The Blue Swimmer Crab

Article | Updated 9 months ago


Image copyright WA Museum 

Description

Blue Swimmer Crabs have large clawed front legs that they use for both hunting and defense. Their back legs are smaller and shaped like paddles, making them powerful swimmers.
The upper part of their shell is known as the carapace (ca-ra-pace). The carapace is broad, flat and edged in sharp teeth-like ridges. The last teeth on each side are pronounced, resembling horns.

Colour
Although each crab is unique in its colour vibrancy and shell pattern, males are always blue whilst females are a mottled brown.
Habitat
Adult Blue Swimmer Crabs inhabit the sandy floors of inshore marine waters, such as bays and estuaries. During the early stages of their life cycle the larvae float at the water’s surface up to 80 kilometres from the coastline.
Distribution
They are found Australia-wide and extend eastwards to the waters of New Caledonia.
Other names
Sand crab, manna crab, blue crab, swimming crab, flower crab, sandy crab, blueys.
Notes
The Blue Swimmer Crab is highly sought after. Professional crabbers use purpose built crab traps whilst amateurs use drop nets or scoop nets. Western Australia has specific fishing regulations to maintain their population levels.

Male or Female?

A male Blue Swimmer Crab

A male Blue Swimmer Crab
Image copyright WA Museum 

There are multiple ways to identify the sex of a Blue Swimmer Crab. One differentiating feature is in their colouring. Males are electric blue, whilst females are mottled brown. Additionally, the males of this species are much larger than the females.

The easiest way to distinguish if a crab is male or female is to inspect the shape of the abdominal flap on its underside. A male’s flap is narrow and angular, while a female’s is wider and more curved.

A female Blue Swimmer Crab

A female Blue Swimmer Crab
Image copyright WA Museum 

Did you know?
A courting male will catch a female and carry her around for up to ten days whilst fighting off other potential male mates.

Life Cycle

The lifecycle of a Blue Swimmer Crab

The lifecycle of a Blue Swimmer Crab
Image copyright WA Museum 

1.    Zoea (Zo-ee-ya)
Blue Swimmer Crabs hatch from their eggs in early summer as tiny larvae known as zoea. During this first stage of growth they are transparent and only 1mm in length. They float on the surface of the water far from the coastline. The zoea is charachterised by a large dorsal spine on its back and has small swimming appendages. The larvae have a very high mortality rate as their size makes them poor swimmers and easy prey for bigger sea creatures. By autumn, only a small percentage will have made their way to the more shallow coastal waters.  

2.     Megalopae (Mega-loap-ah) 
Once in the shallow water, the zoea begins to grow quickly in weight and size and sheds its shell several times. The young megalopae will eventually reach the size of a 10 cent coin.

3.    Juvenile crab
The megalopae continues to moult and begins to look more crab-like in appearance. During the two months spent in this stage the young crab migrates to the coastal floor. The carapace has now grown to be up to 60mm in width.

4.    Adulthood
By winter, the Blue Swimmer Crab has matured into a young adult and its carapace is roughly 90mm wide. The crabs will moult a final time to reach full maturity. At this stage many young females will mate for the first time.

5.    Reproduction
The young female retains the male’s sperm for the remainder of the winter as her ovaries develop. In late spring, the female crab uses the stored sperm to release up to two million fertilised eggs. These eggs are incubated on forked appendages (pleopods) on the mother's abdomen for nearly three weeks. The life cycle begins again when these eggs are released into the water to hatch into zoea.

Did you know?
When female Blue Swimmer Crabs incubate eggs, they are referred to as to being ‘berried’. Berried crabs must be returned to the water if they are caught.

Portunidae Family

A Blue Swimmer Crab underwater

A Blue Swimmer Crab underwater
Image copyright WA Museum 

The Blue Swimmer Crab, formerly known as Portunus pelagicus but now known as Portunus armatus, is a member of the Portunidae family. This family includes other large, edible crabs found in Australia, such as the Mud Crab.

Previously, "Portunus pelagicus" was known as a tropical species that was widely distributed across the Indian and Pacific Oceans. However, recent research looking at the morphology (physical characteristics) and genetics of "Portunus pelagicus" has shown that it is actually a 4-member species complex. The four species are classed as P. armatus (A. Milne-Edwards, 1861), P. pelagicus (Linnaeus, 1758)], P. reticulatus (Herbst, 1799) and P. segnis (Forskål, 1775).

Experts in the 18th and 19th centuries originally considered these four species to be unique identities but this was deemed contentious. In 1968 the four species were pooled together under Portunus pelagicus. However in recent times, the use of molecular, morphological and biogeographical data has provided support for the original theory of four discrete species.

The species now known as Portunus pelagicus occurs across southeastern and eastern Asia, including Japan; P. armatus is distributed from Australia to New Caledonia; P. reticulatus is found in the Bay of Bengal, and P. segnis is confined to the western Indian Ocean.

The research that helps us understand the composition of these species also aids in the creation of effective sustainable management and conservation plans.


The Museum's Research on Crabs and other Crustaceans

Inside the WA Museum Collections and Research Centre

Inside the WA Museum Collections and Research Centre
Image copyright WA Museum 

The animals known as crustaceans include more than 52,000 species worldwide. They are diverse in their body shapes, colours and lifestyles; they include barnacles, shrimps, fish lice, rock lobsters (marine crayfish), slaters, water fleas, beach hoppers, marron and scampi.

Although there are many different types of crustaceans in Australian coastal and inland waters, our knowledge of them is still very limited and is mainly confined to a few commercial species, especially prawns, crabs and rock lobsters. There are hundreds of other crustaceans that have not yet been studied and many that have not even been named or described by scientists. There is much work to be done on both the taxonomy (naming and classifying animals into groups) and the ecology of these lesser-known species.

The Western Australian Museum cares for and manages the State collections, including the Crustacean Collection. Scientists in the Crustacean Department are responsible for collecting, documenting and researching the marine and freshwater crustaceans of Western Australia. They conduct fieldwork throughout the state and have named and described many new species. As well as finding new crustacean species, our scientists document the population levels of these species. They present this information in varying formats including print publications, exhibitions, documentaries, lectures and online resources.


Further Reading:

Jones, D. & Morgan, G. 2002. A Field Guide to Crustaceans of Australian Waters

Western Australian Museum. 1988. Life in our Western Seas. Blue Manna Crab