It doesn’t bite but has quite a pinch! Coming up, Jonathan investigates the crabby world
of marine Arthropods! Welcome to Jonathan Bird’s Blue World!
There are more than a million species of Arthropods on Earth—more than any other phylum--mostly
because of these: bugs! I mean insects. 90% of the arthropods are bugs. I mean insects.
I’m going to leave the bugs to my buddy Coyote Peterson.
I’m Coyote Peterson, and I’m about to enter the sting zone with the Tarantula Hawk.
AHHHHHHHH!
Umm, no thanks! I’m going to stick to the other Arthropods, which are mostly underwater,
in my world!
All Arthropods share certain characteristics making them unique in the animal kingdom.
Some well-known marine arthropods include crabs, lobsters, shrimp, horseshoe crabs,
barnacles—yes, barnacles—and more obscure things like sea spiders (which are not spiders)
and copepods, which are not insects, even though they look like bugs. But they are kind
of like underwater bugs.
The most obvious characteristic of an Arthropod is the external skeleton (which is technically,
not a shell). Not only does this exoskeleton protect the animal like a very tough skin,
it actually does serve as the animal's skeleton. The muscles of an Arthropod are connected
to the inside of this exoskeleton. There is no internal skeleton.
Because the exoskeleton cannot grow, the animal must periodically shed its exoskeleton, a
process called molting. When it first molts, the new exoskeleton is soft. Over a couple
days, it expands to a larger size and hardens. Once it hardens, the animal is stuck at this
same size until it molts again.
Until the new exoskeleton has hardened, the animal is quite vulnerable and tends to stay
hidden.
Lobsters caught during this period of time are sometimes called "soft-shelled" because
they feel noticeably squishy.
Lobsters often eat part or all of their old exoskeleton because it’s a good source of
calcium required for the hardening of the new exoskeleton.
The name Arthropod means "jointed-foot." In order for an Arthropod to move in such
a rigid body, it has numerous joints in its exoskeleton. Like door hinges, they allow
bending in only one axis.
Still, a lobster is quite flexible when necessary, and can manage to rotate its claws sufficiently
to defend itself effectively--or take a selfie.
Most arthropods also have compound eyes. Each eye is composed of many smaller light-sensitive
organs. Together, they form a single working eye. The compound eye is not specialized so
much for detailed sight as for the detection of motion.
The Taxonomy of this phylum is complicated, with so many different kinds of Arthropods.
The Sub-Phylum Crustacea contains the Arthropods most people can identify. Lobsters, crabs
and shrimp for example are all Crustaceans. But so are barnacles, and copepods.
Most Crustaceans have five pairs of appendages. Usually the front pair, called Chelipeds,
have claws on them, while the remaining four pairs are walking legs. In many species, the
second and even the third pairs of legs have smaller pincers on them as well.
The more advanced forms of crustaceans, like lobsters, crabs and shrimp, have the body
divided into two regions: a cephalothorax, and an abdomen. The cephalothorax--meaning
"head-body”--is so called because it contains the "head" as well as the main body in one
piece. The abdomen (which many people erroneously call the tail), on the other hand, is mostly
muscular and used for swimming.
Barnacles are one of those crustaceans that don’t look like they belong in the same
group.
The body form of the barnacle is Crustacean, but it isn't very obvious unless examined
at the larval stage—where they swim around in the water column as plankton. But as they
mature, they settle to the bottom to begin their next stage of life.
Attached to a hard surface, they create their own little "houses," called carina, which
look like tiny volcanoes. Barnacles get their food by waving their limbs in the water to
catch drifting plankton. They’re basically glued to the rock by their head.
Another Crustacean that looks nothing like a crab: the remipede. It looks more like a
bristle worm, but these animals live only in caves. They are so rare that they weren’t
even discovered until the late 1970s, by who else but cave divers!
Horseshoe crabs--which are not crabs or horseshoes or crustaceans--look quite different from
other Arthropods. They belong to an ancient group of Arthropods going back 450 million
years.
Circa 1870, the horseshoe crab was given the name "horse-foot crab," which is a better
description of its shape than "horseshoe." However, somehow the name became corrupted
between then and now.
Arthropods reproduce by creating thousands of eggs. With crabs, shrimp and lobsters,
the female carries them around in a cluster on her tail until they hatch.
Once they hatch, the babies drift in the ocean as planktonic larvae for a while until they
grow a bit and settle down. That’s how they spread far and wide.
Arthropods live in every environment on Earth. And even if you take out the insects, and
things like land crabs, there are thousands of marine Arthropod species, from the ocean
to fresh water.
The shallows to the deep sea.
And even in deep inside caves where there is no light.
Given that they go back more than 500 million years, Arthropods aren’t going anywhere.
They will probably be here long after humans are gone, roaming the Blue World.
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