Why arthropods molt

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BR Molting shedding or ecdysis of the outer cuticular layer of the body is a process vital to arthropods, including insects and crustaceans. See also: Arthropoda ; Crustacea ; Insecta An adult cicada emerges from its year nymph stage, molts, and arises as a winged adult.

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About AccessScience AccessScience provides the most accurate and trustworthy scientific information available. Download the flyer Get Adobe Acrobat Reader. However, the circumstances of fossil preservation should also be considered. Environmental conditions such as fast-flowing river currents or decay processes at the time of fossilization may result in disarticulation of a complete carcass that looks like moulting.

Subsequent deformation can also produce repeated crushing and overlapping features, making carcasses look like moults. For some arthropod groups, it is particularly difficult to study moulting Fig. Horseshoe crabs Xiphosura and scorpions Scorpiones, both part of Chelicerata moult using a horizontal gape suture at the front of the animal, without requiring disarticulation of any body segments Fig.

The suture closes after exuviation, often leaving a completely intact moulted exoskeleton that may be mistaken for a carcass. In a scenario such as this, it is important to carefully compare numerous fossil specimens, and to identify any repeated limb orientations or segment preservation styles. For example, splaying and extension of the limbs in modern moults has allowed researchers to describe the moulting behaviour of extinct scorpions from museum collections Fig.

The successful identification of moults in the fossil record has allowed for the reconstruction of moulting behaviours in many arthropod groups. Some of these, such as trilobites or eurypterids sea scorpions , have no living relatives, and the moult fossil record therefore provides the only available source of behavioural and developmental data. For other groups, studies of moulting have shown that highly specific anatomy-linked behaviours were ancestral and evolved long ago.

For example, crabs that lived around 70 million years ago moulted like modern crabs. Studying fossilized moults enables us to trace the origins and prevalence of different moulting strategies, and the effects that these had on the long-term evolution of morphology body shape , behaviour, development and ecology in arthropods. The evolutionary histories of moulting in a few key arthropod groups are discussed below.

Decapoda, a group of crustaceans, includes the shrimp, lobsters and crabs. At least two moulting behaviours can be observed for living decapods, and these extend far back into their early fossil record. Shrimp and lobsters, which have a long body formed of a carapace shell and segmented abdomen, moult through a gape suture formed between these two parts. The animal then withdraws its limbs from in front of the body, and emerges forwards and upwards from this gape Fig.

Figure 6 — Video of a moulting juvenile rock lobster. Creative Commons licensed. Crabs, which have a rounder dorsal back carapace, also break along the back edge of the shell, disarticulating it from the abdomen at the rear end. This often involves the carapace rotating away or flipping forwards towards the head. To exit the old exoskeleton, the crab moves backwards, withdrawing its legs last from in front of the body Fig. Understanding moulting in crabs has had an impact on the food industry.

We can also use these descriptions of moulting in living representatives to identify fossilized moulted exoskeletons, when we see similar gape sutures. For all these crustaceans, this style of moulting can be seen throughout their history. Figure 7 — Video of a moulting spider crab. The group Chelicerata includes the spiders and scorpions Arachnida , and the horseshoe crabs and extinct sea scorpions together, the Merostomata.

Spiders, potentially including fossils from the Devonian million to million years ago and Permian million to million years ago periods, moult using a similar behaviour to crabs. Their similar anatomy means that they use the same backwards movement out of an opening at the back and side of the old exoskeleton, leaving behind moults with disrupted carapaces and clear stepwise body-size increases.

This can be seen in images of moulted pet tarantula that are reminiscent of a horror movie Figs 2C, 5A. We can use knowledge of moulting behaviour in groups with living representatives to describe the behaviours of entirely extinct groups. For example, sea scorpions are similar in appearance and closely related to scorpions. They probably moulted in a similar way, also opening a gape suture on the belly surface, between the limbs. The head shield was pushed upwards, often disarticulating it and one or more body segments Fig.

Trilobites were one of the most successful groups of Palaeozoic arthropods. They were an extremely anatomically, geographically and ecologically diverse group Fig. Their ultimate demise in the mass extinction at the end of the Permian period followed a series of declines in diversity.

This means that we can only reconstruct their moulting behaviour through comparison to extant groups such as those described above, and through the preservation of fossilized moults. For trilobites, moulting is one of the few direct behaviours that we can reconstruct, and it is important for helping us to understand their ecology, evolutionary relationships and morphological evolution. Trilobite moulting is particularly interesting to look at because it separates these animals from the rest of Arthropoda.