Slide 1
A rigid shell limits growth
A crab's hard outer skeleton protects its body but limits expansion. Growth requires shedding that old covering through moulting.
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The shell is an exoskeleton, a skeleton on the outside. Replacement creates a different growth pattern from an animal whose internal bones and soft exterior enlarge more continuously.
Slide 2
Preparation starts underneath
Before moulting, a crab separates its old shell from underlying tissues. It also recovers some mineral material from the covering.
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A new covering forms underneath while still flexible. The crab prepares its next body boundary before escaping the old one, rather than waiting exposed to begin making a shell.
Slide 3
Leaving the old shape
The crab emerges from its old exoskeleton and takes up water. Expansion creates space before the new covering becomes rigid.
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The water-filled space can later be occupied by growing tissues. A larger outer size after moulting does not mean the crab instantly produced the same amount of new muscle.
Slide 4
Protection takes time to return
A newly moulted crab is vulnerable while its covering hardens. Shelter is especially valuable during this part of its life.
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Risk is a cost of obtaining room to grow. Feeding opportunities and environmental conditions influence the balance, making moulting an ecological event as well as an anatomical change.
Slide 5
A shell is not a birth certificate
Crabs repeatedly discard their outer covering as they grow. Counting marks on the current shell cannot simply reveal their age.
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Researchers study size, growth rates and moulting history to understand populations. Shell condition may indicate time since a moult, which differs from the crab's total lifespan.
Sources
- Alaska Shellfish Growth Research
NOAA Fisheries
- Fun Facts About Intriguing Invertebrates
NOAA Fisheries