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Corals, Medusae, and Hydroids (Phylum Cnidaria)

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• General Description
• Status
• Key Locations
• Summary of Threats
• Typical Habitats
• State of Information
• Significance for Maori
• Key References
• Figures

Dennis Gordon and Franz Smith

General Description

Corals, medusae, and hydroids belong to the phylum Cnidaria. They are all characterised by small stinging cells (nettle cells), called nematocysts or cnidae, which are unique to the phylum, have a body plan with radial symmetry, and only two cellular layers, separated by a layer that can be very thick and jelly-like. The basic body shape is sac-like, with a mouth surrounded by tentacles and no anus. The tentacles contain many thousands of nettle cells, sometimes in dense clusters. These cells are extraordinarily complex structures that are essentially inverted coiled tubes within a cell. When triggered, the tubes are explosively everted like microscopic harpoons, their barbed tips penetrating cells of other creatures and discharging toxins. Cnidarians are basically carnivorous, whether or not they feed on large prey like fish, or tiny plankton. Another distinctive feature of the phylum is that many species are dimorphic, having two body forms, polyp and medusa.

Worldwide there are almost 9500 recognised species of Cnidaria. Owing to recent molecular research, the phylum is nowadays divided into five classes. The largest is the Anthozoa (corals and anemones), with about 6000 species. They have only the polyp phase. Many Hydrozoa (hydroids and hydromedusae) and Scyphozoa (swimming jellyfish) have both phases or the polyp phase is lost. In the Staurozoa (stalked jellyfish) and Cubozoa (box jellies) the microscopic polyp is transformed directly into dominant medusa, which is normally the sexual phase in cnidarians. The ability of the polyp phase to bud asexually in many cnidarians allows them to form large colonies, for example stony corals, sea fans, black corals in the Anthozoa and the planktonic siphonophores in the Hydrozoa. In the siphonophores and some other groups, the polyps can be differentiated into separate feeding, defensive, reproductive, and other individuals, a phenomenon known as polymorphism, which is better developed in the Cnidaria than in any other phylum.

Cnidarians are conspicuous organisms in marine ecosystems, whether as planktonic medusae and siphonophores, or as sea-floor-dwelling corals and anemones. They can be abundant from the intertidal zone (sea anemones and hydroids) to the deep sea and from polar regions to the tropics, where they can form reefs covering thousands of square kilometres. Large forms, like stony corals, black corals, gorgonian octocorals, and some hydrozoan species can provide habitat and shelter (as well as a food source) for many fish and invertebrates.

A total of 1044 cnidarian species are known in New Zealand waters (five of them freshwater). At least 218 species are endemic but the real number is likely to be higher as there are 326 marine species that have not yet been fully identified or described. Unfortunately, there are no cnidarian experts in New Zealand. Around 17 species are introduced aliens. Most of these are associated with wharf pilings and vessels' hulls in ports and harbours. Overall, New Zealand's cnidarian fauna is relatively diverse, with the calcareous stylasterid hydrocorals (55 species), soft corals (gorgonians, alcyonaceans, and sea pens) (243 species), and sea anemones and kin (106 species) particularly well represented. Overseas specialists estimate that perhaps another 250 cnidarian species remain to be discovered in New Zealand's exclusive economic zone.

Although jellyfishes are easily recognised and widespread in the world's oceans, there aren't that many different kinds, with fewer than 300 species worldwide, distributed in three classes. In New Zealand, only twenty-five scyphozoans have been reported, three species of stalked jellyfish (one not seen since its discovery at Port Chalmers in the 1880s), and one box jelly (tiny Carybdea sivickisi, found in Cook Strait).

The Anthozoa is the only class of Cnidaria that has completely lost the medusa stage. Anthozoans are primarily suspension feeders of a broad range of plankton groups and larval forms of invertebrates and fish, while the larger sea anemones are predators of mobile invertebrates and fish. Many sea anemones and all reef-forming corals can also derive energy from symbiotic algae (dinoflagellates known as zooxanthellae). There are several distinctive orders within this class, all with different life-forms and ecologies. Order Actiniaria comprises the sea anemones), exemplified by olive-green intertidal Anthopleura aureoradiata and the wandering anemone, Phlyctenactis tuberculosa. Order Corallimorpharia, commonly referred to as "jewel anemones", are distinguished from other anemones by the bulbous cnidae-studded tips of their tentacles, and can form brilliantly coloured aggregations of clonal individuals, as in Corynactis australis. Order Zoanthidea, the zoanthids, can be distinguished by smooth tentacles in one or two rows and can form clusters of clonally reproduced individuals. Several species of zoanthid can be found living in association with black corals and sponges. Order Scleractinia, the stony corals, comprises hermatypic (zooxanthellate and mostly reef-forming) and ahermatypic (non-zooxanthellate, non-reef-forming) species. Hermatypic corals are found at Raoul Island on the Kermadec Ridge (but do not actually form reefs there), while the remaining species in New Zealand are ahermatypic corals. Some ahermatypic species grow as a single polyp and are commonly referred to as "cup corals", such as red-orange coloured Monomyces rubrum. Others are colonial and can form large, intricate thickets. Order Ceriantharia includes the tube anemones, which live in sand-encrusted mucous tubes and have two sets of tentacles, one surrounding the mouth and another elongated set extending from the edge of the oral disc. Species of the order Antipatharia, the black corals, form tree- or whip-like colonies supported by a rigid skeleton made of a horny material. Small polyps are arranged on a thin veneer of flesh covering the skeleton.

All of these anthozoans are known as hexacorals, with internal body partitions and tentacles in multiples of six, contrasting with eight in the remaining anthozoans, the octocorals. There are three orders. The Alcyonacea include dead men's fingers and related forms, the Pennatulacea the sea pens, and the Gorgonacea the gorgonians - sea fans, bamboo corals, bubblegum coral, and their kin. Most of New Zealand's rich octocoral fauna has not been named or described.

Relatively few cnidarians pose threats to New Zealanders, unlike in tropical waters, and reports of stings are sporadic. The Portuguese man-of-war or bluebottle, Physalia physalis, can sting, as can the species of lion's-mane jellyfish Cyanea, and the siphonophore Apolemia uvaria in the Hauraki Gulf.

Although New Zealand has no coral reefs, several species do form important habitats for other marine life, including the large fleshy octocorals and hermatypic stony corals at Raoul Island and the large thicket-forming cold-water corals on the Chatham Rise and other deep-water locations. Carbon-14 dating of several large species in New Zealand waters have demonstrated some remarkable longevities. Giant bubblegum coral, Paragorgia arborea, can attain 400 ± 100 years and more than 5-6 m in height and a species of bamboo coral, Keratoisis, achieves the same age. Black coral, Antipathella fiordensis, in Fiordland achieves 5 m in height and lives for 200-300 years. Other species are known to be dependent on these large corals for their existence. Orange roughy is known to be associated with cold-water coral banks in New Zealand waters, which makes the coral vulnerable to trawl-fishing for orange roughy.

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Status

All Antipatharia (black corals) and Stylasteridae (hydrocorals) are protected under the Wildlife Act and an application has been made (March 2006) by the Royal Society of New Zealand Committee on Biodiversity for formal protection of giant bubblegum coral, Paragorgia arborea, in New Zealand waters under this Act. All corals in marine reserves are automatically protected according to the articles of the Marine Reserves Act. In particular, this is important for the hermatypic (though non-reef-forming) corals at Raoul Island.

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Key Locations

Cnidarians are important components of some parts of the shallow subtidal benthos at Raoul Island (Kermadec Islands Marine Reserve), as they are on rock walls at the Three Kings and the Poor Knights Islands, and at Mimiwhangata. At all of these locations, large colonies of stony corals, alcyonaceans, or gorgonians (eg, yellow Euplexaura sp. and an unidentified pink primnoid) occur. Some parts of the Chatham Rise and some seamounts carry significant thickets of intricately branching stony corals, in particular Goniocorella dumosa, Solenosmilia variabilis, Oculina virgosa, and Madrepora oculata. Antipatharian corals occur in Fiordland, on rock walls at the Three Kings and the Poor Knights islands, and on some seamounts. The latter also carry populations of large bamboo corals (Keratoisis and related taxa) and Paragorgia arborea. Some very large hydroids occur in Spirits Bay, including Nemertesia elongata and Crateritheca novaezelandiae, and they are important substrata for some associated epibionts, in particular a new endemic genus and species of barnacle, Chitinolepas spiritsensis.

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Summary of Threats

Bottom trawling poses the greatest threat to large, three-dimensional epifaunal

cnidarians anywhere on the sea floor. Some attractive corals have been collected by divers in the past but some key species and localities are now protected in marine reserves. The most accessible black coral in New Zealand, Anthipathella fiordensis in Fiordland, is not suitable for jewellery-a reason for opportunistic collecting of some antipatharians in the past (but they are all now nominally protected in New Zealand).

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Typical Habitats

Cnidarians occur almost anywhere in the sea, from the intertidal to abyssal depths and in the plankton. In the warm waters of Raoul Island, zooxanthellate corals occur. Around mainland New Zealand, rock walls and other hard substrata in clean current-swept locations are ideal for many taxa. The soft sea floor accommodates some burrowing or rooted species (sea anemones, cerianthids, and sea pens). Some taxa are intimately associated with seaweeds and have cryptic colouration. In shallow harbours and estuaries, especially in summer, swarms of jellyfish and siphonophores can occur.

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State of Information

Since 1991, NIWA has published several monographs on New Zealand cnidarians, including all hydrozoan groups except siphonophores, and stony corals. Major gaps in monography include gorgonians, antipatharians, sea anemones, zoanthids, and jellyfish.

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Significance for Maori

No specific use or significance of Cnidaria to Maori is known. The fourth edition of Powell's Native Animals of New Zealand (David Bateman, 1998) gives the Maori name "poku kawekawe" for coelenterates in general.

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Table 20: Marine Corals, Medusae, and Hydroids (Phylum Cnidaria) in New Zealand

Class	Order	Endemic Species	Other Species	Total Species
ANTHOZOA	ACTINIARIA	19	66	85
	ALCYONACEA	5	40	45
	ANTIPATHARIA	14	44	58
	CERIANTHARIA	0	2	2
	Corallimorpharia	4	3	7
	GORGONACEA	33	134	167
	PENNATULACEA	0	31	31
	SCLERACTINIA	17	111	128
	ZOANTHIDEA	0	12	12
Total		92	443	535
SCYPHOZOA	CORONATAE	1	6	7
	Rhizostomeae	2	3	5
	Semaeostomeae	0	13	13
Total		3	22	25
STAUROZOA	Stauromedusae	1	2	3
Total		1	2	3
Cubozoa	Cubomedusae	0	1	1
Total		0	1	1
HYDROZOA	Anthoathecata	42	112	154
	CALYCOPHORAE	0	33	33
	CYSTONECTAE	0	1	1
	LAINGIOMEDUSAE	1	0	1
	LEPTOTHECATA	78	168	246
	LIMNOMEDUSAE	0	2	2
	NARCOMEDUSAE	0	14	14
	PHYSONECTAE	1	10	11
	TRACHYMEDUSAE	0	18	18
Total		122	358	480
Grand Total		218	826	1044