Background
The Landlocked Crab
The species of crab studied in CSU’s Crab Lab is Gecarcinus lateralis, commonly known as the blackback land crab. These are fist sized, terrestrial crabs decorated with bright orange legs and claws with a splash of deep purple on their backs (hence the common name, “blackback”). These creatures inhabit the beaches of the Dominican Republic and other Caribbean islands, thus they are very accustomed to a tropical climate. Because G. lateralis is adapted to terrestrial conditions and does not require an aquatic environment, it is an ideal species to study in a laboratory setting. Each year hundreds of these unique crabs are collected and shipped to the CSU Crab Lab where they are continuously cared for and studied by students in the lab. Down in the laboratory basement they remain, feeding, growing and going about their seemingly simple lives; all the while researchers upstairs work diligently to uncover the mysteries that lie underneath their shells.
Natural Transformations
Mutare means “to change” in Latin, the base language of science. When we think of change in the animal world we see fluffy baby owls growing into bold predators, scrawny caterpillars transforming into vibrant butterflies and massive snakes leaving behind a crumpled shadow of themselves. Scientists refer to each of these transformations as a molt (a term directly derived from the Latin root.) Molting is the process of shedding old skin, feathers or skeleton to make way for new growth. The molting process of crabs is of particular interest to the CSU Crab Lab, as researchers delve into the specific processes and mechanisms that control this seemingly simple, but very complex event.
An Insight into the Animal World
What exactly does molting mean in crabs? Crabs belong to a broad class of animals known as Arthropods. This group includes insects, scorpions, lobsters, and a wide variety of others. A unifying trait for all Arthropods is their outer layer of hardened skin that covers the animal like a suit of body armor. This covering is known as an exoskeleton, and enables these creatures to move about the world while staying well protected. But how do these animals grow with such a rigid outer layer encompassing their bodies? This is where the molting cycle comes in.
A Little Warrior
Imagine a tiny warrior fitted in a strong coat of armor. He will run about his business, and his armor will protect him from harm. But what happens when the warrior starts to grow bigger inside his body suit? Obviously a coat of armor is too tough to stretch with the warrior’s growing body, thus he is left to go to the blacksmith and get fitted for new armor. Since crabs do not have the luxury of a blacksmith to forge them a new exoskeleton, they must make one for themselves.
Forging the Protective Barrier
A crab, being a crustacean with two large pinching claws rather than dexterous hands, cannot assemble a suit of armor like a blacksmith would. The way the new exoskeleton is made and established is a sophisticated process that the crab’s body innately does (which, if you think about it, may be even more impressive than if the little creature were to sit down at a wood burning stove and start crafting metal tools with a mallet). Instead of the blacksmith method, the entire synthesis of the new exoskeleton occurs within the crab’s body, deep within the tissues. This synthesis coincides with the degradation of the old exoskeleton, ensuring that the soft-bodied crab has a protective covering soon after it sheds its old one. What goes on to make all of this happen in a timely manner is a series of actions and reactions by the crab’s cells, which in turn elicit changes in different parts of the animal. There is a lot going on at once, but basically all of the components of the crab are working together like parts in a machine, to facilitate the replacement of the exoskeleton. This process is essential for the crab’s growth, protection and survival.
The Underlying Controls
A hidden, but extremely essential aspect of this process is a series of cellular regulators and inhibitors that work together to manage the flawless timing of the molt. First, we’ll look at the inhibitor portion of this combination. There is an organ in the crab that contains a molting inhibiting hormone (or MIH). MIH is a chemical that when secreted from the cells, halts the molting process in the animal. The organ is called the X-organ, and is located in the crab’s eyestalks. If we look at a different organ in the crab called the Y-organ (found in the crab’s main body cavity), we will find a hormone that has opposite effects on the molting cycle. The Y-organ is the main growth regulator in the crab, and secretes chemicals known as ecdysteroids. When ecdysteroids are secreted the crab’s molting process is initiated.
In these two separate organs we have two different chemicals with opposite effects on the molting cycle. How then, does the molting cycle occur? What goes on physiologically in the crab is a steady release of ecdysteroids by the Y-organ, which instruct the crab to grow and molt. The crab will carry on molting and growing until it is unfavorable to do so. At unfavorable conditions (during times of environmental stress) the X-organ will secrete the molting inhibiting hormone, and the molting process will cease. When it is time to molt again, the X-organ will stop secreting MIH, and the process will continue.
The two hormones work in synchrony with each other to regulate when the crab undergoes the molting cycle and when it ceases. The stages described below are an outline of the changes that happen not just with the cuticle layers but also with the different hormone levels associated with the X and Y-organs.
Initiating the Cycle
When the crab is growing too big for it’s shell, cell pathways are triggered and messages are sent telling the molting process to begin. As mentioned before, there is a lot to this process, and it doesn’t just entail the molt, or shedding of the exoskeleton. The process is a cycle and requires a lot of preparation and post molt activity in order to maintain continuous protection and growth throughout the animal’s life.
The first step in the molting process is referred to as the premolt stage; it involves preparation steps needed before the shell can be shed. The cells in the outer skin layers and exoskeleton are told by the messengers to loosen connections between the two layers, so the exoskeleton can be detached. While this is happening to the top layers of integument (or skin), new layers of integument are being synthesized underneath. Once the top layer is shed, it is this newly synthesized layer that becomes the new exoskeleton.
Another very important step of the premolt phase is called “claw muscle atrophy.” Atrophy means to decrease in size, and in order for the crab’s body to squeeze out of the exoskeleton it must first get smaller.
Out with the Old, In with the New
Once these preparation steps have occurred, the actual shell shedding is ready to happen. The soft-bodied crab crawls out of its old shell, and is left exposed and vulnerable to the outside world until a new shell has formed and hardened. During this vulnerable stage when the outer covering has not hardened, the crab will take in as much water (if it’s an aquatic species) or air (if it’s a terrestrial species) as it can, inflating the new shell to increase body size. This allows the shell to harden at its maximum size and leave room for tissue and muscle development. Following the molt, the crab will eat the exoskeleton it has just shed. Ingesting this calcium rich shell allows the animal to stock up on nutrients needed to synthesize the next shell.
Toughening Up
Directly following the molt, the crab’s soft body is unprotected as the newly synthesized exoskeleton takes form. Stored nutrients are used to calcify or harden the skeleton, so the crab is protected again. This hardening takes roughly a couple of weeks, and is actually finishing up during the next premolt stage. Once completed the shining new suit of armor will serve as the crab’s exoskeleton for a variable amount of months, depending on species. This span of time between molt cycles is referred to as the intermolt stage. The crab will continue to grow until it once again becomes too big to fit inside its exoskeleton, at which point the process will repeat itself.