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.
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.
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.
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.
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 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.
c) Callorhinchidae (Plough-nose chimaeras):
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.