The Ecology of Rock PoolsAuthor(s): Mabel ClaphamSource: The Irish Naturalists' Journal, Vol. 1, No. 3 (Jan., 1926), pp. 48-51Published by: Irish Naturalists' Journal Ltd.Stable URL: http://www.jstor.org/stable/25531169 .

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48 The Irish Naturalists' Journal. [Vol. 1.

THE ECOLOGY OF ROCK POOLS.

By Mabel Clapham, B.Sc.

On rocky shores there is not a more fascinating study, for a

naturalist, than the flora and fauna of the pools left when the tidal waters recede. The fauna has its own interests, and the flora has an interest not only of curious, beautiful or quaint form but also of habits and habitats.

The chief factors determining the presence or absence of

particular species and the extent and locus of their development in any particular rock pool seem to be elevation above low tide

level, depth, drainage, insolation, temperature, and, to a lesser

degree, the size of the pool. Just above high water mark, pools which lie in the spray

zone show an extensive development of green sausages (Entero

morpha intestinalis) and the dark green tufted threads of Clado

phora sericea, both seaweeds which grow well in brackish waters. These pools are replenished by rain and by salt spray in rough weather, and are therefore distinctly brackish. Another form which appears in the more salty of such pools is a brownish, soft, cord-like red seaweed, Dumontia filiformis.

At lower elevations, within the tidal range, a large number of species may be found in the pools. Such pools, in fact, seem to have been the best localities for the evolution of the

higher types of seaweed, and many species are to be found there which are not found in the more turbulent and less favourable conditions of the open shore.

ZONATION. The series of forms which occur in a pool near

Bangor, Co. Down, may be taken as an example of this in

teresting phenomenon. A high vertical rock wall just above the

pool shows a more or less typical succession. Its upper three feet have isolated patches of the yellow rock lichen (Xanthoria parietina), while below this there is a two-feet wide band of another lichen (Verrucaria maura), giving the rock the appearance of being coated with black paint. The coast being sheltered, the belt of channel-wrack (Pelvetia canaliculata) extends only for six

inches, growing in the zone of black lichen. Then comes a well

defined band of the curly or spiral wrack (Fucus spiralis) about a foot in depth, and below this to a little below the water level of the pool extends the zone of noded sac-leaf (Ascophyllum nodosum). The sac-leaf is more sheltered here, on account of

the formation of the surrounding rocks, than it is near some of the higher pools, and thus it has a lower seaward range. (Fig. .1.)

Around this particular pool this belt of sac-leaf has a well defined sub-vegetation, a ground flora, so to speak, in the forest of the larger sac-leaf. Downwards, from eighteen inches below its upper limit, and sheltered by the overhanging brown seaweed are found the pink tufts of Callithamnion byssoides, the graceful red plumes of Plumaria elegans, the broad green

" leaves

" of

the sea lettuce (Ulva latissima) and green tufts of Cladophora rupestris, with, at the lower levels the jointed juice branch or

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January, 1926.] 49

Fig. 1.

A?Xanthoria.

I f\ ^?Verrucaria.

jn C?Pelvetia. D?Fucu&.

K l\ E?^ scophyllum. I F?Plumariat Lomentaria, Ulva, Ceramium, Callithamnion.

J Cladophora. A Cx?Ulva, Lomentaria, Delesseria.

V, p H?Asperococcus, Ditsea, Cystoclonium, Laminaria (young)

^ k Delesseria, Griffithsia, Ulva.

I / T\ I?Laminaria.

j J?Laminaria.

Y' K?Halidrys.

j L?Ulva, Ceramium, Corallina, Chondrus, Delesseria,

j \ Dilsca. CL / M?Fucus.

j\ N?Ulva and Entcromorpha.

spiders' legs (Lomentaria articulata), a variety of dulse [Deles seria alata) and the red filamentous tufts of Ceramium rubrum.

The majority of the species of this ground flora have the thallus of nne, much-branched threads which retain the moisture when

the tide is out. This association also occurs here in patches which are not even shaded by overhanging Ascophyllum, but

apparently in the pool the shaded aspect of the rock face which is towards the north, together with the water-holding capacity of the spongy tufts, is sufficient protection against the possibility of inter-tidal desiccation.

The rocks below some of these pools are extensively colonised by barnacles, but broad sheets of purple laver

(Porphyra umbilicalis), mixed with Cladophora rupestris, tufted small green sausages (Enteromorpha comprcssa),

sea lettuce, the

curly brown Laurencla pinnatifida and the yellow balloons of Leathesia tuber if or mis), are co-dominant with the barnacles to

just above low water. Here dense growths of the grape-stone carrageen (Gigartina mamillosa) and coralline (Corallina officinalis) merge into the Sea Belt (Laminaria) Association of the sub-littoral region.

SEASONAL CHANGES. Taking the same pool as an

example, it was found that in February the red seaweeds were dominant in the pool, growing without shade from the larger

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50 The Irish Naturalists' Journal. [Vol. 1.

brown algae. Wavy-leaved dulse (Delesseria sinuosa) and

carrageen or Irish moss (Chondrus crispus) were most noticeable. Two very well-grown plants of the filamentous rosy red Grifhthsia setacea were at that time observed growing in the open, but they had both vanished by the middle of March, and afterwards

Griffithsia was found growing only in the dense shade.of other

algae. It seems probable that the increased light intensity was the destructive agent in this case. In February also, old worn thalli of digitate sea belt (Laminaria digitata) were fairly evenly distributed through the whole of the pool and these remnants

were infested with brown tufts of Sphacelaria cirrhosa. These

specimens of sea belt showed marked rejuvenation from April onwards, until in June they reached a length of about three feet in some cases.

The plants colonising the sides of the pool in more or less unshaded positions showed little or no change as the season

advanced; e.g., Irish moss, coralline, spiders' legs, the flat fork

ing brown Dictyota dichotoma and the brown curly tufts of Laurencia. There was some slight increase in the development of the various species of Ceramium which form pink to red spongy tufts, and the real dulse (Dilsea edulis) grew rapidly, up to six inches in one month. Similarly new growth occurred steadily in other varieties of the dulse-like Delesserias, until in May the

typical summer form was attained. Dumontia filiformis made its first appearance in May, the plants being at first small with few branches, but increasing in numbers of individuals and of

branches. Even in June, however, the whole thallus was rarely

over four inches in length. EPIPHYTES AND PAKASITES. This pool is not so rich

in these forms as are some of the pools higher up, but the fila mentous brown Ectocarpus grows luxuriantly

as an epiphyte on

sac-leaf, etc. The epiphytic forms, Sphacelaria, Elachisia, Ulva,

Asperococcus, Enteromorpha compressa and Cladophora lanosa

were also found here, usually on the stalks of sea belt, wrack or

sac-leaf. Other epiphytic forms, occurring in the higher pools, are Cystoclonium, Litosiphon. Leaihesia and Chordaria.

SPOELINGS. The lower pool appeared to be eminently suited for the early development of very numerous sporlings. In addition to the supply from the plants growing in the pool, it is probable that the currents in the channel, upon the edge of which this pool was situated, conveyed further supplies amongst the debris which was left by the falling tide. The pool was also sheltered from direct sunlight to a considerable extent by the above-mentioned rock-wall, and this shade also would be favour

able to the sporling growth. DEAINAGE. The saw-edged wrack (Fucus serraius) is

specially luxuriant around the point where drainage takes place as the tide falls. This phenomenon is probably correlated with the similar development of this kind of wrack in zones of

moderately strong wave action, rather than in quieter

zones.

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January, 1926.] 51

Saw-edged wrack is more tolerant of exposed conditions than is the sac-leaf, and it occurs on many shores as a belt connecting the littoral with the sub-littoral regions. At another point where the drainage is only slight the thalli of the sac-leaf trail and over

hang in their usual fashion. INSOLATION. The whole of this lower pool was practically

always shaded during the early months, but the marked effects of insolation can be seen in other pools where the vegetation of the shaded and the unshaded sides shows significant differences.

Apart from the rocks, shade is also provided by saw-edged wrack and sea-oak (Halidrys). These grow luxuriantly, but are not found much beyond the zone exposed to mid-day

sun. Sea

belts also occur shaded by this dense growth, with the usual

ground flora of small red sea-weeds such as the various dulses,

spiders' legs, and Irish moss.

In a dee^p pool, situated about half-tide level, the vegetation on the shaded side was noted as poorly developed, but with red sea-weeds dominant. These included Gelidium carneum,

Delesseria sanguinea, D. alata, Plocamium and RhodomeJa sub

fusca; all forms intolerant of exposure to sunlight, but all growing here unshaded by larger sea-weeds because shaded so well by the rock. Wavy dulse (D. sinuosa), also found in small patches in more exposed positions, covered areas of considerable extent

in the shaded zone, with the more or less shade-loving Dilsea, Cystoclonium, Chondrus, Asperococcus, Punctaria and Ceramium

intermingled. TEMPERATURE. Observations during the summer of

1925 indicate that the surface water of rock pools may have a

temperature several degrees higher than that of the bottom water of such pools, the difference varying with the wrarmth of the day and the length of exposure to the sun during low tide.

The luxuriant growth of seaweeds around the upper parts of rock pools is, it seems, largely due to this relatively high tempera ture and the strength of the illumination which both tend towards increased photosynthesis and growth. The action of other factors

is, however, indicated by the sharp transition seen in some deep pools, where the dense belt of vegetation extends downwards to a depth of about two feet and then abruptly ceases.

TALKS ABOUT MOSSES.

By Bev. W. B. Megaw, B.A.

III. If the question be asked?" Where do mosses grow?" the

answer is?" everywhere." They are to be found at all altitudes

from the lowest swamps and turbaries to the verge of perpetual snow. Several species occur on the snow line in the Alps. In

Spain, on the summits of the highest Sierras and on the loftiest elevations of the Scandinavian peninsula these hardy plants are

found. The range in space of individuals is often extensive. One

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