These animals are also plants … wait, what? - Luka Seamus Wright
Take a good look at this slug.
No, not that— that’s a leaf.
This slug.
There we go.
Elysia chlorotica may not look like much—
okay, it looks like a bright green leaf—
but it’s one of the most extraordinary
creatures around.
Living in salt marshes
along the east coast of North America,
it can go about a year without eating.
During that time, it lives like a plant.
Generally speaking, animals are
what are called heterotrophs,
meaning they can’t produce their own food—
they’re consumers of other life.
Plants, meanwhile, are autotrophs,
or producers:
they can synthesize their own fuel
from sunlight, CO2,
and other inorganic compounds.
Plants do this by using organelles
called chloroplasts,
which give them their bright colors
and convert sunlight into food
through photosynthesis.
Elysia is what’s called a mixotroph:
it can both consume food, like animals,
and produce its own
through photosynthesis, like plants.
In fact, Elysia steals its ability
to photosynthesize
from the algae it eats
by piercing the algal cells
with specialized pointy teeth,
called radula.
It sucks the cell empty and digests
most of its contents,
but the chloroplasts remain intact.
They’re incorporated into the epithelial
cells lining Elysia’s digestive system
that branches throughout its flat body.
This makes the slug look
even more leaflike,
providing camouflage as well as food.
As incredible as this adaptation is,
there are more than 70 species of slug
that steal chloroplasts from their food.
What makes Elysia
and a few closely related species
in the Mediterranean and Pacific unique
is how long they can hold
onto chloroplasts—
most other slugs keep them
for a few weeks at most.
This longevity seems to be due
to the survival abilities
of both plastids and slugs.
Specifically, the chloroplasts
of certain algae
can repair their own
light-harvesting systems,
while most chloroplasts are thought
to rely on their host cell
and its genes for repairs.
This makes the chloroplasts
able to sustain themselves
for longer inside the slug.
Meanwhile, the slug adjusts its gene
expression
to improve its relationship
with the chloroplasts
and removes damaged plastids
to avoid accumulation
of potentially damaging chemicals.
Though few species can steal organelles
from another species’ cell,
these slugs are far from alone
in getting an assist from plants.
Organisms as diverse as corals,
giant clams and sponges have symbiotic
algae living inside their cells,
supplying them with organic compounds
through photosynthesis.
In turn, they supply their little helpers
with shelter and inorganic compounds.
Some of these mixotrophs even transmit
the algae to their offspring.
Without the aid of these algae,
filter-feeding corals, clams, and sponges
would not gain enough nutrition
in the nutrient-poor tropical ocean,
and the dazzling coral reefs they build
simply would not exist.
Mixotrophy even cuts both ways:
an alga called Tripos furca can consume
several microscopic animals a day,
allowing it to survive
in darkness for weeks.
Tripos is in turn eaten
by other mixotrophic algae,
providing frequent opportunity
for exchange of organelles
such as chloroplasts.
This seems to allow some algae
to survive in parts of the dark ocean
such as the Mariana Trench,
which plants otherwise wouldn't
be able to inhabit.
The processes by which Elysia
becomes photosynthetic
and Tripos switches between feeding modes
are reminiscent of what scientists believe
led to the origin of all plants.
Single-celled animals preyed
on cyanobacteria.
Some of these tiny plants were not
digested and lived on in the animal cells,
eventually giving rise to chloroplasts.
But these first eukaryotic plants were
soon consumed by other animals,
which hijacked the precious chloroplast,
just like Elysia.
And following the example
of eating and being eaten,
we’ve seen in the case of Tripos,
this chloroplast heist happened
up to three times,
giving rise to plastids
with four membranes
and the ocean’s most productive
plants and forests.
