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Patiriella regularis, otherwise known as the New Zealand common cushion star, is an endemic species to New Zealand. As its common name suggests, it is one of the most common Asteroids in New Zealand; Asteroids are more popularly known as starfish or sea stars and are found in abundance along the coastline in the sub and intertidal zones.

Taxonomy
Patiriella regularis was first described by Addison Emery Verrill in 1867 under the name Asterina regularis; Verrill created a new genus of the Asteriniae family called ‘Patiriella’ in 1913. Subsequently, the cushion star was defined as Patiriella regularis which was accepted.

Description
Patiriella regularis is pentagonal in shape, with five short arms, although individuals have been observed with four or six arms. P. regularis can grow to 6cm in diameter and can appear in a variety of colours above, from olive-green mottled with yellow to orange and browns to more muted colours below. The dorsal surface of the sea star is covered in overlapping plates that give a granular appearance due to its ‘thorny spines’ that run along the arms.

Morphologically Patiriella regularis is similar to Patiriella mortenseni and has been found in the same habitat (Auckland). The major distinguishing characteristics are P. regularis’ bright colouring, wet surface appearance and the crescent-shaped form of abactinal plates that run along the arms of the star.

Natural global range
Patiriella regularis is endemic to New Zealand and primarily located throughout the rocky intertidal zones of coastline.

P. regularis is also found in Tasmanian water. It is thought to be introduced in 1920 through the introduction of Bluff oysters. P. regularis dispersal in Tasmania follows the same model as its distribution in New Zealand, in that it is found in large numbers throughout the habitat.

New Zealand range
As P. regularis’ common name suggests, it is found in abundance throughout the shallow waters of both islands of New Zealand. P. regularis can also be found around Stewart, Chatham Islands and in Milford and Doubtful sounds of Fiordland.

Habitat
Patiriella regularis is found in a variety of coastal habitats in the intertidal and subtidal zones New Zealand’s coastlines. Coastal habitats such as marine cobble fields and rocky shores.

Life cycle/Phenology
Patiriella regularis has an annual reproductive cycle; the species spawns in the summer period from December to February in which females and males will both release gametes.

It takes Patiriella regularis 9-10 weeks from fertilisation to grow from egg to adult form. Patiriella regularis has small eggs that are 150μm in diameter. Their eggs are negatively buoyant, which allows their sticky jelly coating to adhere to substratum (rock & shells) in their habitat. The eggs will then develop into planktotrophic larvae. P. regularis has two stages of larval development. The first is bipinnaria, which is a free-swimming feeding larvae. The second stage is brachiolaria in which the larvae develops three arms (brachia) and a centrally located adhesive disc.

Metamorphosis is the next phase in which the larval body develops into its adult form. The brachiolaria larvae are shortened into a thin attachment stalk, from which the cushion star forms its five armed shape. Tube feet used for adhesion and locomotion form on the star’s surface. Once free of the attachment stalks, the juvenile stars measure 450-500μm in diameter. The star continues to grow until it reaches maturity at a length of 6cm.

Diet and foraging
Patiriella regularis is an omnivore, feeding on organic particles in its habitat. It feeds on crustose coralline algae, micro-organisms that it can digest. Whilst P. regularis can survive on these food sources it needs to feed on mussel carrion, crushed up mussel to gain weight and grow.

Patiriella regularis is a grazing deposit feeder. P. regularis does this via its cardiac stomach. The cardiac stomach is extended out through the mouth, allowing the cushion star to digest food outside of its body and sucking other available nutrients into its stomach.

Predators, Parasites, and Diseases
Patiriella regularis is an oviposition host for caddisfly (''Philanisus plebeius). P. regularis'' lay their eggs via their ovipositor. P. plebeus’ ovipositor is long and narrow enough that it fits through the papular pores on the surface of Patiriella regularis and into the coelom of the cushion star. Once the caddisfly eggs have been incubated in the coelom, the larvae leave the star via the papular pore or through the stomach wall.

Other Information
A genetic variance has been identified in P. regularis populations in the South and North islands. The geographic barriers and coastal upwellings are thought to provide a barrier to gene flow among the New Zealand populations. Upwellings can do this through larvae exposure to cold upwellings outside of optimum temperature and transporting larvae offshore impacting larval survival.