Giant Shore Anemone

Oulactis magna

Description 8

Common large anemone.
Bottom of tidal pool in sand.

Pedal disc
Basal disc wider than column, forming a flange around the base.

Usually fawn or cream, sometimes dull green or light orange. Wider at top, covered with white to grey or green verrucae bumps which adhere to grit and shell fragments. At the base 70 to 90mm diameter, and up to 100mm high but usually buried in sand so only the top is showing.

Oral disc
About 100mm diameter - wider than column, so creased into a number of lobes. Usually a single colour but might fade towards the outside. May be red, brown, pink, yellow-green, or purple-blue.
At the top margin of the column the verrucae take on a finely branched form which gives the appearance of a white ruff between the tentacles around the outside of the disc (usually white but may vary) . The fronds may have a spherule at their tip.

Short, about 10mm, up to 190 in 4 whorls. May be the same colour as the oral disc, or a contrasting colour or there maybe two colours of tentacles.

Endemic, throughout NZ.

Edited version of Records of the Canterbury Museum, Vol 6, No 1, p117-9, Nov 1951
Article: The Actiniaria of New Zealand - a check-list of recorded and new species, a review of the literature, and a key to the commoner forms Part 1
By G. Parry. :

Genus ISOCRADACTIS (Carlgren, 1924) Actiniidae with well-developed pedal disc. Column cup-like with adhesive verrucae arranged in longitudinal rows, and increasing enormously in number a short distance below the tentacles. Several verrucae, in bunches projecting from a common stalk, here form in each intermesenterial compartment, 'frond'-like formations; these are surmounted by a spherule.

Tentacles numerous, short, conical, hexamerously arranged, the inner a little longer than the outer. Oral disc very wide, folded.

Isocradactis magna (Stuckey, 1909) The basal disc is very well-developed, being a little wider than the column, and forming a flange round the base; it is of thin epithelium and closely applied to the substratum, making it very difficult to remove intact. The column is pillar-shaped, widening towards the distal end. It is covered with numerous hemispherical verrucae, which function as suckers, attaching to themselves all manner of small stones, shells, pieces of weed, etc., so that in contraction, the animal looks like a small pile of stones. These verrucae are arranged in longitudinal rows, but are often so numerous, especially in the upper part of the column, that they appear to have no regular pattern. Towards the basal end, the warts become larger, fewer, and more regularly placed; towards the oral disc they become fused, and are small and closely packed. At the margin they form compound structures, the ' fronds '. Each frond is rather like a tree with branches on one side only, the oral face being straight, the outer branching. Each frond is made up of 30-40 warts if it lies above an exocoel; of 130-140 if it lies above an endocoel. There is no marked difference between these warts and those of the column. They are usually surmounted, at the tip, by a spherule, often white, sometimes coloured. The effect of these fronds is to form a lacy white ruff round the tentacles—it is usually white, but may be variable (see below).

The tentacles are numerous, short, and rather blunt. They are arranged in 4 cycles, but may exceed this, up to 192 tentacles (Carlgren). Their arrangement is hexamerous, and there is little difference in size between the different cycles. They are perforated at their tips, like the verrucae, both compound and simple, and it seems a common reaction, when the animal is stimulated, for contraction to be accompanied by the discharge of fine streams of water from these perforations.

The disc is considerably broader than the column, so that its edge is thrown into deep folds. In expanded specimens the disc is flat, or more often, rather concave, with the tentacles held over it. There is a peristome very slightly above the level of the disc, the two siphonoglyphs being marked by differently coloured tubercles. The sides of the actinopharynx are grooved.

The column is generally light grey, or buff, or occasionally a pale orange, or a dull green. The verrucae are mostly white, but again may be grey, cream, or green. Stuckey reports the column varying from brownish yellow, pinkish yellow, yellowish green, and pale green. I do not think that the range is quite so great, though the North Island specimens may be more brightly coloured. The disc is much more variable. In expansion, the colour appears concentrated at the centre, round the peristome, where it has a velvety texture. The colour fades towards the tentacles, so that further out, the mesenterial insertions show as darker lines, the exo- and endocoels as more translucent areas. Between the bases of the tentacles, the epithelium is almost colourless. There is usually only one colour in this, in contrast to O. muscosa, which has a radiate pattern of red, brown and white. The colour in /. magna may be almost anything, and the most astonishing colours are seen—brown, olive green, yellow green, dark red, cerise, yellow brown, purplish blue—these are only a few of the colours observed. The actinopharynx is white or cream, with a cream tubercle above each siphonoglyph. The tentacles may be the same in colour as the disc; or they may be a contrasting colour; or they may be in two colours. For example, one specimen with a claret coloured disc has olive-green tentacles; another has a cerise disc, inner tentacles orange-brown, the outer ones darker brown.

Size : Height of column 10 cm. or more, width of lower column 7-9 cm., width of disc greater than 10 cm., length of tentacles 1 cm., length of fronds 1 cm.

Stuckey records this animal as being commensal with the common shore crab, Halicarcinus planatus, but I think this must be a mistaken observation. Certainly the two inhabit the same region of the shore—low water, and generally where rocks and sand are mixed—but since I have very commonly found broken pieces of the carapace and legs of this species of crab within the coelenteron of the anemone, it does not seem that the crab is in any way immune to the predatory habits of the anemone. In view of the fact that the actinian is as big and as powerful as the British Tealia felina, which is commonly known to eat and digest quite large fish, it can be reasonably assumed that O. magna can capture Halicarcinus, Collected from Plimmerton (Stuckey), Cape Maria van Diemen (Carlgren), Tauranga; Sumner, Taylor's Mistake, and Menzies Bay, all in Bank's Peninsula (author).

References 8


  • Cradactis magna
  • Isocradactis magna

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  5. (c) Avenue, some rights reserved (CC BY-SA),
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  7. (c) University of Otago, some rights reserved (CC BY-SA), uploaded by Tony Wills,
  8. (c) Tony Wills, some rights reserved (CC BY-SA)

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