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Home › Sea Wildlife › Marine Organisms › Vertebrate › Flatfish

Fascinating Facts about Flatfishes

[Flatfish Phylum: Chordata] [Class: Actinopterygii] [Order: Carangiformes] [Suborder: Pleuronectoidei]

Despite being a ray-finned fish with a vertebral column (backbone), most of the species that belong with the order Pleuronectiformes have an unfamiliar body shape and strange life cycle.

This section contains some of the lesser-known facts about flatfishes (Pleuronectoidei), including where they live, what they eat (and how), and their peculiar reproductive system.

Flatfishes Geographical Range and Habitats

We have an extensive marine vertebrates list, yet only a handful of the fish species exist in all oceans.

For example:

Most flat ocean fish, in one form or another, reside in the Arctic Ocean, Antarctic Ocean, and even more so in the tropics, especially the Indo-Pacific and the Caribbean Sea.

Still, most species of flatfish are difficult to spot underwater, even near shallow dive sites, because they can change their skin colour to match sand and muddy seabeds.

Many of the right-eyed flounders, such as the American plaice (Hippoglossoides platessoides), thrive better in deep water environments.

But, most of the common flatfish create their habitats on continental shelves, sloping terrains, and close to shallow coastal areas.

Interesting Fact: Some of the flat faced fish are known to swim into brackish water, and several sole fish species (Achiridae) and tonguefishes (Cynoglossidae) spend their whole life in fresh water.

Marine Flatfish Characteristics

Even though flatfish species vary in size and shape, they share many characteristics and behavioural traits. So for example, besides being a fish that lays on its side:

• After hatching into juveniles, they are symmetrical and shaped like "normal" ray-finned vertebrates, with one eye on each side of the head. But, one eye migrates to the other side during the process of metamorphosis into the adult form.
• Even though they are "handed" (meaning righteyed flounders have both eyes on the right side and lefteyed flounders have on the left), adult flat fish always come to rest with the blind side facing down to the seabed so that both eyes are facing upwards.
• Unlike dorsoventrally flattened ray fish (meaning top-to-bottom), flatfishes have a laterally flattened body (meaning side-to-side).
• The upper surface (side with eyes) is almost always deeply pigmented and heavily camouflaged. In contrast, the underside (the blind side) tends to be more of a drab pale shade of white or grey.
• Having an ability to change their body colouration and pattern to match that of the ocean floor makes them a master of stealth with aposematic attributes.
• The majority have extra long dorsal and anal fins running along the majority of their body length. These help to create stability when they bury themselves and hide in benthic substrates.
• The size of flat fish in the ocean varies between the different species. According to Guinness World Records, the smallest flatfish is the large-scale righteye flounder (Nematops grandisquama) which grows to a maximum length of nine (9) centimetres (3.5 inches). Whereas, the Atlantic halibut (Hippoglossus hippoglossus) and the Pacific halibut (Hippoglossus stenolepis) can grow to a whopping 2.4 metres wide (8 feet) and weigh in excess of 226 kilograms (500 pounds).

Common Flatfish Behaviour

Because they spend most of the day lying flat and concealed in sand, mud, or gravelly seabeds, they have become very efficient predators and excellent bottom-feeders.

With only their two eyes exposed above the sediment, these ambush predators burrow themselves in and wait quietly to strike their favourite prey - shrimps.

Some of the plaice and halibut species are known to migrate seasonally - mostly according to their feeding and spawning patterns.

Here's the thing:

Flatfish are mostly a nocturnal species. They are more active at night and choose to hunt under the cover of darkness for extra safety.

They have a unique swimming style. Instead of swimming upright, like most marine vertebrates, they use their fins to propel themselves in a "wave-like" movement - often with great speed when they're actively hunting or if they feel threatened.

Pro Tip: Wondering how to spot a flatfish while scuba diving? It's better if you have the night dive certification, then you'll need to look closely at the sandy or muddy bottom and watch for sudden movements because they usually bury themselves with only their eyes sticking out.

What Do Flatfish Eat?

Flatfishes are carnivorous predators with a varied daily diet. But, they tend to feed most on weak fish, segmented worms, sea urchins, small crustaceans (e.g. pink shrimp species), and marine molluscs, especially squids.

Types of Flatfish Species

How Do Flatfish Reproduce?

A lot of marine organisms reproduce through the process of external fertilisation, often called broadcast spawning, and reproduction in flatfish follows a similar approach.

Most flatfish will reach sexual maturity after the age of two (2) and before the age of five (5) sometimes migrating long distances to reach their spawning grounds.

But wait - there's more:

Males release their sperm and females release millions of eggs into the open water, usually close to the seabed and sometimes spreading across wide areas.

Flatfish eggs are buoyant and pelagic, so they drift with ocean currents before eventually hatching into symmetric, free-swimming larvae.

Metamorphosis in Flatfish

As the larvae start to grow and develop into bottom-dwellers, one of their eyes migrates across the head to join the eye on the other side. During this process, the body starts to flatten and the fish starts swimming on its side.

Tropical flatfish, such as the peacock flounder (Bothus mancus) and leopard flounder (Bothus pantherinus), tend to reproduce in multiple small spawning events throughout the year. By comparison, most of the cold-water flatfishes (halibut, plaice, and turbot) rarely spawn later than spring or early summer.

Fun Fact: The general timing for spawning is seasonal and is often triggered by a rise in water temperature and availability of food for the larvae. Some of the vulnerable flatfish species spawn in schools to maximise successful fertilisation.

Flatfish Predators and Threats

Unlike similar marine species, especially ray fish with stingers, flatfish do not have any specific defencive weapons per se. Hence, they rely mainly on camouflage, stealth, and quick bursts of speed to avoid predation.

As a consequence of that, the natural predators of flatfish are plentiful and include cod, haddock, monkfish (genus Lophius), sharks, large rays, and even some large invertebrates, such as crabs and cephalopods.

Furthermore, some seabirds (e.g. cormorants, gulls) and marine mammals are known to eat flatfish, especially sea lions, dolphins, and seals.

Other contributing factors for the decline in a few vulnerable areas include certain types of human behaviour (e.g. coral reef destruction and degradation, overfishing).

Pro Tip: According to a recent assessment released by the IUCN Red List of Threatened Species, the conservation status of most flatfishes worldwide is of "Least Concern" (LC).

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