Deep sea chimeras are cartilaginous fish that inhabit the dark, cold depths of the world’s oceans. Their common names, including ghost shark, spookfish, and ratfish, are inspired by their ethereal appearance and deep-sea existence. The name chimera references a mythological creature made of various animal parts, suiting a fish with features reminiscent of other animals.
These fish are relatives of sharks and rays, belonging to the class Chondrichthyes. They represent a distinct evolutionary path, having split from their shark relatives approximately 400 million years ago to form their own order, Chimaeriformes. Because they live in remote environments, they remain one of the ocean’s more mysterious groups of fish.
Unveiling the Ghost Shark: Anatomy and Survival
A ghost shark’s anatomy is highly adapted for life in the deep ocean. They possess large heads and long bodies that narrow to a thin, whip-like tail. Many species propel themselves through the water with large, wing-like pectoral fins, creating a gliding motion that contributes to their spectral name. Their entire skeleton is composed of cartilage rather than bone.
Unlike the rough, tooth-like scales of sharks, the skin of an adult chimera is smooth and scaleless. Their coloration is usually dark and uniform, such as brown or blue-grey, which provides camouflage in low-light conditions. To navigate and hunt in the darkness, they have developed specialized sensory systems, including exceptionally large eyes suited for detecting the faint glow of bioluminescent organisms.
Ghost sharks also have a prominent lateral line system, visible as stitch-like lines along their bodies, which detects vibrations and pressure changes in the water. They possess electroreceptors in small pores around the head that can sense the weak electrical fields produced by prey. For defense, most chimeras have a sharp, venomous spine located just in front of their first dorsal fin.
Male reproductive anatomy is distinct. They are equipped with claspers for internal fertilization, a trait shared with sharks. Many male chimeras also feature a retractable appendage on their forehead called a tentaculum. This structure is covered in sharp, tooth-like projections and is used to grasp the female during mating.
Dwelling in the Abyss: Habitat and Distribution
Chimeras are inhabitants of the deep ocean, thriving in zones with immense pressure and near-total darkness. They are most commonly found in the bathyal zone along continental slopes, and some species venture into the deeper abyssal zone. Their depth range is between 200 and 2,600 meters (about 650 to 8,500 feet).
The distribution of ghost sharks is nearly global, with species found in most of the world’s major oceans, excluding the coldest polar regions. While the group is widespread, individual species often have more limited ranges. For instance, certain species are frequently observed off the coasts of New Zealand and southern Australia.
These fish are associated with specific seafloor environments where their food sources are abundant. They glide over soft, muddy, or sandy bottoms on the ocean floor. They can also be found congregating around underwater geological features like seamounts and oceanic ridges, which support rich biological communities.
Life in the Dark: Diet and Reproduction
Diet
The diet of a ghost shark consists of organisms that live on or in the seafloor. They are adept hunters of bottom-dwelling invertebrates, using their specialized senses to locate prey in the absence of light. Their meals include crustaceans like crabs and shrimp, mollusks, marine worms, and echinoderms such as sea urchins and brittle stars.
To consume their prey, chimeras have a unique dental structure. Instead of rows of replaceable teeth like sharks, they possess three pairs of permanent, plate-like teeth that grow continuously. These durable tooth plates are designed for crushing the hard shells of their invertebrate prey. Their sense of smell and electroreception are the primary tools for locating these food sources.
Reproduction
Chimeras are oviparous, meaning they reproduce by laying eggs. Females deposit large, leathery egg cases directly onto the soft sediment of the seafloor. These cases, sometimes called mermaid’s purses, come in various shapes and can take a very long time to hatch in the cold deep-sea environment.
The life history of these fish is characterized by slow growth rates and reaching sexual maturity late in life. These traits, combined with what are believed to be long lifespans, are common among deep-sea organisms. This slow reproductive cycle makes their populations particularly susceptible to disturbances and slow to recover from decline.
A Family of Phantoms: Types of Chimeras
‘Ghost shark’ refers not to a single species but to the diverse order of fish known as Chimaeriformes. This group is organized into three main families, distinguished by the shape of its snout. These families represent the surviving lineages of a once much more diverse group of cartilaginous fish.
The family Callorhinchidae are known as plownose chimeras or elephantfish, recognized by their flexible, hoe-shaped snouts used to probe the seafloor for food. The Rhinochimaeridae, or longnose chimeras, are characterized by their elongated, pointed snouts. The Chimaeridae family includes the shortnose chimeras or ratfish, which have a more rounded, blunt snout.
Scientists are still discovering new species of chimeras. Expeditions into the deep sea continue to reveal the full extent of their diversity, underscoring how much remains unknown about these fish. The recent identification of the Andaman shortnose chimaera highlights this ongoing process of discovery.