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Patiriella regularis, also known as New Zealand common cushion star and Pātangatanga, is a sea star of the family Asterinidae, endemic to New Zealand. There are 2 species of Patiriella endemic to New Zealand and 9 native to Australia. They come in a wide range of colours and have 4-6 arms with an average radius of 80mm.

It is one of New Zealand's most common rocky shore starfish and is abundant in shallow water around both mainland islands, Stewart Island, and the Chatham Islands. They were also introduced into waters in south east Tasmania, where they have become well established.

They are opportunistic omnivorous scavengers which have a varied diet including algae, carrion, barnacles, molluscs and crustaceans. It intakes nutrients by pushing its stomach out of its mouth like a cushion and digesting the nutrients outside of its body.

They breed in the southern hemisphere summer months and produce planktotrophic larvae which take 9-10 weeks to develop in the water column into juveniles.

Taxonomy
The New Zealand common cushion star was originally described as a new species by American invertebrate zoologist Addison Emery Verrill in 1867. He named this species Asterina regularis. In 1913 Verrill revised the genera of starfishes in the sub-family Asterininae, and renamed Asterina regularis to Patiriella regularis. It was previously thought that there was only one species of Patiriella endemic to New Zealand, P. regularis, however this has been reviewed and there are now considered to be two species of Patiriella endemic to New Zealand. There are 9 species of Patiriella which can be found in Australia.

Description
The New Zealand common cushion star is usually a pentagonal shape with 5 arms but has been found with between 4 and 7 arms. The arms are merged with a central disk which is approximately two thirds the length of the arms or less. The average adult radius of Patiriella regularis is 80mm. The upper surface of the sea stars body is covered in overlapping, irregularly arranged, scale like plates. These plates are crescent shaped along the centre of the arms. The colouring of P. regularis is highly variable. Predominately it is olive green but can be found in a range of colours including yellow, orange, mauve and brown. The colouring can also be mottled. The mouth of the sea star is located on its underside which is much paler than the upper side of the sea star.

Distribution
Patiriella regularis is one of the most common rocky shore sea stars in New Zealand. It can be found in the shallow waters of both the North and South Islands, as well as Stewart Island and the Chatham Islands. P. regularis is found in a wide range of coastal habitats but most commonly lives in rocky intertidal to subtidal areas generally down to a depth of 30 meters. The deepest recorded depth they have been observed at is 100 meters. It can survive in estuaries where the water has low salinity as well as the surface water of fjords in the southwest of New Zealand. P. regularis is the most tolerant known sea star to low salinity conditions. They generally live in water temperatures colder than 20°C. These sea stars prefer places where the water currents bring it food, be it from the shore or from deeper water. P. regularis has also been introduced to Tasmania, with local records of it dating back to only 1952. There are concerns that they may be an invasive species in Tasmania.

Life cycle
Patiriella regularis undertake sexual reproduction. They spawn in the summer months in what appears to be synchrony. Like fish they spawn by releasing their eggs and sperm into the water column in their thousands, allowing for the external fertilization and development of the eggs. The eggs are around 0.2mm across. The eggs develop into planktotrophic larvae, which feed on phytoplankton and zooplankton while they live in the water column. They live as planktotrophic larvae for 9-10 weeks, metamorphosing in several stages. First it flattens out into its 5 armed shape. Then it develops the beginnings of its spiny skeleton and 3 pairs of tube feet along each arm. At around 67 days old and 0.3mm across it has reached the juvenile stage and looks like a miniature version of the sea star. At 1 year old the juvenile P. regularis will be around 4mm across the arms. P. regularis become sexually mature at around 3 years old. The lifespan of P. regularis is around 10 years, and most are thought to die from old age rather than being eaten.

Interactions
Patiriella regularis is an opportunistic omnivorous scavenger with a varied diet. It is most often observed eating crustose coralline algae and carrion but also eats other foods such as barnacles, molluscs and crustaceans. P. regularis absorbs food outside of its body by pushing its stomach lining out of its mouth to cover the food source. The food, which is trapped beneath its stomach, is then digested using digestive enzymes before the sea star retracts its stomach into its body, swallowing very little. It can take P. regularis up to half a day to digest a snail. As omnivores they have the potential to strongly influence the dynamics of the communities they live in. This is because omnivory adds complexity and lessens fluctuations in the food web, stabilizing the web depending on the versatility of the omnivore in question. Being a scavenger also enhances the sea stars importance in the food web as it contributes to the cycling of nutrients. This ability to impact the stability and dynamics of its community is a cause of concern in Australia where P. regularis has been introduced. There are concerns it could out compete native species in Australia and have a negative impact on the ecosystem.

There is little evidence of this sea star having predators once it has grown to a reasonable size after leaving the larvae stage. This is probably because they are inconspicuous and small, with gritty leathery skin, and very little meat on them as they are 90% water.

Further information
A genetic variance is found in P. regularis from the North and South Islands of New Zealand. Geographic barriers, and coastal upwellings might keep various types of P. regularis from being able to spread or comingle.