Page 293 - (DK) Ocean - The Definitive Visual Guide

ARTHROPODS 291

Growth HUMAN IMPACT

Crustaceans can only develop and grow by molting and replacing their KRILL DECLINE
exoskeleton with a larger one. The moulting process, called ecdysis, is
1
controlled by hormones and occurs repeatedly during adult life. The Antarctic krill are only 2 ⁄2 in (6 cm)
exoskeleton is produced from the layer of cells situated immediately long but are among the most abundant
crustacean arthropods. Numbers in the
below it. Before a molt starts, the exoskeleton detaches from this cell Southern Ocean have fallen over the
layer and the space in between fills with molting fluid. Enzymes within past few decades, partly due to rising
this fluid weaken the exoskeleton so that it eventually splits at the water temperatures and melting of ice.
weakest point, often somewhere along the back. The new exoskeleton Krill feed on algae that grow beneath
is soft and wrinkled, so it needs to expand and then harden. Marine and within the sea ice, and shelter
arthropods absorb water rapidly after molting to expand their new under the ice to avoid predation.
Over-harvesting of krill for human
protective covering. Those that can remain hidden for hours or days, as and animal feed also poses a significant
they are more vulnerable to predation until their exoskeleton hardens. threat with potential to disrupt the
Antarctic food web (see p.295).
THE MOLTING SEQUENCE
This sequence shows a harlequin
shrimp molting. The exoskeleton has
split just behind the neck joint, allowing
the shrimp to pull out its head. The rest of
its body quickly follows as the split enlarges.
It takes only a few minutes for the shrimp
to free itself completely, after which it rests
for a few seconds.
The new exoskeleton is soft, since it
must be flexible to buckle up to fit inside
the older, smaller skeleton. It stretches
to accommodate the increased size of the 1 The old exoskeleton splits along the 2 The shrimp emerges further 3 Molting is complete, and the old
shrimp. Complete hardening of the new back behind the harlequin shrimp’s head. and struggles to free itself from exoskeleton lies beside the shrimp, as
exoskeleton will take about two days. It eases out backward. the old exoskeleton. the animal rests. OCEAN LIFE