How do Lobsters Grow? All about Lobster Molting 

Estimated reading time: 6 minutes

Lobsters grow by molting. This process entails them struggling out of their old shells while simultaneously absorbing water that expands their body size. To grow, a lobster needs to shed its hard carapace and replace it with a newer larger carapace. The shell of a lobster, called the “exoskeleton”, provides the lobster’s shape as well as an excellent armor protection from enemies. Learn learn why and how a lobster molts from the experts at LobsterAnywhere.

How Lobsters Molt
Ana Castillo Ericksen illustrates a lobster growing.

How Do lobsters shed their shells or molt?

Before shedding its old shell, a new soft shell is formed under the old shell. To get out of its old shell, the body shell splits and the skin between the tail and body shell opens. Next the lobster roll over on its side and bends itself almost in half to pull itself out of its old shell. The new shell lobster then pumps itself with fluid to enlarge the very soft shell. Once a lobster has moulted, it  absorbs sea water to rehydrate as quickly as possible which lowers the concentration of protein in the lobster’s hemolymph (or blood).The new shell is paper thin and the lobster is very vulnerable to predators at this time. The lobster then focuses on feeding and “filling” up that new shell and, as it does that, the protein level increases.

In most cases, lobsters mate after the female moults. Before that stage, the female release pheromones (chemicals) into the water to let nearby males know she is preparing to moult and mate. Her chosen mate will take the female into his hideawy and protect her while she is vulnerable due to a fragile shell.

Lobster molting
A lobster molting into a new shell. Cartoon by Ana Castillo Ericksen

Lobster Molting Facts

How big does a lobster grow after each molt?

A lobster increases in size by about 20% with each molt.

How long does it take a lobster to molt?

It takes a lobster about 30 minutes to molt.

How often to lobsters molt or shed?

Most lobsters shed every year but less frequently as they grow older and larger. Within the first five to seven years of a lobster’s life, a lobster may molt (shed) its shell up to 25 times. As the lobster gets bigger, the number of molting events decrease. Adult lobsters only molt about once a year (males) to once every two years (females).

When lobsters molt, why don’t you see a cast-off shell?

After molting, lobsters eat voraciously, often devouring their own recently vacated shells. This replenishment of lost calcium hastens the hardening of the new shell which take about 14-30 days from the actual loss of the old shell.

What are the lobster size requirements? What is a legal size?

Lobsters take about 5-7 years to mature (called chickens) and will weigh about 1 pound each. A chicken lobster will measure about 3 1/4 inches in length from the eye to the top of the tail. Learn more about what is a legal size lobster.

When do lobsters stop growing?

Lobsters never stop growing. In fact, the heaviest one ever caught reached 44 lbs in Nova Scotia, Canada in 1977. On average, the American lobster reaches 8-24 inches long and weigh 1-9 pounds. The longest American lobsters have a body (excluding claws) of 25 inches long.

How many different types of lobsters are there?

There are only 2 main groups of lobsters but with various species — clawed lobsters (30 species) and Spiny or Rock lobsters (45 species). The best known clawed lobsters are the American and European lobsters that both thrive in chilly, shallow waters. Marine spiny lobsters, typically a warm water crustacean, are typically found off the coast of California, Florida and in the waters of the Caribbean Sea.

What were lobster used for centuries ago?

As far as 2000 years ago, in North America, lobsters were so plentiful that Native Americans used them to fertilize their fields and to bait their hooks for fishing.

Lobster Meat and the Molting Cycle

Soft-shells are lobsters that have molted recently, shedding their old hard shells so they can grow into new, larger shells with growing room. That room fills with water, making the yield slightly less per pound. Soft shell lobsters, or shedder lobsters, may be sweeter but contain less meat, cannot survive long-distance shipping, so are generally eaten only close to where they are caught.

There is less meat in a new shell lobster. In time, more muscle (meat) grows until it’s time to molt again. After molting and before the lobster’s shell becomes “hard” again is when the terms “soft shell”, and “new shell” are used to describe the Maine lobster.

Hard shell lobsters haven’t shed yet but have reached capacity in their shells and are packed with very full and tasty meat. These are the only lobsters shipped by LobsterAnywhere because soft shell lobsters will not survive in transit because they are too fragile. Learn more about the differences between hard shell and soft shell lobsters.

You followed the cooking instructions exactly, but your lobsters became waterlogged? There are a couple of theories about why the cooked lobster came turned out water-logged: (1) allowing lobster to cook too long, boiling lobster (vs. steaming them) or (2) the lobster recently molted. Chris Kimball, one of the founders of America’s Test Kitchen is convinced that it is the molting cycle of the lobster that determines whether or not the meat will be firm and dense or soft and water-logged.

How to can you tell a lobster that just molted?

Marine Scientist, Joseph Kunkel explains that you can identify when a lobster molted by the presence of hemolymph. This is the blood protein present in lobsters. A newly molted lobster does not accumulate enough of this protein because it is needed to build its new exoskeleton. This is also why you will not find the presense of the white gelatinous substance in a cooked soft-shell lobster. The hard shell lobster, on the other hand, will have more meat and hgher hemolymph protein levels as evidence by congealed blood when cooked.

Shell Color of Molted Lobsters

The color of a hard-shell lobster’s meat is also different than that a soft-shell lobster. You will notice that a soft-shell lobster has a brighter red color, due to the carotenoid pigments becoming more concentrated right before, or right after, a lobster molts. A hard-shell lobster’s meat is also red, but it’s a bit duller in color.

Some prefer hard shells for their meaty taste and firm, dense texture. While others prefer the sweeter, tender taste of soft shells. Learn more about the differences in hard shell lobsters versus soft shell lobsters.

Lobster Molt
New Shell lobster cartoon by Ana Castillo Ericksen

What type of lobster does LobsterAnywhere sell?

Here at LobsterAnywhere , we sell the American Lobster, aka Homarus Americanus, a species of lobster found on the Atlantic coast of North America. These cold water lobsters are straight off the Maine coast, only have two large claws and whiter meat than warm water lobsters, and are what people think of when they refer to Maine lobster. The Maine lobster is available as hard shell lobsters, which is what we ship live.

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