The New Zealand pea crab (Nepinnotheres novaezelandiae) is a parasite that spends its adult life within a mussel shell. However, the larval stages of its life cycle take place in the open ocean.
As a postgraduate research student at Leigh Marine Laboratory, Jessica Feickert was interested in understanding the pea crab life cycle, particularly the larval stages. She was also interested in the settlement cues (signals) that larval crabs use to help them locate a new mussel to call home.
Adult life: housed by a mussel
Throughout their adult lives, pea crabs live within green-lipped mussels. They lead a solitary life, with just one crab usually inhabiting each mussel shell.
Adult pea crabs rely on their mussel hosts for both food and protection. When mussels catch phytoplankton on their gills (as part of the process of filter feeding), pea crabs intercept some of the phytoplankton and eat it themselves. Also, the hard shell of the mussel helps keep pea crabs safe from predators.
A female pea crab never leaves her host mussel. As she is protected from predators throughout her adult life, she has no need for her own hard shell. Instead, her shell is soft and rounded. Males, on the other hand, do leave their host mussel on occasion (to seek out a female and fertilise her eggs), so they have a hard shell and are relatively flat from top to bottom. This shape helps them with getting into and out of mussels through the narrow gap between shells that is used by mussels to pump seawater in and out. Their shell colouring also provides camouflage to avoid predation by fish such as spotties, which are common around mussel reefs and mussel farms.
Huge numbers of eggs
Pea crab females produce far more eggs than non-parasitic crabs. Their protected lifestyle means they don’t need to move to find food or escape from predators, so they can use their extra energy on egg production. Unlike non-parasitic crabs, female pea crabs produce eggs constantly all year round.
After her eggs have been fertilised by a male crab, the female holds the eggs under her abdomen while they develop. As they grow, they form into an enormous egg mass that’s as big as the crab’s body – it’s so large that she would have trouble moving even if she wanted to! After about a month, the eggs hatch and the swimming larval crabs move out of the mussel into the surrounding seawater.
The larval stages: all at sea
Unlike adult pea crabs, larval crabs live in the open ocean, without the protection of a host. Until recently, no one knew all the stages pea crab larvae went through or even how many there were. Now, though, Jessica Feickert has identified every larval stage by growing the larvae from eggs to juvenile crabs in the lab.
Jessica found that, after hatching, pea crab larvae pass through three distinct stages known as the first, second and third zoea (only the first zoea had been seen before). During these zoeal stages, the larvae are less than 1 mm long. Jessica showed that pea crab zoeas are rounded and soft and lack the large spines that other crab larvae have. She also discovered, to her surprise, that pea crab zoeas are vegetarian! They grow on a diet of phytoplankton (whereas many other crab larvae are predators and require an animal food source to grow).
Jessica also identified the pea crab’s final larval stage – the megalopa. This stage is the one that seeks out a mussel host to live in. Like other crab megalopae, pea crab megalopae have pleiopods (swimming appendages) along their tails, which help them move towards potential hosts. They also have very large chemoreceptors (which detect chemicals in the seawater) on their heads, which aren’t seen in other crabs.
The return home: identifying settlement cues
To locate a new mussel home, megalopae need some kind of signpost to point them in the right direction. Some non-parasitic crabs find their adult home by listening for sounds (acoustic cues) – the noise of a reef, for instance – but when Jessica exposed the pea crab megalopae to reef noise, there was no response.
Next, Jessica exposed her megalopae to ‘mussel-flavoured water’ (seawater that had had live mussels stored in it). She was excited to see that, in her first experiment, they responded rapidly. Within a day, they had metamorphosed into adult crabs (a sign that they are ready to settle).
Jessica points out that this is a preliminary result that has yet to be confirmed. However, it does suggest that the megalopae were detecting chemicals released by mussels in the seawater (perhaps by using the large chemoreceptors on their heads). If this is the case, it is likely that pea crab megalopae are sniffing out their mussel hosts then swimming to find them.
Learn about the settlement cues of non-parasitic crabs in the article Crabs finding home.
