Marine Crabs at the South Coast
Care should be taken when looking for crabs, especially under rocks and cobbles during the day. Crabs can be easily crushed, so take care that you replace crabs under a boulder safely, preferably where you found them. Do not disturb egg masses. Remember that this is a marine reserve and crabs, like any sea critters, deserve respect – and to be left in a safe, natural place.
Crabs, shrimps and lobsters belong to the Order Decapoda, part of a larger grouping named Arthropoda. Arthropods are amongst the most specialised of invertebrates, having specialised eyes, paired appendages (antennae, arms, legs), and an exoskeleton. In order to grow, arthropods just periodically moult, shedding the entire exoskeleton at once. At such times and until the new exoskeleton grows hard, the organisms are vulnerable to predation.
Decapods show further specialisation, with the head and thorax parts of the exoskeleton fused into a single carapace. They have 3 pairs of maxillipeds for feeding. Their five pairs of remaining appendages are specialised for crawling or swimming – hence ten legs, or decapods!
Crabs belong to the Group Brachyura, while the hermit or false crabs belong to Anomura. While true crabs have a broad carapace, the latter group are characterised as having long antennae and reduced walking legs and with a tail fin like shrimps and lobsters.
Crabs, like other decapods predate worms, shellfish and other crustaceans, or scavenge of plant and animal detritus. They are most active at night – during the daytime they may be found under rocks or in rocky crevices. Or in estuarine locations, buried in sand or mud. At night they may be almost likened to a cleaning squad coming out to clean up the detritus of the day on seabed and intertidal zone.
There are separate sexes in crab species, with some showing greater or lesser differences in shape or appearance between males and females, for example, some males may have larger front claws than the female. They mate in late winter or spring. The female carries egg masses on the underside of the body, showing up as a bright red or orange mass. Eggs are incubated often for some months and hatch into a planktonic stage, or in some cases into a juvenile stage having a better chance at growing through moulting stages into adulthood.
Crabs, like many marine organisms, adopt various strategies to help reduce the threat of predation. While the feeding maxillipeds may be large and used for defence, crabs will commonly use camouflage through colour or decoration, or burrow rapidly into seabed substrates, retreat rapidly under cobbles or, with hermit crabs, carry around a ‘house’ empty gastropod shell into which they may quickly hide. The larger crabs are perhaps better able to defend themselves and are less likely to use such masking defences in adulthood.
Crabs can be found in the deep waters off Cook Strait, or right up to and above the high tide mark.
Here are some examples of the crabs that may be seen at the Taputeranga Marine Reserve:
Decorator or camouflage crabs – Leptomithrax or Notomithrax species
These species range in size up to 100 mm, or 90 mm carapace width. Leptomithrax australis is often hauled up in lobster pots. In shallower water it can be seen to camouflage with red algae, but does not seem to bother in deeper water. Notomithrax species may be covered in sponge as a permanent disguise, or may use a temporary mask by picking up pieces of green or red algae and attaching them to hooked hairs on the back of their carapace. Notomithrax ursus is the most common of this genus, being covered in a ‘velcro’ of golden brown hooked hairs. When fully masked these crabs cannot be readily seen unless they move, though it is a major task each day to keep the masking in place.
Paddle crab – Ovalipes catharus
This is found commonly on open, sandy beaches, but also amongst the more limited sandy areas in the marine reserve. In this species the last thoracic appendage is modified into a swimming paddle, hence the name. These paddles enable a rapid retreat into the sand as a defence mechanism.
The species can grow up to 140 mm and is quite easily caught in nets or crab pots, and there is a small annual harvest of paddle crabs mostly in the warmer months. They are particularly equipped to lever open bivalves in feeding, but in turn are predated by many commercial fish species, including snapper, blue cod and moki. Egg masses have been found to contain up to 850 000 eggs on one female.
Big handed crab – Heterozius rotundifrons
This species is one of the most frequently found on the coast, frequenting the underside of boulders in the mid to low tide zone. It is characterised by the very large claw on adult males. They are small crabs, around 10 – 15 mm across the carapace, and are a smooth, yellowy green colour, with yellow ‘big hand’. It’s defence when disturbed is to simply remain still, perhaps in the hope that it will not be seen. Females may be seen carrying orange egg masses at almost any time.
Yellow shore crab – Leptograpsus variegatus
A large and fast moving crab which is difficult to handle if picked up. It is about at its southern limit as a species on the Wellington south coast. Adult crabs feed at low tides nocturnally, by scraping rock surfaces and crushing barnacles, chitons, mussels and gastropods. The egg mass is an iridescent green when close to hatching.
Purple rock crab – Hemigrapsus sexdentatus
Careful with this one – it runs fast and nips! It is fairly large and can be almost as big as the paddle crab. Usually it is only seen at night, emerging from crevices and moving surely to food sources – which are many. It is a good scavenger.
Red rock crab – Plagusia chabrus
You will find this crab from low tide down to about 25 m depth. Look out for these under crevices and overhangs – more or less the same hidey-holes as rock lobster. This is a powerful crab which moves fast over the substrate. It is easily distinguished as it is deep red in colour. They are feisty, spending up to 25% of their time fighting amongst themselves. Broad feeders, including bird carcases and cannibalism.
False crab or porcelain crab – Petrolisthes elongates
These false crabs tend to inhabit the same crevices and boulders as the big hand crabs, and are especially abundant where there are mussels. They are dark olive green, with orange antennae. While not a true crab, they are very crablike in appearance. It appears to be a suspension feeder, filtering out large particles from the sea bottom.
Hermit crabs – Pagurus spp.
These false crabs are asymmetrical in shape, so that their soft and vulnerable hind parts will fit within the coiled shape of empty marine gastropod shells. While hind leg appendages are much reduced the tail fin is calcified so as to get a good grip on the gastropod shell. The hermit crab uses the interior of the snail shell to give protection not only to its soft parts but also to its egg mass while it is incubated. As hermit crabs grow they must move on to increasingly larger shells. There are reports overseas of hermit crabs lining up to try out and maybe move into the newly vacated shell, in turn leaving another empty shell.
It has long been an interest to know how hermit crabs apparently screw their bodies into snail shells. Otago University has offered hermit shells glass ‘snail’ shells abd some have readily used transparent new ‘homes’. See more at http://www.sciences.otago.ac.nz/gbu/gbu-hermitcrab.html
Source: This article is largely a summary of Robert Wear’s discourse on decapods in chapter 21 of ‘The Taputeranga Marine Reserve’, second edition, edited by Jonathan Gardner and James Bell, First Editions, 2008.