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Atrina zelandica, one of several species known as the horse mussel, is a large species of saltwater clam in the family Pinnidae, the pen shells. This species is found around New Zealand.

Description
The shells of the Atrina zelandica are triangular elongate blackish with the inside being purplish pearly with valves and convex with rather close longitudinal ribs armed with semi cylindrical hollow spines this differs from P. squamosa in that it is smaller, black and in the end being more truncate. The body of Atrina zelandica is bilaterally symmetric meaning it is possible to divide the animal into two nearly identical halves .the body of Atrina zelandica has a length of 280 mm volume of 541 cm cubed.

= Geographic Distribution and habitat = Atrina zelandica is native to New Zealand, Atrina zelandica can be found all along the coastlines of the country, thriving in the marine environment surrounding both islands. They inhabit depths of up to 50 m and are widely distributed across the New Zealand. They primarily inhabit muddy sand substrates in inter tidal and sub tidal shallows within sheltered waters. Atrina zelandica is the largest of all bivalve species in New Zealand, with some individuals of Atrina zelandica growing up to 400 mm in length. Most individuals of Atrina Zelandica measure between 260 and 30 mm in length and a width of 100 to 110 mm. Atrina zelandica form patches from 10 m2 or even greater they do this by embedding their shells into the substrate by their pointed anterior end.

= Life cycle/Phenology = Atrina zelandica can spawn throughout much of the year and are classified as dioecious broadcast spawners which means they release a large amount of sperm and eggs into the water and are absent during raising of their young. Atrina zelandica spawns during the summer months there is currently no available research regarding ages and sizes at which Atrina zelandica begins breading. Once Atrina zelandica hatches it begins swimming until if finds a suitable location to settle this can sometimes take several weeks. It is still not known whether these locations are the same as where adult Atrina zelandica reside or if there is a nursery area they inhabit before moving to primary settlements. Atrina zelandcia grows rabidly during its first couple of years with shells of 120 mm in length grew 40 mm in just one year once the shells reach 166 mm there growth slows significantly to the point were larger individuals are between 5 years and 15 years old.

= Diet / feeding = Atrina zelandica exhibits an extraordinary ability to filter and separate minute particles, such as algae, bacteria, and sediment, from the water. The filtered particles are then deposited on the seabed to be utilized by other organisms, thus contributing to the ecosystem's sustainability and functioning.

= Parasites = Atrina zelandica is parasitized by a species of pea crab (Pinnotheres) endemic to New Zealand that is found in two other species of mussel. The pea crabs can cause significant problems for the Atrina zelandica as they parasitise most of the food slowing the growth and causing a deterioration in the health of Atrina zelandica .

= Other information = The shell of the Atrina zelandica has a crenellated posterior edge that extends a few centimeters above the substrate. This helps keep the water intake clear of surface deposits. Additionally, the crenellated edge provides a surface for various algae and invertebrates to attach to it, such as sponges and sea squirts. Atrina Zelandica has experienced frequent die-offs In the Rangitoto Channel, for instance, populations of 200-300 individuals per square meter decreased to just 1-35 individuals per square meter over a period of 2-3 years. Possible explanations for this include damage caused by storms, shell damage leading to subsequent predation, and exceeding the carrying capacity of the environment ;.