Black Swallower

The black swallower, Chiasmodon niger, is a small deep-sea fish found in the North and Southwestern Atlantic, including the Gulf of Mexico. Of its genus, it is the most common species in the North Atlantic. Adults live in the mesopelagic and bathypelagic zones (twilight and midnight zones), up to about 2 miles deep. Juveniles are found in shallower water, from near the surface down to about half a mile. They belong to the family of snaketooth fishes, Chiasmodontidae; even their own genus name means ‘diagonally arranged teeth.’ The common name, however, reflects their eating habits. More on that in just a minute… The first specimen of this talented fish was obtained in 1830 at Madeira (Portugal), at a depth of 312 fathoms (nearly 1900 feet), by Lowe, who neglected to describe it. The species was rediscovered, and described, at the same locality by Johnson twelve years later. Black swallowers have long bodies that are laterally compressed. They reach about 10 inches in length and have smooth black skin (no scales), a long head with a blunt snout, and a large mouth. The lower jaw extends past the upper, and both are lined with sharp, depressible teeth, which interlock when the mouth is closed. Some of the teeth are so long that opposites on the jaw cross when the mouth is closed. They also have a small number of fixed canine teeth. Their eyes are a standard size, which is surprising considering where they live. They have long pectoral fins and two dorsal fins: the first has 10-12 spines, and the second is longer with only one spine. There is also a spine on their underside, just in front of the gills. Their lateral line is continuous with two pores per body segment. I think the best description comes from Frank Thomas Bullen in his book published in the early 1900s, Creatures of the Sea: Being the Life Stories of Some Sea Birds, Beasts, and Fishes – “The next monster down for notice is a very good specimen of the deep-sea chimaera, Chiasmodon niger. It is a veritable nightmare in appearance, being entirely black in color, with a mouth that cleaves the head asunder laterally for its whole length, so that vulgarly speaking, when its mouth is wide open it has no profile.”

Many deep-sea fishes have distensible stomachs. An expanding stomach certainly benefits a fish that must gulp down whatever crosses its path. Such a stomach can even enable a predator to swallow prey larger than its own body. Enter the black swallower. These fish can not only swallow fish whole that are larger than themselves, they can swallow fish twice their length and 10 times their mass. Their upper jaws are articulated with the skull at the front via the suspensorium (bones and cartilage that normally attach just the lower jaw to the skull), which allows the jaws to swing down and encompass objects larger than the swallower’s head. The two prevailing theories on how they accomplish this are 1) they capture prey by the tail, then slowly engulf the fish until it is fully coiled inside the stomach, or 2) they bite onto the head of the prey, suffocating it; their teeth prevent the prey from getting away, and they are eventually able to swallow the fish whole. If the first, one can only imagine the wild ride it’s in for once it has latched onto a larger fish. Due to their backwardly depressible teeth, they’re probably unable to release a victim that has entered the mouth and are forced to swallow whatever they seize, whether they want to or not.

The distensible stomach can stretch so far down that the stomach tissue actually becomes transparent. However, just because you can swallow a fish larger than you, doesn’t always mean you should. Their special stomachs can stretch so much that digestion becomes a race against time — and the swallowers sometimes lose. Overly ambitious black swallowers sometimes swallow prey so large that decomposition sets in before the meal can be digested. This results in a release of gases which inflate and burst the swallower’s stomach. They then float up to the surface, dead. Oops. At the depths in which they live, though, food is relatively scarce, which is perhaps why the black swallower eats so much, a get-it-while-you-can strategy. In the sparsely populated deep, they can take time to digest these larger prey items without much fear of being harassed.

Two early accounts seem to be the basis for most of our current assumptions about the black swallower’s eating habits. One, the aforementioned book by Frank Bullen, and an earlier book published circa 1888 by John Sterling Kingsley and Friedrich von Hellwald, The Riverside Natural History.

Bullen’s account: the “immense mouth is furnished with equally effective teeth, which are not only found in the jaws but on the palate also. Its front teeth are hooked and movable, so that while they may be pushed inward to admit the entrance of prey, they effectually prevent it from coming out. This peculiarity is explained by a slight examination of the creature’s feeding habits. It can and does swallow entire fish actually larger than itself – which sounds impossible, but it is not. For the belly of this atrocious glutton is like an India-rubber bladder which may be expanded amazingly. And consequently by dint of perseverance Chiasmodon can and does draw himself on to the body of another fish, as it were, until the visitor is snugly coiled away in that expanding bag, which being transparent, shows plainly from the outside the position of its occupant.”

Kingsley/Hellwald’s account: “It espies a fish many times larger than itself, but which, nevertheless, may be managed; it darts upon it, seizes it by tail and gradually climbs over it with its jaws, first using one and then the other; as the captive is taken in the stomach and integuments stretch out, and at last the entire fish is passed through the mouth and into the stomach, and the distended belly appears as a great bag, projecting out far backwards and forwards, over which is the swallower with the ventrals dislocated and far away from their normal place. The walls of the stomach and belly have been so stretched that they are transparent, and the species of the fish can be discerned within. But such rapacity is more than the captor itself can stand. At length decomposition sets in, the swallower is forced belly upwards, and the imprisoned gas, as in a balloon, takes it upwards from the depths to the surface of the ocean, and there, perchance, it may be found and picked up, to be taken home for a wonder, as it is really. Thus have at least three specimens found their way into museums – one being in the United States National Museum – and in each the fish in the stomach has been about twice as long, and stouter in proportion, than the swallower – six to twelve times bulkier!”

Black swallowers are oviparous (eggs develop and hatch outside of the body). The eggs are pelagic and measure about 1.1 mm in diameter. Each egg contains a clear oil globule and six dark pigment patches, which become distributed along the newly hatched larva. These patches eventually disappear, and the body darkens overall to black. Eggs are mostly found between June and September off South Africa; juveniles have been found from April to August off Bermuda. Both larvae and juveniles are covered in small spines. Most of what we know about this species come from dead specimens, juveniles, and eggs – though as a creature of the deep sea, we know a surprising amount.

Where I learned about black swallowers, and you can too!

World Register of Marine Species
www.marinespecies.org/aphia.php?p=taxdetails&id=126840#distributions

Ocean Biodiversity Information System
obis.org/taxon/126840

Biodiversity Heritage Library
www.biodiversitylibrary.org/page/4375472#page/20/mode/1up
www.biodiversitylibrary.org/page/28863420#page/39/mode/1up
www.biodiversitylibrary.org/page/60381604#page/193/mode/1up
www.biodiversitylibrary.org/page/37329923#page/617/mode/1up
www.biodiversitylibrary.org/page/58961559#page/118/mode/1up
www.biodiversitylibrary.org/page/12043484#page/270/mode/1up
www.biodiversitylibrary.org/page/2105699#page/364/mode/1up

IUCN Red List
www.iucnredlist.org/species/190310/21914202

Fish Base
www.fishbase.de/summary/10192

Marine Species Identification Portal
species-identification.org/species.php?species_group=fnam&id=1806

Smithsonian
ocean.si.edu/ocean-life/fish/black-swallower

Live Science
www.livescience.com/57302-twitter-famous-deep-sea-fish.html

University of Melbourne Student Union
umsu.unimelb.edu.au/weird-fishes/

National Geographic
www.nationalgeographic.com/animals/article/animals-carry-own-food-deep-sea-fish

All That’s Interesting
allthatsinteresting.com/black-swallower

Exequy’s Blog
exequy.wordpress.com/2015/02/17/black-swallower/

Wikipedia
en.wikipedia.org/wiki/Black_swallower