Anthozoans

Anthozoans are a class of animals belonging to the larger taxonomic group Cnidaria and are exclusively aquatic. They include many colourful organisms such as the reef building ‘stony’ corals, sea anemones, tube-dwelling anemones, sea fans and sea pens. Unlike other cnidarians (such as the jellyfish), Anthozoans live their entire life as polyps attached to a substrate. The name anthozoa means ‘flower like’ and refers to the appearance of the polyp which is a simple tubular structure crowned with a ring of tentacles. There are approximately 7,500 species of anthozoan world wide ranging in habitat from the intertidal to the deep sea. Scientists believe that anthozoa are an extremely old class of organisms, originating in the Pre-cambrian era (> 600 million years ago). However, this is still a highly contested theory.

The main group of anthozoans found in Waikato estuaries are the tube-dwelling anemones or cerianthids. These are physically very similar to other sea anemones but, as their name suggests, tube-dwelling anemones have a fibrous tube like structure below their tentacular crown. The organisms are solitary and can withdraw inside their tube for protection. Sea anemones are carnivores that eat fish, mussels, zooplankton (like copepods, other small crustaceans, tiny marine larvae), and worms. They have few natural predators. Cerianthids live exclusively in soft sediment systems and are often buried, making them difficult to see. They are often found attached to the shell of common bivalves, particularly cockles. Information on the most common anthozoan species we monitor in is given below.

Key features
Anthozoans are morphologically very simple organisms. They consist of a tubular body terminating in a mouth surrounded by a ring of tentacles. The tentacles are often arranged around the edge of the mouth disk and, much like jellyfish, have tiny stinging cells on them known as nematocycts. As the tentacles move in the water column, the stinging cells are used for food capture, manipulation, ingestion and defence. The polyp body wall consists of an internal layer (endoderm) and external layer (ectoderm). These are separated by a jelly like substance known as mesoglea. Some species also have a symbiotic relationship (beneficial to both) with dinoflagelate microalgae known as zooxanthellae which exist within their body wall. Both algae and anthozoan can utilise each other’s waste product as an energy source. The zooxanthellae are also afforded protection by the anthozoan. The bright colours of anemones and coral are also due to the zooxanthellae they harbour. Anthozoans attach to substrates using an adhesive foot known as a pedal disk rendering them immobile.
Life cycle
Some anthozoans have the ability of both sexual and asexual reproduction. Asexual reproduction occurs through budding (where a tiny version of the animal is formed on the side of the animal, then released in to the water). In sexual reproduction Anthrozoan eggs can be fertilised internally or externally through release of sperm and eggs in to the water column. Juveniles undergo a free swimming larval phase (planula), before settling on to an appropriate substrate where it becomes a polyp.

Generalised sea anemone anatomy (Source: Brian McCloskey)

Descriptions of species commonly found in Waikato estuaries

Anthopleura aureoradiata (Photo courtesy of Leon Altoff, www.bluering.org.au)

Anthopleura aureoradiata attached to a pilon with tentacles extended for food capture (Photo courtesy of Michela Mitchell, www.taxonomyservicesaustralia.com)

Common name: Mud anemone, small brown anemone
Latin name: Anthopleura aureoradiata
Scientific group or class: Anthozoa
Description: Anthopleura aureoradiata is a small brown predatory tube-dwelling anemone (up to 10mm in diameter) with a cylindrical body which is wider towards the tentacles. It uses the tentacles for food capture, manipulation, ingestion and defence. The tissues of the anemone support colonies of green plant cells called zooxanthellae.
Habitat: Anthopleura aureoradiata occurs in tide pools and on mudflats in estuaries, often attached to shells (for example cockles), stones or wood. It is intolerant of low salinity.
Distribution: New Zealand wide (estuaries)
Indicator species: Yes
Why is this a good indicator species? Anthopleura aureoradiata tolerates a sediment mud content of up to 40%, with an optimum range of 0-10%. It is intolerant of high turbidity. Therefore Anthopleura aureoradiata is usually found in more sandy habitats. This species is also very sensitive to copper contamination. Where estuarine sediments become muddier (exceeding its optimum range) and/or polluted (with copper in particular), the abundance of Anthopleura aureoradiata is likely to decrease. This makes Anthopleura aureoradiata a good indicator species with which to assess change in the input of sediment and pollutants into our estuaries.

Anthozoans

Key features

Life cycle

Descriptions of species commonly found in Waikato estuaries

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