The largest group of animals on Earth are the Arthropods. There are more than a million species known and probably many millions more that haven’t been discovered yet. They include a wide range of animals including spiders, crabs, ants, beetles and countless others. They have several characteristics in common, but the one I want to focus on here is a process known as ecdysis, or molting.
All members of this gigantic phylum have an exoskeleton covering the body. This is not a calcareous shell like the one found on mollusks, but is protein-based. It does not grow with the animal so an arthropod must periodically shed this outer covering and replace it with a larger one. The process of ecdysis is not simply a physical one, but also involves hormonal changes and diet. Although the basic process applies across the board in arthropods, the exact procedure differs slightly with each group within the phylum. Let’s use three marine animals as examples below: The blue crab (Callinectes sapidus), American lobster (Homarus americanus) and the horseshoe crab (Limulus polyphemus).
Molting actually occurs in three stages. The actual molting stage, when the animal removes the old exoskeleton, is called Ecdysis and involves the animal absorbing water into its flesh which creates pressure on the old exoskeleton, causing it to crack. This fissure point varies among groups: In crabs the fissure occurs laterally along the rear end of the body where the carapace (top part of the shell) meets the lower shell. In horseshoe crabs the crack occurs along the front margin of the shell, and in lobsters the fissure appears on the dorsal surface where the carapace meets the first segment of the tail.
Prior to ecdysis is a pre-molt stage called precdysis. This stage is mostly unnoticable and involves mainly hormonal changes within the body which cause a breakdown of the inner exoskeleton material, causing it to detach from the musculature. This broken down material is recycled and forms a papery shell precursor beneath the existing exoskeleton.
Postecdysis occurs after the old shell is molted and involves the process of growth and hardening of the new exoskeleton. This is the longest of the three stages. Although the exoskeleton becomes noticeably thicker within hours, it may take a couple of days for it to become fully formed. The animal does not eat during the first two stages but it will often consume part of the molted shell to ingest minerals that help the hardening process along (the first body parts to harden are usually the mouth parts for this reason).
Blue Crab
Ecdysis starts early in the life of a crustacean. Blue crab larvae are planktonic for several days before settling onto the bottom, and during this time they undergo a series of molts. The pre-molted, newly hatched larval stage is called a zoea. This stage will molt into a vaguely crab-like plankter known as a megalops. The megalopa will molt a few times before turning into a juvenile crab which settles to the bottom. Here’s a diagram of these stages.
As you can see in the photo above, the fissure along the backside of the body is where the entire animal exits from. Everything is shed in one piece including the legs, claws, antennae and even the transparent eyeball coverings. On most crabs the claws pose a special problem since the diameter of the opening at the base of the claw is much smaller than the claw itself. Crabs solve this problem by having what are called fracture planes at the claw’s base. Like scored glass, these weak points split apart during molting, enlarging the opening the flesh inside needs to pass through. Below we see the next step in the above blue crab extricating itself from the old shell.
Blue crabs live for about three years and shed an average of about twenty times during their lives. Larger crabs shed less often than young ones, thus overall growth rates slow down as the animals age. One puzzling aspect of this whole process is if the crab sheds to grow, how can a larger shell exist beneath the old one? The answer lies in the fact that the underlying pre-exoskeleton is not only very thin, but also elastic. Once the crab is out of the shell it will absorb a tremendous amount of water, swelling the body more than double its previous size. The precursor shell stretches with the expanding tissues and hardens in this new size. Notice in the photo above the crab has just emerged and hasn’t yet inflated itself with water.
Horseshoe Crabs
As mentioned, horseshoe crabs (which are not only not crabs, they aren’t even crustaceans) fissure along the front end of the body. Other than that the process described above applies for the most part. One difference in these animals is how the new shell expands after the shedding process is completed. Rather than swallowing water to increase the size of the body as crustaceans do, the horseshoe crab’s precursor shell is pleated like an accordion. As the new shell slowly emerges from the old one, these pleats expand one by one and harden into place soon after shedding.
An adult horseshoe crab normally sheds once a year, after its winter dormancy and just before spawning in the spring. Molting usually occurs underground and this is usually assumed to be a way of protecting itself from predators during this vulnerable time. Having kept these animals for years I have my doubts that this is the sole reason. I think a more important factor is the combination of the friction the sand has on the shell and the weight of the sand above the buried animal is required to help pull the old shell off the body. I’ve noticed that a horseshoe crab dug from the sand in mid-ecdysis, like the one below, will be unable to continue the process without help.
American Lobster
Like their relatives the crabs, lobsters also go through a young planktonic stage. The larvae molt three times in fairly rapid progression before settling to the bottom for good. In lobsters, the fissure occurs on the top part of the body, rather than along the edges. I’ll use this photo of a lobster with very unusual coloring to illustrate where the shell splits. The part of the body that is all black is the carapace. The shell splits along the top right where the carapace ends and the half-black tail begins. Notice that even the relatively huge claws are also pulled out whole during this process.
Like crabs, young lobsters molt fairly often, but the process becomes less frequent as the animal ages. A large adult may only shed once every ten years while a year-old individual may molt every couple of months. The ecdysis process is very energy consuming and to save strength for egg production the females tend to molt less often than the males do.
I’m including a video of one of my immature lobsters molting below, but before you watch it I’ll need to set this up a bit. First of all, keeping undersize lobsters in captivity is illegal, however because mine are used in educational programs the state issues me a permit that allows me to take six each year. I keep them for one school year and then release and replace them. This individual was captured without any claws (they lose them in territorial fights with other lobsters).
We were teaching a group of third graders about marine life when a student came up to one of my staff to tell her a lobster was dying. She took a look, knew it was molting and grabbed her digital camera to film the event. The quality isn’t all that great since it was hurriedly shot with the video option of the camera, but it is still pretty remarkable. These animals very rarely molt during the day, preferring the relative safety of darkness. She caught just the very end of the process and you can see at the beginning the animal is mostly out of the old exoskeleton, but you’ve got to love the little tail flip it uses to leave the shell completely.
Notice two things as you watch this: First, how she can barely move after leaving the shell behind. The body is so soft it cannot support the animal’s weight even under water. Second, voila! New claws! Click the image to videofy.
Other diaries in this series can be found here.