A female Melanocetus eustales.
Photo: Theodore W. Pietsch
From Wired by Matt Simon
Teen movies are, at their core, veiled studies in evolutionary biology,
with young men and women coming to sexual maturity and either giving
into or resisting what is arguably an animal’s sole purpose on this
planet — to find a mate.
Some decide to wait until they’re married,
others lack the desirable traits to even get that far, and still others
succeed and consequently have to put off college for a while.
But if the deep-sea anglerfish happened
to have the cognitive and physical capabilities required to produce its
own such films, there’d be decidedly fewer plot twists.
Every single
movie would go a little something like this: Boy meets girl, boy bites
girl, boy’s mouth fuses to girl’s body, boy lives the rest of his life
attached to girl sharing her blood and supplying her with sperm.
Ah, a
tale as old as time.
3D scans reveal deep-sea anglerfish's huge final meal
A rare hairy anglerfish that entered the Museum's collections 13 years
ago had perplexed researchers with its massive stomach.
However, the
specimen was so rare they didn't want to cut it open to identify its
last meal.
Now, using micro-CT scanners, the Museum's imaging experts
have finally been able to solve the mystery.
The over
300 extremely varied species of anglerfishes
inhabit everything from shallow to super-deep waters, and are so named
because they are fish that fish for fish using lures, which are actually
highly modified spines of dorsal fins that have migrated to their
snouts.
But among the 160 deep-sea species, only some 25 engage in the
aforementioned biting-fusing-mating, what is known as
sexual parasitism.
In this group, the diminutive male looks like an entirely different
species, lacking the female’s enormous jaws and characteristic lure.
This is because he doesn’t need to hunt.
He only exists to attach to a
female, and according to evolutionary biologist Theodore W. Pietsch of
the University of Washington, mates are so scarce down here that it
might be that only 1 percent of males ever find a female.
The rest
starve to death as virgins — unfortunate guys in a sea that doesn’t have
plenty of other fish.
A female Haplophryne mollis.
Photo: David Shale
But it isn’t for lack of trying.
The
male has the biggest nostrils in proportion to its head of any animal on
Earth, according to Pietsch.
These sniffers are paired with extremely
well-developed eyes, “so we think that it’s kind of a dual approach,” he
said.
“The female emits a species-specific smell, a pheromone, and the
male searches out based on that, and then when the male gets close
enough, the eyes can be used to distinguish the female of the correct
species.”
And with two dozen other species of anglerfishes that engage in this
manner of reproduction, the male had better be damn sure he chooses the
right one.
Luckily, the female
puts on the red blue light — in the form of glowing bacteria living in her lure. Incredibly, some 90 percent of species in the deep utilize
such bioluminescence.
“The bait out there is not only an organ of luminescence, but
structurally it’s species-specific,” said Pietsch.
“Every species of
these 160 forms within this group, they have a pattern of filaments, and
pigment patterns, and probably also light flash patterns, like
fireflies. And they separate themselves out that way so that males can
find females,” distinguishing “the tiny little differences between the
structure of the bait.”
A female Himantolophus appelii.
Photo: Theodore W. Pietsch
Once the male closes in, he bites onto the female, usually her belly,
and their tissues fuse together to permanently join the pair in
incredibly unholy matrimony.
The male’s eyes and fins atrophy away, and
here he will live out the rest of his life nourished by her blood, still
breathing with his own gills and, importantly, still producing sperm.
“This establishes a hormonal connection,” said Pietsch, “so that
probably the maturation of eggs and sperm is synchronized by the sharing
of hormones.
And once the eggs are mature and the male is ready, she
extrudes the eggs” in a kind of gelatinous sheath that can be 30 feet
long.
This acts like a sponge, readily absorbing the water that the male
has released his sperm into.
Keep in mind that this is happening several miles down, where there is little
plankton
for juvenile fish to eat.
So the whole gelatinous mess is buoyant,
slowly making its way to the surface, where the larvae hatch and feed,
ideally growing big and then migrating down to the depths.
The females of these species can live 30 years, according to Pietsch,
and over that time might collect several males, who provide sperm
season after season after season (there is no “not now, honey, I have a
headache” with anglerfishes).
But other than the security of maintaining
a constant source of sperm, why evolve such a complex ritual of
reproduction in the first place?
“The idea is basically that it’s a deep-sea economy measure,”
ichthyologist James Maclaine of London’s Natural History Museum wrote in
an email to WIRED.
“An anglerfish couple requires about half of the
amount of food they would if the male was the same size as the female
(and presumably living an unattached life).
He is stripped down to the
absolute bare essentials, she has to remain big due to the relative cost
of making large eggs as opposed to tiny sperm.”
Where such a size difference between sexes, known as
sexual dimorphism,
gets really interesting is its manifestation in the world at large.
The
famed evolutionary biologist Stephen Jay Gould wrote about this in his
essay
“Big Fish, Little Fish”
— which stars the anglerfishes, of course — arguing that in the
majority of animal species, females are larger than males, because the
latter often never need to fight for the former.
A female Lasiognathus amphirhamphus.
Photo: Theodore W. Pietsch
For creatures like lions and gorillas
and even humans, the largest male has an obvious advantage in the quest
to mate (and therefore better odds at passing along his genes).
But with
deep-sea anglerfishes, the male not only doesn’t fight other males,
he’s lucky to even find a female in the first place.
Hence their puny
size and remarkable mode of reproduction.
He’s really missing out, though, on all those great fishing trips
that the females take, with their gaping maws, needle-like teeth, and
highly expandable stomachs, which are yet more remarkable adaptations to
the abyss.
“At the depths at which many deep-sea anglers live … food rapidly
becomes more scarce the deeper you go,” said Maclaine.
“Therefore, a lot
of fish have evolved various means of being able to tackle large prey
items. When you’ve managed to catch your first meal in weeks, you don’t
want to have to let it go because it’s too big. Therefore big mouths,
fearsome teeth, and elasticated stomachs have all become
common features.”
In fact, many female deep-sea anglerfishes can swallow prey twice
their size, as Maclaine shows in the video above.
They are, after all,
sometimes eating for several freeloading males.
High school boys.
They’re all the same.
