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Phylum Chordata > Subphylum Vertebrata > fishes
Flatfishes
Order Pleuronectiformes
updated Oct 2016

if you learn only 3 things about them ...
They are 'one-sided' fishes as adults, but start life looking like other 'normal' fishes. Their transformation is fascinating.
They can crawl slowly using the fins that border the body edge.
They are well camouflaged and some are tiny. Don't step on them!

Where seen? Like animated leaves, these fishes are encountered on many of our shores. Tiny juvenile soles are sometimes seen by the eagle-eyed visitor on the sand, especially in seagrass areas. These may be as tiny as 2cm long. Some small ones may be mistaken for flatworms! Larger adults (20-40cm) are sometimes encountered too, usually when they are accidentally stepped upon.

What are flatfishes?
Flatfishes belong to the Order Pleuronectiformes. These fishes are flat, with eyes on one side of their body.

Single-sided Fish: Flatfishes undergo an amazing change as they grow up. When it first hatches, a flatfish larva looks like the larva of other 'normal' fish. As the larva matures, it starts to swim on one side of its body.

One eye moves to what becomes the upperside, also called the eyed side. The mouth and one pectoral fin also becomes asymmetrically distorted.

There are also changes in the skeleton and digestive system. The change may be completed within five days.

Here is a fascinating photo of flounder larva and of flatfish hatching on Image Quest 3-D Marine Library

The underside or blind side tends to be flat and pale. The eyed side has camouflaging colours and patterns. Many flatfishes can change the colours and patterns of the eyed side to blend in with their surroundings. Some species are banded like a zebra on the eyed side!

Roving eye:
If the right eye migrates to the left side, the flatfish is left-eyed (sinistral). If the left eye migrates to the right side, the fish is right-eyed (dextral). Left-eyed flatfish include the Family Paralichthyidae (lefteye flounders). Right-eyed flatfish include the Family Pleuronectidae (righteye flounders); Family Soleidae (true soles) and Family Cynoglossidae (tongue-soles). Here's more on how to tell apart flatfish families commonly seen on our shores..

Best on the bottom:
Adapted for life on the sea bottom, flatfishes have been observed 'creeping' along the bottom using the fins that edge the body. They swim by undulating their bodies.

What do they eat? The adult flatfish is an ambush predator. It usually lies just beneath the sediment or sand, with only its eyes sticking out. It snaps up small bottom-dwelling worms, crabs and prawns. It also snacks on buried animals such as bivalves.

Human uses: Many flatfishes are edible and some species are important commercially. Soles are said to retain their flavour for days.

Status and threats: Our flatfishes are not listed among the threatened animals of Singapore. However, like other creatures of the intertidal zone, they are affected by human activities such as reclamation and pollution. Poaching and overfishing can also have an impact on local populations.

All that can be seen of the
huge buried fish are its eyes!
Chek Jawa, Jan 04



Swallowing a very long worm!
Changi, Jul 07



The underside is usually pale
and unmarked.


The pectoral fin on the underside is usually
reduced. The mouth on the underside
may bristle with tentacles and teeth!



Order Pleuronectiformes recorded for Singapore
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.
*Lim, Kelvin K. P. & Jeffrey K. Y. Low, 1998. A Guide to the Common Marine Fishes of Singapore.
+Other additiona (Singapore Biodiversity Records, etc).

  Family Bothidae (lefteye flounders) with list of species recorded for Singapore

  Family Paralichthyidae (large-tooth flounders) with list of species recorded for Singapore
formerly included in Family Bothidae

  Family Pleuronectidae (righteye flounders)
  Samaris cristatus

  Family Psettodidae (halibuts) with list of species recorded for Singapore

  Family Soleidae (soles) with list of species recorded for Singapore

Family Cynoglossidae (tongue-soles) with list of species recorded for Singapore

Links
References
  • Marcus F. C. Ng. 16 May 2014. Juvenile flatfish at Keppel Bay Marina. Singapore Biodiversity Records 2014: 132
  • Lim, Kelvin K. P. & Jeffrey K. Y. Low, 1998. A Guide to the Common Marine Fishes of Singapore. Singapore Science Centre. 163 pp.
  • Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore. National Council on the Environment. 163pp.
  • Allen, Gerry, 2000. Marine Fishes of South-East Asia: A Field Guide for Anglers and Divers. Periplus Editions. 292 pp.
  • Kuiter, Rudie H. 2002. Guide to Sea Fishes of Australia: A Comprehensive Reference for Divers & Fishermen New Holland Publishers. 434pp.
  • Lieske, Ewald and Robert Myers. 2001. Coral Reef Fishes of the World Periplus Editions. 400pp.
  • Bond, Carl E., 1996. Biology of Fishes 2nd ed. Thomson Learning Inc., 750pp.
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