide range in both well- and dimly-lit habitats; and lastly those which are confined to caves and shaded clefts.

Within the Waitemata Sandstone area on the east coast mainland of the Hauraki Gulf, caves are encountered at frequent intervals, often coincident with a fault line at the junction of cliff bases and intertidal platforms. Many are no deeper than a metre or two—mere concave hollows which harbour a seepage or high spring tide community of Lichina, Enteromorpha or Rhizoclonium. Others are hollowed out to form caverns more than 30 metres deep and several metres high. Two caves were selected for observation, one at Red Beach, the other at Stanmore Bay. Both places are in the southern sweep of Whangaparaoa Bay, about 17 miles north of Auckland City (see Chapman, 1950, Fig. 1, for location of the Stanmore Bay cave). Zonation of the dominant species on each wall was recorded by the first author early in February, 1950, at Red Beach, while that at Stanmore Bay was recorded in midwinter of the same year (2.7.50). The caves were re-examined by both authors some two years later (30. 12.52) when a detailed topographic survey was made, and daylight penetration measured with a Weston photo-electric meter. Each meter reading was taken from the surface of a sheet of white unglazed paper held against the wall, 2 metres above the floor of the cave, and at such an angle that maximum deflection of the meter was obtained. The day was a clear, sunny one, and all light readings were taken within an hour of noon. Although the light intensity values as shown in Figure 1 will have no absolute significance except for the time of observation, it is believed that they will indicate reasonably well the gradient of indirect illumination in the caves at most times when direct

sunlight is absent. The principal difference between the two in this respect appears to be that, while opposite walls of the Stanmore Bay cave are very nearly equally illuminated, at Red Beach the north-western wall receives more light than the south-eastern.

The penetration of direct sunlight has been estimated from a table of azimuth and apparent elevation of the sun^* for midsummer (December 22) and midwinter (June 21). The isopleths in Figure 2 indicate the number of hours' exposure to direct sunlight at a level 2 metres (Stanmore Bay) or 1 metre (Red Beach) above the cave floor. The most significant feature indicated by the figure is that, while the Stanmore Bay cave receives the majority of its direct sunlight in summer, the reverse is the case at Red Beach. This is due to the fact that the two caves face respectively the mid-summer (N. 60° E.) and mid-winter (S. 60° E.) sunrises. Maximum light penetration can only occur when the sun is relatively low and virtually all direct light is intercepted by the cave roof for elevations greater than 10 degrees.

When the above data are compared with the distribution of the cave-dwelling organisms, it will be seen from the ensuing description and illustrations that the biological pattern follows more closely the penetration of indirect daylight. No obvious correlation can be inferred between zonation and direct sunlight because, although one cave receives nearly all its direct sunlight in summer and the other in winter, there is no great difference in the pattern of distribution of the dominant plants and attached animals between the two caves.