Introducing: Chimaeras

By Christopher Bird (@SharkDevocean)

In the wake of Shark Week, it is safe to say that we are all that little more familiar with sharks. Lurking in the dark, cold waters of the deep however, hides the sharks distant cousin; the Chimaeras.

Chondrichthyans are a group of cartilaginous fish that are composed of the Elasmobranchii (sharks, skates and rays) and the Holocephali (chimaeras). You may not have heard much about the chimaeras though as they receive far less coverage than their more famous shark cousins. They may never get a whole week of televisual programming but after these 15 neuron stimulating facts, you will agree that they are some of the most beautifully bizarre fish in the ocean. I introducing to you; the Chimaeras: Deep-sea aliens.

The pale ghost shark, Hydrolagus pallidus, captured of deep-sea ROV footage. Image: ifremer

 1. Their name literally means “whole-head”

Holocephali literally translates, from latin, to whole (Holo) head (Cephalic) and is synonymous with their relatively large head compared to their sleek slender body. Other common names include ghostsharks and spookfish, referring to their spectral like appearance. They also have one other common name but you will have to see number 14 for that. Chimera also refers to a Greek mythical monster, but more on that later.

Chimaeriformes are thought to have branched off from Elasmobranchs (sharks, skates and rays) about 420mya. Amended figure from Inoue et al 2010

2. They are some of the oldest fish in the ocean.

The Holocephalans, of which all chimaera belong to, are thought to have originated some 420 million years ago, during the Silurian. If the fossil record is anything to go by, they reached highest diversity during the Carboniferous, about 300 million years ago, but then decreased at the end of the Permian, about 250 million years ago). Due to their cartilaginous skeleton, most of what we know about early chimaeras is known from just tooth-plates and fin-spines.

A Rhinochimera (Harriotta sp.) observed swimming in Hydrographer Canyon. GIF created from video by NOAA Okeanos Explorer Program.

3. Widely distributed deep-sea dwellers

Inhabiting all of the worlds oceans, except the Arctic and Antarctic waters, they are particularly abundant in deep-sea ecosystems between 500m – 3000m, although some species do occur in shallower coastal waters.

Only new born chimaera have denticles. Once mature, skin is devoid of scales or denticles, leaving bizarre ghostly rubbery smooth skin. Image: John Snow/ MexFish

4. They have scaleless skin

Chimaeras do not have any scales or denticles, although newly born hatchlings do have very small denticles that are later shed. Their rubber like skin only adds to their bizarre appearance and it comes in a variety of shades and colours. In the deep sea, most chimaera are dark in colour but shallower dwelling species can have elaborate patterning.

5. There are 3 families, each weirder than the last

There are approximately ~50 known species of chimaera belonging to six genera and three families.

a) Chimaeridae (Shortnose chimaeras): 

The Spotted ratfish (Hydrolagus colliei). Image: Life of Sea

The most common and diverse group of chimaeras, this group contains both Chimeara and Hydrolagus genera. Can occur from 50m down to more than 2000m.

b) Rhinochimaeridae (Longnose chimaeras): 

This distinct group of chimaera are characterised by having an elongated snout that is used for increased sensitivity when foraging for benthic invertebrates in the sediment.

This long-nosed chimaera was discovered in 2003 during the NORFANZ Voyage in the Tasman Sea. Image: NORFANZ

c) Callorhinchidae (Plough-nose chimaeras): 

It is easy to see how Callorhinchus milii is also called ghost shark. Image: Ernst Hoffinger/ fishbase

Almost somewhere in between Chimaeridae and Rhinochimaeridae, plough-nose chimaeras or elephant fish possess an elongated plough-shaped snout that is used to probe the seafloor in search of food. These chimaera are often observed in the shallow waters around Australia.

Aggregation of spotted ratfish (Hydrolagus colliei). Image: Dr. John Butler, NOAA NMFS SWFSC

6. Species, gender and age segregate at differents depths

Like many of their shark cousins, chimaeras of different sex, age and species segregate out by depth. In the Atlantic, mature females tend to occur at deeper depths than mature males. In spite of this somewhat antisocial behaviour, some species are known to congregate into large aggregations in order to mate.

The plough-nose chimaera uses its snout to probe the sediment in search of food. Also notice dentition and nostrils. Image: Fish Index

7. “Primitive” genome is small and slow evolving

The genome of the plough-nose chimaera, Callorhinchus milii (Australian Ghostshark), is the smallest of all known cartilaginous fish and is subsequently used as a model genome for vertebrate evolution studies. In addition to its small size, its genome is also the slowest evolving of all known vertebrates.

8. Permanent rodent like teeth

Unlike sharks that can continually replace rows of teeth throughout their life, chimaeras possess only three pairs of hyper-mineralised tooth plates. They use these teeth to crunch through bivalves, crustaceans and echinoderms associated with the seafloor. It is their peculiar dentition, along with their long thin rat-like tail, that gets them their name ratfish [edited] or rabbitfish.

Preceding the first dorsal fin is a large venomous spine that protects it from predation. Source: National Geographic News

9. Armed with toxic fin spines

When you spend your days swimming on the sea floor searching for food and mates, you leave yourself vulnerable to attacks from above. Chimaeras protect themselves from such threat by having a large toxic spine located just in front of the first dorsal fin. In addition to inhibiting marine based predation, the spines have also been known to cause serious injury to deep-sea fishermen that often get lodged in lower limbs and hands. Ouch!  [Edited] Although definitive proof of spine toxicity is lacking in the literature, medical reports suggest fishermen have experienced burning and numbing sensations for weeks after puncture traumas from chimaera spines.

Using a motion similar to birds flapping their wings, chimaera propel themselves through the water with large pectoral fins. Image: Andre’ Hagestedt

10. They swim using large pectoral wings

Unlike most sharks that generate propulsion using their tail, chimaeras use large, wing-like, pectoral fins to swim through the water, giving them characteristics more suited to a bird than a fish. 

Chimera are the only fish with true respiratory nostrils. Also note the deep lateral line pits used to detect pressure differences. Image:Edward Farrell/ IEG

11. Hidden, unique breathing apparatus

You may have noticed that chimaeras do not appear to have any gills. This is because their four-gill openings are hidden behind a fleshy covering called an operculum. It gets weirder still. The holocephalans are the only group of fish that possess true nostrils. Respiratory water is drawn in through the “nostrils” and passed over the gill arches. They can now add mammalian traits to their arsenal of assorted characteristics.

Chimaera egg cases are adapted to being deposited on the sediment. Source: wellentheorie

12. They lay elaborate egg cases on/in the sediment

All chimaeroid fishes are oviparous, meaning that they lay eggs (often associated with or buried in the sediment). Egg capsules are easily distinguishable from other chondrichthyan species due to a prominent frilled edge. For the spotted ratfish (Hydrolagus colliei), egg capsules can take 18-30 hours to be extruded (ouch) and then take more than 12 months to mature. This incubation period is thought to be even longer in deeper species and also why populations may be sensitive to fishing mortality.

 

This cephalic clasper is used to attach onto female pectoral fins during mating. Image: John Snow/ MexFish

13. Sexual appendages on their head!

The mature males of some species have a club-like appendage on their head, which is covered in rows of spikey denticles. This cephalic clasper or frontal tenaculum is used by male chimaeras to grasp onto the pectoral fins of females during copulation.

Appearing to be the concoction of various other animal characteristics, these almost frankenstein fish are not only intriguingly beautiful but also biologically fascinating. Image: NOAA

14. A Greek mythical monster

In Greek, a Chimera is “a fire-breathing female monster with a lion’s head, a goat’s body and a serpent’s tail.” The word chimera is also more generally used for any organism that is formed from the parts of various other animals. With the tail of a rat, the teeth of a rabbit, the nostrils of a mammal, the fin-spine of a porcupine, the sometimes elongated snout of an elephant and pectoral fins comparable to that of bird wings, it is understandable how these fish got their name.

Hydrolagus pallidus (Pale chimaera). Note the relatively large head and apparent lack of gills ©Peter Wirtz/ fishbase

The deep-sea still holds so many secrets and we still know so little about these bizarre chimaeras. With new species discovered on a regular basis and the advent of new technologies that are capable of deeper ocean exploration, who knows what other beautifully intriguing beast are out there yet to be discovered.

The chimaeras, sharks cooler deep-sea cousin.

N.B: All photos are credited to original source where possible. Clicking on image will take you to respective origin.

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