Chimaera

Chimaeras are a unique group of cartilaginous fish in the order Chimaeriformes.

They are often called ghost sharks, rat fish, or rabbit fish. While they are closely related to sharks and rays, they are a distinct lineage. Their last common ancestor with sharks lived nearly 400 million years ago. Today, there are over 50 living species divided into three families: Callorhinchidae, Rhinochimaeridae, and Chimaeridae. Most species live in the deep ocean, though some, like the rabbit fish, inhabit shallower waters.

The anatomy of a chimaera is quite different from most other fish. Their skeletons are made entirely of cartilage rather than bone. This belongs to the class Chondrichthyes, which includes sharks and rays. Chimaeras have bulky heads and long, tapered tails. Their pectoral fins are large enough to generate lift while swimming. This movement gives them the appearance of flying through the water.

Some species even have a venomous spine on their first dorsal fin for defense.

One of the most remarkable features of the chimaera is its feeding mechanism. Unlike sharks, which have many replaceable teeth, chimaeras have only six large, permanent tooth-plates. These plates are arranged in three pairs. One pair sits at the tip of the lower jaws, and two pairs are on the upper jaws. They form a beak-like structure used for crushing and grinding.

This structure is similar to the teeth found in rodents. These plates grow continuously throughout the animal's entire life.

The biology of these teeth is scientifically unique among vertebrates. The plates are made of a soft material called osteodentin. However, the active edges are reinforced with a special tissue called pleromin. Pleromin is an extremely hard, enamel-like tissue. Its hardness comes from the mineral whitlockite, which crystallizes within the teeth as the fish matures. This specialized mineralization allows them to eat hard-shelled prey like molluscs and crustaceans. This diet makes them known as demersal durophages, which are animals that live near the bottom and eat hard things.

Chimaeras also possess advanced sensory capabilities to navigate dark environments. Many species have a bulbous snout that acts as a sensory organ. This organ uses electroreception to detect the electrical signals of prey. Small pores on the snout lead to specialized electroreceptor cells.

Their skull structure is also distinct through a process called holostyly. In this arrangement, the upper jaw cartilage is completely fused to the cranial cartilage. This is a major difference from modern sharks, which have movable jaws.

The evolutionary history of chimaeras is a subject of intense study. The oldest known relative is Protochimaera, which lived in Russia during the Carboniferous period. For much of history, chimaeras likely lived in shallow waters. Recent studies suggest they only moved into the deep ocean after the Cretaceous-Paleogene extinction event. They reached their highest ecological diversity during the mid-Cretaceous period. Now, they primarily inhabit temperate ocean floors, with some species living at depths exceeding 2,600 meters.

Despite their deep-sea homes, chimaeras face significant modern threats. Many species are vulnerable to overfishing through bycatch, which is when fish are caught accidentally in nets. Some species, such as the Australian ghostshark, have faced severe overfishing in the past. Others are targeted for their meat or liver oil.

Habitat destruction from deep-sea mining and trawling also poses a risk. Additionally, climate change may impact near-shore species by warming the water. Warmer temperatures can disrupt the stable environments needed for their eggs to hatch successfully.