For most of us, peering into rockpools was our entry into the weird and wonderful world of our oceans. Hours spent watching crabs dart in and out of their narrow crevices, investigating the patterns and textures of limpets and seaweed, delighting at the shape of sea stars at the water’s edge, and glimpsing tiny fish flitting around at the corner of your eye. It was heaven.
But nothing could quite beat the sensory thrill of gently sticking your finger into a strange, soft, flower-like creature attached to the walls of the rockpool – its instant recoil usually eliciting a squeal of horror and delight! To celebrate Seaweek and its theme of ‘Connecting with our Seas’, we needed no further excuse to celebrate sea anemones of the Hauraki Gulf Marine Park/Ko te Pātaka kai o Tīkapa using NIWA’s amazing marine identification e-guides. But first, let’s get the pronunciation right – “An M O Knee!”
Anemones are not only found in rockpools, but also under rocks in calm bays, in the splash zone of the rocky shore, in mudflats, and also in the deepest reaches of the ocean in our abyssal trenches. Some even detach themselves and float around in the water column to catch their prey. Globally, anemones are found in tropical waters right through to the freezing waters of our poles. Here in Aotearoa, we are lucky to have anemones in all shapes, colours and sizes.
Anemones belong to the Cnidaria family (pronounced ‘naidaria’), which also includes jellyfish and corals. All species belonging to this family have radial symmetry to their body shape which means their body parts are arranged around a central axis, like pieces of a pie. The name Cnidaria is derived from the Latin word ‘cnidae’, meaning ‘nettle’, providing a clue about cnidarians: all have stinging cells (nematocysts) which they use to capture prey and protect themselves against predators. Sea anemones use their nematocysts to sting and immobilise small animals and plankton, using their tentacles to transfer prey to their mouth.
So, what is the basic structure of an anemone? Anemones all have a column with a cavity inside called a coelenteron to digest food, absorb nutrients and respire – essentially a stomach that they can also breathe through! This column is connected to an oral disc with a mouth in the middle, and tentacles around the outside. Some have a pedal disc that is used to firmly or loosely fix onto a surface, while burrowing anemones have a physa, an inflatable bulb that helps them to anchor themselves in sand or mud. Simple, but beautiful.
Spawn, split or spew
What makes sea anemones particularly fascinating are the differing ways they reproduce. As with many marine species, sea anemones perform broadcast spawning where they release eggs and sperm into the water column (sexual reproduction), and the resulting larvae then swim around until they find a suitable habitat to settle. But sea anemones can also reproduce asexually, either by fission (splitting in half or fragmenting), or by budding from the base. Amazingly, some species are viviparous, reproducing through internal fertilisation, like humans. They hold on to their young inside their body cavity (or in a special brood pouch in some species), releasing the mini anemones out of their mouths!
It’s also worth noting that shallow water anemones have an important mutualistic relationship with certain single-celled algae that live in the tentacles and oral disc of the anemone. These algae photosynthesise, providing the anemone with oxygen and food, and in turn the algae benefit from reliable exposure to sunlight and protection. The colour of the algae determines the colour of the sea anemone.
There are four subgroups of sea anemones, or anemone-like creatures (more on that later!) which NIWA’s Adorable Anemones e-guide covers off including Actiniaria (sea anemones), Corallimorpharia (mushroom or jewel anemones), Spirularia (tube anemones) and Zoantharia (zoanthids). In the Hauraki Gulf / Tīkapa Moana you can find many sea anemones belonging to the Actiniaria family, as well as one rather delightful jewel anemone from the Corallimorpharia family. Here are a few:
Life among the rocky Gulf shore
The giant shore anemone / kōtore tino nui (Oulactis magna) is, as you would imagine, a large and beautiful sea anemone at 10-12cm wide that is easy to spot and very common in rockpools. It has up to 190 tentacles, arranged in whorls of four, all in one colour which can range from white, pink or purple. Its wide oral disc contrasts in colour to the tentacles in hues ranging from cream to brown to fluorescent green. Interestingly, its short column is usually covered with small pebbles and shell hash, a strategy that is thought to reduce erosion. Giant shore anemones feed on shrimp, small fish and small crabs, which they suck into their central cavity and break down with digestive enzymes, expelling the indigestible items back out through their mouths. Recent research suggests that they may live for decades.
The beadlet anemone / kōtore moana (Actinia tenebrosa) is a tough little nut that puts up a fight for its spot on the rocks. Depending on low or high tide, you will observe them in two distinct forms: a blob of dark purple jelly when out of water at low tide, and a beautiful red anemone when submerged (measuring around 7cm). It gets its colour from the algae that lives within its tentacles and oral disc and is one of the rare anemones that is viviparous, meaning its offspring are held in the gut cavity and released from the mouth. This sea anemone will fight with other species, including anemones, for their space, using an extra set of blue/white tentacles beneath the main ones called spherules or acrorhagi. The beadlet anemone is an easy one to spot at our beaches and rockpools, but a handy trick is to look in shadier areas, under ledges, in cracks and underneath boulders.
Getting stuck into our muddy shores
Mudflats are habitats found in bays and estuaries throughout the Hauraki Gulf / Tīkapa Moana, formed by sediments that have been deposited by tides or rivers. Here you can find the tiny (1cm) mudflat anemone (Anthopleura aureoradiata), partially buried in the muddy sand or attached to mudstone. You may even find them attached to cockles, preying on the larvae of the trematode parasite that can infect these shellfish. Much like its environment, the mudflat anemone is light brown to grey in colour, its tentacles banded or spotted with white. It reproduces asexually by budding – get the kids to inspect mudflat anemones nearby; if the patterns found on the oral disc are identical, they are clones. This anemones nutrition is provided by both symbiotic algae found in the cells lining its digestive cavity, as well as minute planktonic animals it filters from the water.
Snorkelling for a wart-covered wanderer
The wandering anemone / hūmenga (Phlyctenactis tuberculosa) has to be seen to be believed! Resembling a pile of fantastical baked beans more than a flower, the wandering anemone (9cm) is found in the shallow subtidal zone off rocky coasts among seaweed. This is an anemone on the move for its food, with a base that is slightly adherent so that it can float or crawl and reattach to surfaces easily. The wandering anemone feeds mostly at night, drifting, rolling and tumbling with its tentacles extended, slumbering during the day attached to seaweed. You can mostly just see its column which is entirely covered in simple, smooth, blister-like warts (verrucae). These ‘warts’ can be orange to olive-green, to red-brown, to pale pink and light to dark grey-blue. Its tentacles, when not retracted, are short and arranged in six cycles, coloured yellow, brown or orange.
Diving for a jewel
The jewel anemone (Corynactis australis) is a beautiful anemone that comes in a stunning array of colours ranging from pink, brown, fluorescent green, yellow and apricot. They are called jewel anemones for the small ball found on the end of their translucent tentacles, called acrospheres. However, this anemone isn’t really an anemone at all – it belongs to the Corallimorpharia family, making it more closely related to a cup coral. The only clue to its true nature is its calcareous basal plate which corresponds to the beginnings of a stony coral-like cup. This species reproduces asexually by splitting apart to form several new animals, so you are likely to find them in close groups together. If you are lucky, you may find them in deep tidal pools, but you will have better luck diving in subtidal areas on the open coast, looking for them on shaded vertical walls and under ledges in places such as Aotea/Great Barrier Island.
So, go on, for Seaweek, reconnect with you inner child, using NIWA’s Adorable Anemones e-guide, and confirmed sightings on the iNaturalist website, to find and observe these incredible flower-like animals of the sea. If you want to find out even more about sea anemones in Auckland / Tāmaki Makaurau, check out diver Matthew Jones blog shapeandtheidea.com.
Love it, Restore it, Protect it.