Author: Bee Smith
In honor of spooky season this month we are going to focus on the strange forgotten cousins of sharks, the ghost sharks. In fact, October 3oth is International Ghost Shark Day!
Key Features & Appearance
Chimaeras are cartilaginous fish, meaning their skeleton is made of cartilage (you have this squishy material in the tip of your nose!) rather than bone. They are one of the oldest fish, evolving from the same common ancestor as sharks and rays but diverging off around 400 million years ago.
Chimaeras are unique-looking fish, reaching around 60-150cm, they have cone-shaped bodies, large eyes, long whip-like tails, and large pectoral fins - they flap these wing-like fins to move, more akin to how birds fly than how sharks generate population with their tail! As well as the apparent differences in how they look and move, they differ from sharks and rays in many other aspects of their basic anatomy. One recognizable way they differ is that they have smooth, scaleless skin rather than the rough skin made of dermal denticles (basically skin teeth) that sharks and rays have. Also, while sharks have five pairs of gill slits (or six/seven in a few species), they have 4 pairs of gills with only one external opening. A slightly weird difference is that chimaeras have nostrils, which they use to ‘breathe in’ water and pass it over their gills, in fact they are the only group of fish to have true nostrils! They also differ in that whilst sharks have many teeth arranged in rows that get constantly replaced, they have three tooth plates that they keep for life. And another difference is that whilst sharks jaws are not attached to their skulls, their jaws are fused to their skull. In fact chimaera's scientific name “Holocephali” comes from their fused head as “Holocephali” means whole-head. Their common name “Chimaera” comes from the Chimera monster in Greek mythology who was a hybrid of multiple creatures, because of their strange features. Their other common name, “ghost shark" is from their often ghostly appearance due to their silvery white body and dark eyes and they are also sometimes called “rattails” due to their long thin tails (though confusingly, this name also refers to a species of bony fish…).
There are around 50 species of chimaeras that we know of (with over 20 described in the last 20 years), and they present a huge range of diversity of the central body plan. There are three main groups of chimaeras, divided up by their noses...these are the plough-nosed (Callorhinchidae), the short-nosed (Chimaeridae), and the long-nosed (Rhinochimaeridae) - whose snouts can be as long as 50% of their body length! We will be focusing on the “Rabbit fish” which is a species of short-nosed chimaera. Most chimaera species are very understudied but luckily some research has been done on the rabbit fish so we know some fun facts about it!
Image Credit: Andy Murch
Habitat & Distribution
Chimaeras were once an abundant and diverse group that dominated the planet's early oceans. In fact, many well-known “cool prehistoric shark species” were actually chimaeras, such as Helicoprion (that shark who had a saw on its face). Early chimaeras mainly lived in the shallows but it is thought that they have been gradually outcompeted by more modern sea life and forced to move down into the deep. Today chimaeras are mainly confined to deep water, being found from 200m, but mainly 500m, to as deep as 3000m. They live near the seafloor and so are often found on the shelf and slope off continental margins and they inhabit all of the world's oceans with the exception of Arctic and Antarctic waters.
The rabbit fish is a bit of an exception to the deep sea rule, being one of the only chimaeras known to occasionally be found shallower, spending most of its time at 300-500m but occasionally coming up to 40-100m. And it is found in the eastern Atlantic Ocean and western Mediterranean Sea.
Diet
Due to the chimaeras' deep sea nature, they are a tricky species to get access to in order to study. Scientists have to rely on occasional sightings by underwater vehicles or mainly on specimens that are caught and brought up by deep water fishing nets. Because of these rare opportunities to study them, we have limited information about specific species diets, mainly relying on stomach contents analysis from dead specimens. We know that generally they tend to eat bottom dwelling crustaceans such as crabs and molluscs - crushing through their shells with those tough tooth plates.
Fortunately, the rabbit fish’s diet has been studied in the Mediterranean and it has been found to specialise on a specific crustacean, feeding almost exclusively on the couch rubble crab regardless of sex or maturity.
Reproduction
Like sharks and rays, chimaeras reproduce internally. Male sharks and rays have two mating appendages known as “claspers”. Chimaeras also have claspers but theirs are fused together at the base, and they also differ in that they have additional retractable sexual appendages known as “tentacula”. A tentacula is a club-like appendage covered in spiky denticles that is used to grasp a female for mating purposes, and they have one of these on the top of their head! It has been found that chimaeras of different sex and age tend to segregate out by depth, with sub-adults and females tending to occur deeper. Sexual segregation often occurs in species as a method of mate avoidance, as frequent mating can have high costs for females, and considering the extra appendages of male chimaeras I think females have good reason to want to stay away!
Chimaeras are oviparous, meaning that they lay egg cases, as some sharks and rays do. They lay two egg cases at a time and do so onto the seabed. Their eggs are similar to sharks, being small and leathery with a golden colour but are distinguishable due to having a prominent frilled edge. They can take a long time to hatch, as long as 12 months, and it is thought that this period is even longer in deeper species. Though there have been limited studies on the life cycles, due to their close relation to sharks and rays it is believed they have slow growth and late maturity. We have little knowledge about their lifespans with estimates from 10 to 100 years, however, the lifespan of rabbit fish has been studied and is believed to be up to 30 years.
Threats
Chimaeras are caught by fisheries, by directed fisheries in a few limited places, but mainly as bycatch by deep water fisheries targeting other species. Even if they are discarded after being accidentally caught, they have been brought up from such depths and have experienced such a change in conditions that they have a really low chance of survival. But because we know so little about their life history and populations, we struggle to prove whether this fishing is threatening them. However, as we believe they likely have very slow life cycles like sharks and rays, they are probably vulnerable to any increased mortality above natural levels. Also, their sexual segregation may mean fishing results in different exploitation of the sexes which could really impact their ability to breed and so lead to population declines.
Status
The rabbit fish is considered “vulnerable” by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species.
Fun Fact
Most species of chimaera have a large venomous spine in front of their dorsal fin which they use to protect themselves from predators. These spines are no joke, though the venom is not lethal to humans, the sharp spines can cause a serious wound! And these spines have been often seen embedded into the skin of their predators, such as leopard seals… ouch!
Image Credit: Andy Murch
Works Cited
Berkovitz, B. and Shellis, P. (2023) ‘Chondrichthyes 2. rays and chimaeras’, The Teeth of Non-Mammalian Vertebrates, pp. 57–81. doi:10.1016/b978-0-323-91789-6.00001-7.
Calis, E. et al. (2005) ‘Preliminary age and growth estimates of the rabbitfish, chimaera monstrosa, with implications for Future Resource Management’, Journal of Northwest Atlantic Fishery Science, 35, pp. 15–26. doi:10.2960/j.v35.m501.
Finucci, B. et al. (2020) ‘Ghosts of the deep – biodiversity, fisheries, and extinction risk of Ghost Sharks’, Fish and Fisheries, 22(2), pp. 391–412. doi:10.1111/faf.12526.
Shark Devocean (2014) Introducing: Chimaeras, Shark Devocean. Available at: https://sharkdevocean.wordpress.com/2014/08/26/introducing-chimaeras/ (Accessed: 13 October 2024).
Tamayo, M. et al. (2021) ‘Highly specialized feeding habits of the rabbitfish chimaera monstrosa in the deep sea ecosystem of the northwestern Mediterranean Sea’, Journal of Applied Ichthyology, 37(6), pp. 868–874. doi:10.1111/jai.14241.
Van der Linde, K. et al. (2021) ‘Leopard seals (Hydrurga leptonyx) in New Zealand waters predating on Chondrichthyans’, Frontiers in Marine Science, 8. doi:10.3389/fmars.2021.795358.
W, G. (2024) The Curious Chimaera with Brit Finucci, Armatus Oceanic. Available at: https://www.armatusoceanic.com/interview/brit-finucci (Accessed: 13 October 2024).
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