The New Zealand sand flounder (Rhombosolea plebeia) is a righteye flounder of the genus Rhombosolea, found around New Zealand in shallow waters down to depths of 100 m.
Common names
New Zealand dab, pātiki, diamond, tinplate, square flounder.
Description
Like other flatfish, the larval sand flounder begins its life with an eye on each side of its head and a round body shape, swimming upright through the midwater. As it grows out of this larval stage entering the juvenile stage one eye moves to the right side leaving the other blind and it takes on a flat diamond shape swimming flat/parallel to the ground. On the right side, the fish is a greenish brown dark colour or grey with faint mottling and on the left side (the side it lies on without eyes) it is white. The average length of an adult sand flounder is 25–35 cm with the maximum being 45 cm. In the day time, they lie on the seabed camouflaged almost perfectly in sand or mud; they have special pigment cells on their skin that can change colour to match their background, their protruding blue-green eyes being their only giveaway. They swim in a flowing style with an undulating movement of the side fins and when threatened by predators their tail is used for propulsion. Technically the adult swims on its side with the continuous dorsal fin fringing one edge of its diamond shaped body and its extended anal fin on the other. It has no swim bladder and only leaves the seabed for courtship and spawning activities.
The New Zealand sand flounder can be distinguished from other flounder species in New Zealand by its yellow underside.
Distribution
Range
Sand flounder are endemic to New Zealand. New Zealand sand flounder is found in a majority of coastal waters around New Zealand. Its largest population is found at Tasman Bay and on the East Coast of the South Island. Around New Zealand they can be found in harbours, inlets, bays and open water.
Habitat preferences
They prefer coastal areas and are found in waters up to 50m deep but rarely deeper. They can be found in harbours, inlets, bays and open water. They are common on mudflats but seem to have no preference of bottom substrate as they are also found on sand, clay, pebbles and gravel bottoms. They also can be found in estuaries. When they are juveniles they are found in sheltered inshore areas such as estuaries, mudflats and sand flats where they will stay for around two years. They also prefer a temperate climate.
Life cycle
The geographic location of the New Zealand sand flounder determines its spawning period. In the north, it has a long spawning period from March to December. In the south, spawning largely occurs in the spring. A study in the Hauraki Gulf found sand flounder lay between 100,000 and 500,000 eggs when spawning. The variation in the number of eggs laid was attributed to the difference in size of the female laying the eggs.
After a period of time dependent on the temperature of the water (usually around a week), the larval sand flounder hatches. Larval sand flounders have a large yolk sac attached to their underside, providing nutrients to the fish until it is large enough to feed itself. At this stage, they are less than a half a centimetre in length. They have an eye on each side of their head and swim upright, as most fish do. As sand flounders grow they begin utilising external food sources such as seaweed spores and algae, and as they grow further, small shrimp and plankton.
The extra nutrients they receive from these new food sources enables them to grow to around one and a half centimetres by the time they are three weeks old. Above each eye of the sand flounder is a bar of cartilage, and at this stage of its development the cartilage above the left eye is absorbed and the eye begins to move from the side of the head, until it is next to the right eye. The unusual, twisted shape of the mouth of the sand flounder is due to the movement of the skull and bones as the left eye migrates to the right side of the body. While this slow process is occurring, the sand flounder begins to grow out to the side and flatten, losing its rounded shape. This metamorphism makes swimming as the larval sand flounder was able to difficult and exhausting. The now juvenile sand flounders sink to the bottom and begin swimming as adult flatfish do, by undulating their side fins and for rapid acceleration, use their tail.