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when most of us think of crabs we
probably picture marine animals living
in the ocean
eating fish and things the life aquatic
but there are also quite a few crabs
that have basically abandoned the water
we're talking
land crabs not just semi-aquatic beach
crawlers but crabs that spend most of
their lives
up here and there are even some that
live in trees this process of a species
evolving from living in water to living
on land is called terrestrialization and
depending on how you count it's happened
at least five times in crustaceans as a
whole several groups even seem to be
undergoing it
right now but exactly what's pushed
crabs to make the jump is a bit of a
puzzle
and it could involve anything from food
to predators to oxygen availability but
one way or another
it wasn't an easy switch fresh
realization requires nearly everything
to change
from how an animal eats to how it has
babies so here's
how crabs manage it when you're moving
from water to land probably the most
obvious thing that has to change
is the whole breathing part like many
other animals aquatic crabs get oxygen
they need from a series of gills
feathery structures that are chock full
of blood vessels and can extract
oxygen from the water but if you've ever
seen a fish flopping around on land
you'll know that gills often don't work
so well in the air the main problem
is those feathery structures that make
up the gills can collapse in on
themselves
leaving the animal unable to breathe and
therefore dead but land-going crabs have
found ways to adapt around this
some herbert crabs use the water in
their shells for gas exchange for
instance a more impressive adaptation
though is lungs
which show up in many land-bound crabs
like the robber crab which actually only
breathes air they'll enter the water to
drink or release eggs but they can't
stay too long or they'll drown those
crabish lungs aren't exactly like ours
though
they're modified branchial chambers
which is also where the gills are so
these crabs breathe in
through openings near the base of their
walking legs then out through their
mouths instead of the in and out mouth
breathing we're used to
in land crabs the tissue on the inside
of the chamber has become convoluted and
spongy
increasing surface area and allowing for
gas exchange so their lungs aren't
entirely new organs they're more like
heavy modifications to the structures
that were already there
meanwhile the gills also tend to change
a bit although how
depends on the type of crab in some
cases gills may get smaller
stiffer and have spacers on them all of
which help prevent that collapsing
problem we mentioned
a lot of crabs can also kind of switch
between gills and lungs as needed
to optimize oxygen delivery based on
their level of activity and environment
another species the gills mostly stick
around to regulate the crab's internal
ph levels
and to get rid of waste products they
also do that as a secondary function in
marine crabs but in these land crabs it
becomes their main job
of course just being able to breathe air
isn't enough to get around on land i
mean it's important but you also need to
be able to support yourself since
buoyancy is much less of a thing when
you leave the water that's why if you
put a whale on land
the force of gravity eventually crushes
its tissues thankfully though
crabs are not whales as you may have
noticed most
importantly here crabs don't use the
same internal bony skeletons whales and
other vertebrates use
they have hard kitenous exoskeletons
instead in the ocean
this is great for protection against
predators but it also really comes in
handy for walking out of the water in
general the crab shell is pretty well
built and more than enough to deal with
gravity and mechanical strain
but that's not to say there's no
challenge the biggest problem is what
happens when land crabs
outgrow their exoskeleton and molt when
this happens
the crab sheds its exoskeleton unveiling
a new
soft version that will harden and expand
over the next few days the problem is
that during that time they lack the
strong support system that's been
holding them up
and they become at risk of turning into
a crab pancake or a
crab cake if you will if this happens
their new shell will harden in the wrong
shape
impeding further molting and likely
killing the crab the way they've adapted
to this is through what's sort of their
other skeleton
the hydrostatic skeleton it's another
way crabs usually support themselves
involving pressurized internal fluids
that help keep the crab
inflated think less knight and armor
more
balloon animal all crabs water bound or
land bound have this but to stand up to
the force of gravity on land
the fluid pressure has to be much higher
in land crabs and that comes with some
trade-offs for one thing the greater
internal pressure puts them at higher
risk of the newly grown exoskeleton
rupturing it also becomes more expensive
energy-wise to move around compared to
marine crabs how strong this hydrostatic
skeleton can get might actually be the
major limit on the size of crabs like
coconut crabs which is lucky or unlucky
i suppose depends on how much you like
giant crabs anyway that's skeleton
and gravity once an animal's gotten onto
land walked around a bit and taken in
the air
it's eventually going to get hungry and
it turns out that open-air dining is a
bit different than eating underwater
marine crabs mostly eat algae and small
animals
but for land crabs the food source that
seems to come easiest to them is
vascular plants
fruits leaves that kind of thing these
kinds of foods are a lot tougher than
what's in the sea
compare a nice soft clam to something
crunchy and tough like celery they can
also be full of tough hard to digest
fibers like cellulose and even compounds
that are potentially toxic to the crab
like tannins and they can be pretty
nutrient poor compared to seafood too
especially when it comes to calories and
nitrogen to deal with this
herbivorous land crabs have developed a
number of adaptations for one
land crabs tend to grow more slowly and
live longer which lets them deal with
the low nitrogen they'd otherwise need
to grow like the coconut crab can live
to be 60 years old they also have
enzymes in their gut that can help
digest the cellulose and other fibers
some of these enzymes may be made by the
crabs themselves but one of the keys to
exploiting this new food source may
actually be
microbes we're still in the very early
stages of research on land crab
microbiomes the
communities of microbes that live in
their guts but we know that other land
arthropods depend on their microbiomes
for instance in isopods which are crab
relatives like roly-polies
it's been shown that their microbiomes
are critical to digesting plant material
microbes in their guts produce enzymes
called carbohydrate active enzymes
which help digest and get energy from
woody plant fibers they may also help in
other ways
like reducing water loss and digesting
nitrogen so breathing
walking eating these adaptations all
help individual crabs survive on land
when it comes to continuing entire crab
lineages though
there's a whole other set of adaptations
involved i mean
those 60 year old coconut crabs are
impressively old
but the crab lineage has been on land
for millions of years
to achieve that they need to reproduce
and that leads to some new challenges
in this case land crabs have mostly got
themselves a pass that's because they're
essentially still at what you might call
the
amphibian stage of conquering the land
in other words they still depend on
water for their larvae that said
many have had to change up how they do
things because you can't just release
your eggs whenever you want if you live
kilometers from the ocean
for example one species of hermit crab
can live near high cliffs
and throws clumps of hatchlings over the
cliff edges
to reach the ocean some other species
have solved this through brooding where
they use
pouches to carry their young until the
time is right kind of like marsupials
the abdomen actually folds under to
create a brood chamber a kind of
sealed water-filled chamber for the
embryos or at least a super-humid one
other crabs like the mandarin vampire
crab let their babies ride around on top
of them but some of the most remarkable
species
actively care for their young and raise
them away from streams or oceans
notably the bromeliad crabs raise their
young in bromeliad plants
in the spaces where water collects
between the leaves and the stem the
mother keeps
the water quality high stirring it for
oxygen adding snail shells to tweak the
ph
and killing predator larvae they feed
the offspring for several months and
they can even have
later broods in the same plant making a
family group meanwhile
jamaica's snail crab keeps its young and
water-filled snail shells the mother
refills the water turning the snail
shells upside down to catch rainwater or
carrying water to the shells
she also feeds the offspring for several
months these couple of species are
special
because they're the only known cases of
active brood care
and probably the result of one
evolutionary jump by an ancestor so
yeah crabs have had to change a lot in
order to make it onto land and we've
only been able to touch on some of the
challenges
because moving to a whole new world is
complicated but
move they have and they've done it time
and time again
understanding crab evolution requires
pushing boundaries and expanding our
ideas about the world
and that's the heart behind the album
music for scientists patrick olsen wrote
and recorded these songs for scientists
and science communicators
and they were all inspired by the beauty
of science they celebrate exploration
understanding and also the people who
push those fields forward if you want to
check out the album you can stream it on
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