Moulting Biology of Shrimp and Crayfish

How shrimp and crayfish moult: stages of the moult cycle, hormonal control, where calcium for the new shell comes from, and the documented causes of failed moults.

Why crustaceans moult

Shrimp, crayfish and other crustaceans belong to the Ecdysozoa, animals that grow by shedding their cuticle. An exoskeleton cannot stretch, so to grow the animal must replace it. The process is called ecdysis, and the discarded shell is the exuviae (or exuvia). Right after a moult the new exoskeleton is soft and only hardens over the following hours to days. In crustaceans, moulting is needed not only for growth: broodstock of farmed shrimp also go through a reproductive moult before mating, and female ornamental shrimp of the species Neocaridina davidi release their eggs only after moulting.

Young animals moult far more often than adults. A Pacific white shrimp (Penaeus vannamei) moults about 50 times in its life, 11–12 of them during larval metamorphosis. A 2 g shrimp has a moult cycle of about 5 days, and a 15 g shrimp one of about 11 days. Juvenile Neocaridina davidi likewise moult more often than adults.

Stages of the moult cycle

The crustacean moult cycle is usually divided into four main phases, labelled with letters. In Penaeus vannamei the premoult phase is further split into substages based on changes in the tail appendages.

1. Intermoult (C): the hard-shelled phase between moults, when the animal feeds and grows tissue.
2. Premoult (D, with substages D0–D3): the animal prepares to shed. Hormones start the process, the old cuticle separates from the underlying tissue, and a new cuticle forms beneath it.
3. Ecdysis (E): the actual shedding, which lasts only seconds to minutes.
4. Postmoult (A and B): the new exoskeleton takes up minerals and hardens.

Hormonal control

Moulting is controlled by the neuroendocrine system. The X-organ–sinus gland complex in the eyestalk releases molt-inhibiting hormone (MIH), which suppresses the production of ecdysteroids, the moulting hormones made by the Y-organs from cholesterol. When MIH levels fall, the Y-organs release ecdysteroids and premoult begins. Ecdysteroids bind to a receptor complex (EcR with RXR) and switch on a cascade of genes, including chitinase, that carry out the moult. Other eyestalk neuropeptides, such as crustacean hyperglycemic hormone (CHH), adjust energy metabolism and the progress of the moult. After ecdysis the hormone bursicon helps harden the new cuticle.

Calcium and the new shell

Calcium is the key mineral for hardening the exoskeleton, and in fresh water it is scarce. Freshwater crayfish deal with this by storing calcium in gastroliths, stones in the stomach that serve as a calcium source for the moult. LSU AgCenter research on soft-shell crawfish describes where calcium for the new shell comes from: the dissolving gastroliths, the hepatopancreas, the blood (haemolymph), and water and food once feeding resumes. In that work, shell hardening normally took about 72 hours, and the longer after the moult, the more gastrolith calcium had moved into the shell.

Crustaceans also recycle their old shell. Crayfish and Neocaridina davidi eat their exuviae to recover the minerals it contains, which for crayfish means calcium and phosphates. Crayfish can absorb calcium directly from both food and water. In a 2023 feeding trial with juvenile red claw crayfish (Cherax quadricarinatus), adding calcium-rich powdered eggshell or seashell to the diet improved growth, moulting frequency and the calcium content of the carapace.

Vulnerability after the moult

Until the new shell hardens, the animal has little protection. In shrimp farming, moults that do not happen at the same time across a group leave freshly moulted animals open to attack and cannibalism by their tank mates. Crayfish are known to eat one another. In laboratory studies of marron and yabbies (Cherax), researchers provide PVC pipe shelters specifically to prevent cannibalism during moulting.

Causes of failed moults

Mass deaths caused by moults that do not complete properly are described in the literature as moult (molt) death syndrome (MDS). The sources reviewed document several contributing factors.

• Ammonia: ammonia is toxic to crustaceans and can disrupt moulting, up to MDS. In juvenile crabs, raised ammonia lowered moulting rates and disturbed moult hormone signalling, although effects differ between species.
• Poor nutrition: in crayfish farming, high mortality during moulting has been linked to poor nutrition and general condition, and calcium supply affects moulting frequency.
• Temperature, salinity and light: in crabs and shrimp, these factors change how moult hormones are made and received. Low temperature can prevent moulting, high temperature shortens the moult interval, and unsuitable light intensity can suppress moulting.
• Chronic stress: long-term stress in crustaceans inhibits growth, reproduction, moulting and metamorphosis.
• Toxins: toxins from bacteria or fungi have been shown to disrupt moulting in copepods.