Chimaera

Chimaeras are special kinds of fish called cartilaginous fish, which means they have cartilage instead of bones, like sharks and rays. They belong to a group known as Chimaeriformes, and people often call them "ghost sharks," rat fish, spookfish, or rabbit fish. These names can also refer to other fish groups, so it's important to know which one people are talking about.

In the past, chimaeras were very common and diverse, as we can see from fossils. Today, most living chimaera species live deep in the ocean, far from shore. They are closely related to sharks and rays, but their last common ancestor with these fish lived almost 400 million years ago.

Chimaeras look quite different from sharks and rays. Some have long, whip-like tails, while others have unique features like large heads or unusual body shapes. They are fascinating creatures that help scientists learn more about how different kinds of fish have evolved over time.

You can find out more about chimaeras by looking at the cartilaginous fish or reading about the Chimaera (genus), the Chimera (mythology), and other related topics like rattails, Opisthoproctidae, and Siganidae. There's also a fun film called Ghost Shark if you're interested in stories that use this name.

Anatomy

Chimaeras are soft-bodied, shark-like fish with big heads and long, thin tails. They can grow up to 150 cm (about 4.9 feet) long. Unlike sharks, their skeletons are made entirely of cartilage, a flexible material.

These fish have special features that help them move through the water. Their large chest fins can help them glide, making them look like they are flying. They also have smaller belly fins and, in some types, a fin near the tail. Some chimaeras even have a sharp, stinging spine on their back for protection.

Chimaeras have a unique way of sensing their surroundings. Their snouts can detect electric signals from prey. They also have strong, permanent tooth-plates that work like a beak, perfect for crushing food. These teeth are special because they contain a very hard material that helps them stay sharp.

Behavior

Chimaeras, also called ghost sharks or ratfish, live on the ocean floor, often in very deep water. Some species, like the rabbit fish and the spotted ratfish, can sometimes be found in shallower water and are seen in public aquaria. They live in all oceans except the Arctic and Antarctic.

Chimaeras eat small sea creatures like crabs, brittle stars, and molluscs. They usually live close to the bottom of the ocean.

When it’s time to have babies, chimaeras are similar to sharks in some ways. The males use special body parts to help with mating, and the females lay eggs in tough, spindle-shaped cases.

Conservation and threats

Some chimaera species may be threatened by being caught accidentally when fishing for other fish or by being caught on purpose for their meat. Although no chimaera species are listed as endangered, several are considered vulnerable or near threatened because there isn’t enough information about them. Many of these fish live in specific areas, and we don’t know much about how they move.

Certain chimaeras are caught for their meat, and rules now help control how many can be caught. However, some populations have dropped sharply. Other species are caught accidentally at high rates, which may also cause their numbers to fall. Chimaeras are mostly not threatened by the trade that takes fins from sharks.

Habitat loss is another concern. Building along coastlines and activities like deep-sea mining and trawling can damage the places where chimaeras live. Near-shore species may also be affected by climate change, as stronger storms and warmer water can harm their eggs.

Classification

Main article: List of chimaeras

Chimaeras are special kinds of fish that are part of a group called cartilaginous fishes, which also includes sharks and rays. Sometimes they are grouped together, and other times they are given their own special group. They share a few traits with regular bony fishes, too.

Scientists have been studying deep water and looking at old museum samples, which has helped them find many new kinds of chimaeras in the early 2000s. There are now more than 50 different living types of chimaeras, split into three families. These families are Callorhinchidae, Rhinochimaeridae, and Chimaeridae, with Callorhinchidae being the most basic group.

Evolution

Studying how chimaeras evolved has been difficult because there are not many good fossils. Scientists now mostly use DNA sequencing to learn about how these fish developed.

Chimaeras and their close relatives are thought to have split from Elasmobranchii, which includes modern sharks and rays, during the Devonian period, over 380 million years ago. The oldest known chimaera-like fish, Protochimaera, lived in the Early Carboniferous period in Russia. Modern chimaeras appeared in the Early Jurassic in Europe. Unlike today’s deep-water chimaeras, older ones often lived in shallower waters. Many modern groups evolved during a time called the Mesozoic Marine Revolution. They were most diverse in the mid-Cretaceous period, developing many different types of teeth.

Scientists used to think chimaeras moved to deep water to survive mass extinctions, but newer research suggests they mostly lived in shallow water and only moved deeper after a big event at the end of the Cretaceous period. The plough-nosed chimaeras are the only group that still live in shallower waters like their ancestors.

Taxonomy

Extinct groups of chimaeras include:

• †Suborder Echinochimaeroidei Lund, 1977
  • †Family Echinochimaeridae Lund, 1977
    • †Genus Echinochimaera Lund, 1977 United States, Lower Carboniferous (Serpukhovian)
• †Suborder Squalorajoidei Patterson, 1965 (Lower Carboniferous–Early Jurassic)
  • ?†Genus Sulcacanthus Itano & Duffin, 2023 United States, Lower Carboniferous (Viséan)
  • Family †Squalorajidae Woodward, 1886
    • †Genus Squaloraja Riley, 1833 Europe, Early Jurassic (Hettangian–Sinemurian)
• †Suborder Myriacanthoidei Patterson 1965 (Late Triassic–Late Jurassic; possible Carboniferous records)
  • †Family Chimaeropsidae
    • †Chimaeropsis Zittel 1887 Belgium, Early Jurassic (Sinemurian), Germany, Late Jurassic
  • †Family Myriacanthidae Woodward 1889
    • †Acanthorhina Fraas 1910 Posidonia Shale Formation, Germany, Early Jurassic (Toarcian)
    • †Agkistracanthus Duffin and Furrer 1981 Austria, England and Switzerland, Late Triassic–Early Jurassic (Rhaetian–Sinemurian)
    • †Alethodontus Duffin 1983 Germany, Early Jurassic (Sinemurian)
    • †Halonodon Duffin 1984 Belgium and Luxembourg, Early Jurassic (Sinemurian)
    • †Metopacanthus Zittel 1887 Posidonia Shale Formation, Germany, Early Jurassic (Toarcian)
    • †Oblidens Duffin and Milàn 2017 Hasle Formation, Denmark, Early Jurassic (Pliensbachian)
    • †Myriacanthus Agassiz 1837 United Kingdom, Late Triassic-Early Jurassic (Rhaetian–Sinemurian)
    • †Recurvacanthus Duffin 1981 United Kingdom, Early Jurassic (Sinemurian)
• †Suborder Protochimaeroidei Lebedev & Popov in Lebedev et al., 2021
  • †Family Protochimaeridae Lebedev & Popov in Lebedev et al., 2021
    • †Genus Protochimaera Lebedev & Popov in Lebedev et al., 2021 Moscow Region, Russia, Lower Carboniferous (Viséan–Serpukhovian)
• Suborder Chimaeroidei Patterson 1965
  • †Eomanodon Ward and Duffin 1989 United Kingdom, Early Jurassic (Pliensbachian)
  • Family Callorhinchidae Garman, 1901
    • †Brachymylus A. S. Woodward 1894 Germany, Early Jurassic (Pliensbachian)
    • †Bathytheristes Duffin 1995 Posidonia Shale Formation, Germany, Early Jurassic (Toarcian)
    • †Ottangodus Popov, Delsate & Felten, 2019 France, Middle Jurassic (Bajocian)
    • †Moskovirhynchus Russia, Upper Jurassic
    • †Pachymylus United Kingdom, France, Middle Jurassic
  • Family †"Edaphodontidae"
    • †Ischyodus (40 species) Worldwide, Middle Jurassic–Miocene (also placed in Callorhinchidae)
    • †Elasmodectes Europe, Jurassic–Cretaceous
    • †Elasmodus Worldwide, Cretaceous–Paleogene
    • †Edaphodon Worldwide, Cretaceous–Neogene
    • †Ptyktoptychion Australia, Early Cretaceous
    • †Lebediodon Europe, Cretaceous
  • Family Chimaeridae Bonaparte, 1831
    • †Canadodus Popov, Johns & Suntok, 2020 Sooke Formation, Canada, Oligocene
  • Family Rhinochimaeridae Garman, 1901
    • †Amylodon Europe, Late Cretaceous–Oligocene