Figure 4.—Schematic diagram of zonation on south-east wall of Red Beach Cave—summer. Vertical scale = twice horizontal.

considerably owing to a curve of the west wall so far in towards the centre of the aperture that there is scarcely room to squeeze sideways into the innermost recesses. The south wall is tiered in a series of terraces 0.2-0.5 metres wide from above E.H.W S. to about M. T.L for the first 10 metres, the wall then becoming steep and more or less vertical. This portion is continually moist, probably through seepage from the cliffs above, and the moisture is responsible for reflecting a certain amount of light across to the more seaward facing north wall. Differential indirect lighting effects between walls are less pronounced than in the Red Beach cave; however, the flat ledges between M. H.W N. and E. H.W. S. on the south wall, together with the more constant moisture, encourage growth of certain algae—e.g., Bostrychia arbuscula and Ptilothamnion pectinatum, which are absent from the vertical northern side.

The absence of Peyssonelia from the south wall could conceivably be related to the occurrence of direct summer sunshine on that wall, but this seems unlikely in view of the fact that the same species is known to occur in sunlit pools in the open midlittoral. Among the crustose red algae Peyssonelia and basal Corallina seem less shade-tolerant than Hildenbrandtia, which is the last algal survivor as far in as 24 metres. extending from the cave roof to just below high water mark, as far as could be judged. At this depth it was necessary to shine a torch or light a match to examine the walls in mid-winter. Beyond the narrow crack was found a scattered assemblage of tiny Micrelenchus dilatus, a univalve with an opalescent shell typically living among kelp at extreme low water. Such a population may be expected to range over almost any part of the cave according to its immediate preference or requirements. The same applies to Melaraphe, Lepsiella (and Onchidella), the ranges on the charts being those at the time of observation. Shells of Elminius plicatus show a marked

Figure 5.—Schematic diagram of zonation on south wall of Stanmore Bay Cave—winter. Vertical scale = four times horizontal.

(b) Differences between caves.

It has been mentioned already that direct sunlight appears to play a subsidiary role to indirect daylight in regulating the distribution of cave-dwellers. By contrast a much more direct correlation can be inferred between the zonation of the cave biota and the relative incidence of indirect daylight, which, though altered in intensity by the degree of sunlight outside each cave, is a more consistent and therefore a more easily measurable factor. These statements will now be examined more fully in conjunction with Figures 1—6.

(a) Direct summer sunshine.

If sunlight were to operate as a presence or absence factor, then one would expect certain species to occur only on rocks which receive direct illumination from the sun's rays. Examination of the first two horizontal metres on the north wall of the Red Beach cave (cf. Figs. 2A, 3) fails to show the presence of a single species which does not occupy a comparable position on the opposite wall. The south wall at Stanmore on the other hand, receiving direct sunlight on early mornings in summer to a distance of 30 metres, shows one or two discrepancies with its opposite face—namely, in the presence of Bostrychia* spp. and Ptilothamnion,^* and absence of Elminius plicatus. It is possible that summer sunlight affects the reproductive cycle of the red algae—a hypothesis that needs checking

Figure 6.—Schematic diagram of zonation on north wall of Stanmore Bay Cave—winter. Vertical scale = four times horizontal.

from more frequent collections of these species. Mention has already been made, however, that angle of substrate, and perhaps salinity, seem to affect their distribution as markedly as light. Absence of Elminius plicatus from the south wall is less easy to explain, for this barnacle is a characteristic cave-dweller, in its modified growth form.

(b) Direct winter sunshine, affecting only the south-east wall of the Red Beach cave.

Again no direct effect can be observed from the zonation pattern; but the possibility must be envisaged that life cycles and seasonal abundance of red algae such as Rhodochorton are influenced in some degree.

(c) Indirect daylight penetration.

A glance at Figure 1 and then at Figures 3–6 indicates at once that the pattern of zonation follows quite closely the gradient of indirect daylight. At Red Beach on both walls, regardless of varying amounts of direct summer and winter sunshine, there is an abrupt cessation of Calothrix, Lichina and Ralfsia, classed in Table I as stenophotic, light-demanding species. There is a more gentle gradient of indirect daylight in the Stanmore cave, because of its wider mouth. This allows a more gradual transition from one dominant to another. Moreover, all the stenophotic light demanders are absent about the entrance to this cave.

Between 500 and 100 metre candles (approx.) there is a transition zone or ecotone where euryphotic, shade-tolerating species such as Chamaesipho columna, C. brunnea, Sabellaria and basal Corallina overlap with wider ranging shade-lovers like Peyssonelia and Hildenbrandtia. The greater penetration of Sabellaria,

Figure 7.—Range of tolerance of organisms to variation in intensity of indirect daylight within caves at Red Beach and Stanmore Bay.

Volsella and Elminius plicatus on the Red Beach north-west wall is probably a direct response to the brighter indirect daylight as far in as 10 metres, at which level the south wall is in almost total darkness. Stenophotic forms are included within and confined to this ecotone—e.g., Nodularia and Bostrychia. Rather surprising is the abrupt termination of Gelidium pusillum not far in from the mouth of the Red Beach cave, for it is so often confined in the open midlittoral to moist and shady clefts.

Below 100 metre candles the truly shade-demanding species come into prominence in both caves. In this category may be listed Ptilothamnion, Rhodochorton, Hildenbrandtia, Tethya and Mitella.

In Figure 7 an attempt has been made to summarize the relative ranges of tolerance to the variations in daylight observed and recorded in mid-summer, 1952 When this chart is correlated with Table I, it may appear anomalous to regard Calothrix, Ralfsia and Lichina as stenophotic, seeing that their ranges in Figure 7 are as wide as others classed as euryphotic. The classification into enry- and stenophotic light and shade preferring species in Table I has been made not only from the facts arising out of the cave survey, but through the writers' knowledge of their behaviour in the littoral. The “normal” daylight range may be quite wide but there is, in the case of Lichina, Calithrix and Ralfsia a sharp lower limit beyond which they cannot penetrate. This limit is high in comparison with most other species, except possibly Saxostrea. Similarly other shade preferring forms—e.g., Bostrychia and Tethya, have a sharply regulated upper limit of light tolerance. Other organisms, however, do not fit