Leslie Massey
Spider crabs are one of the most iconic and mysterious species of the Great Southern Reef in Australia. They are easily recognised by their long, thin legs and unusual body shapes. The largest species of spider crab found in Australia is from the genus Paranaxia, specifically the Paranaxia serpulifera, which has a body length of up to 18 cm, with claws and legs that can reach almost 30 cm. Giant spider crabs (Leptomithrax gaimardii) congregate annually in Port Phillip Bay, with their gatherings often containing thousands of individuals, forming towers that can be as tall as a person.
| Characteristics | Values |
|---|---|
| Species name | Leptomithrax gaimardii |
| Common name | Giant Spider Crab |
| Size | Up to the size of an average adult's hand |
| Shell | Hardened shell, which must be shed to grow bigger |
| Habitat | Found throughout the Great Southern Reef, in deeper waters, and near the shore in winter |
| Congregations | Congregate in large numbers, forming underwater piles up to the height of a person |
| Paranaxia serpulifera size | Body length of up to 18 cm, with claws and legs reaching almost 30 cm |
| Paranaxia keesingi habitat | Found in depths of up to 175 m, from the Houtman Abrolhos Islands to northern Queensland and Indonesian waters |
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What You'll Learn
Spider crabs can grow as big as an average adult's hand
Spider crabs are one of the most iconic species of the Great Southern Reef. They are easily recognised by their long, thin legs and unusual body shapes. Spider crabs can grow to the size of an average adult's hand, with a body length of up to 18 cm and claws and legs that can reach almost 30 cm.
The size of spider crabs is limited by their hard shell, or carapace, which acts as a suit of armour. To grow larger, they must shed their old shell, expand their soft flesh, and form a new shell. This process, known as moulting, leaves them vulnerable to predators such as rays, seals, dolphins, and birds.
Moulting typically occurs during the winter months, with spider crabs congregating in shallow waters near the shore. The aggregation of crabs provides safety in numbers, reducing the risk of predation. These gatherings can be quite spectacular, with piles of crabs reaching heights of up to a person's height.
The moulting process takes about an hour, and all the crabs in an aggregation moult simultaneously. It is during this time that they are most vulnerable, as they are soft and uncoordinated. Once their new shells harden, the spider crabs return to deeper waters and their solitary lives.
While spider crabs can grow to a substantial size, there is still much to learn about their behaviour and life cycle. Scientists are working to better understand these fascinating creatures through research and citizen science initiatives.
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The largest species found in Australia is from the genus Paranaxia
Spider crabs are members of the superfamily Majoidea, and are not your average-looking crab. They are characterised by their long, thin legs and unusual body shapes. Majoids are also known as decorator crabs because they camouflage themselves by attaching other organisms, such as seaweeds and sponges, to the spines and hairs on their legs and carapace.
One of the largest species of spider crabs found in Australia is from the genus Paranaxia. Until recently, there was only one recognised species: Paranaxia serpulifera, found in the north of the country, from Western Australia to Queensland. This species has a body length of up to 18 cm, with claws and legs that can reach almost 30 cm.
However, during recent surveys in the north-west of Western Australia, several specimens of Paranaxia were collected, and it was noticed that some of them had much longer and thinner legs than others. This led to the discovery of a new species, Paranaxia keesingi, which was named in honour of Dr John Keesing from CSIRO, in recognition of his contributions to the knowledge of Western Australian biodiversity.
Paranaxia keesingi is the second species in the genus Paranaxia, and it differs from P. serpulifera in several ways. It has longer and more slender ambulatory legs, a sharper spine on the posterodistal angle of the cheliped merus, and relatively shorter chelipeds. It is also found in deeper waters than P. serpulifera, with a recorded depth of up to 175 m. Like P. serpulifera, female P. keesingi exhibit direct development with females carrying juvenile individuals under the pleon, making them good mothers by crab standards.
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Paranaxia serpulifera has a body length of up to 18 cm
Spider crabs are one of the most iconic and mysterious species of the Great Southern Reef. They are arthropods, which means they have jointed legs and a hard shell, also known as a carapace. Spider crabs can grow to be as big as an adult's hand. However, the exact size of spider crabs can vary, and some can be quite large. One of the largest species of spider crabs found in Australia is from the genus Paranaxia. This species, known as Paranaxia serpulifera, can reach body lengths of up to 18 cm, with claws and legs that can span almost 30 cm.
Paranaxia serpulifera is a species of brooding spider crab endemic to Australia and can be found as far south as Perth in Western Australia. It is one of only a few marine crab species that develop embryos directly into juveniles, rather than going through a planktonic larval stage. The juveniles remain under the abdomen of the mother for at least one moult cycle before dispersing. This species inhabits shallow waters, often seen stranded in rock pools at low tide, and is typically not found deeper than 20 meters.
The size of Paranaxia serpulifera can vary, with measured lengths ranging from 48.0 to 99.5 cm for females and 48.7 to 83.0 cm for males. The size of the crab may also depend on its location, as there are differences in the size and placement of spines on the carapace and claws, as well as variations in the sculpture on the underside of the carapace. These differences have led to the identification of two distinct species within the Paranaxia genus: Paranaxia serpulifera and Paranaxia keesingi.
Paranaxia serpulifera was first described by Guérin in 1832, and the species name serpulifera refers to the presence of a spine in the pterygostomian region, which is typically larger than the branchial and preorbital spines. This spine is always found above the suture line between the pterygostomian and subhepatic regions in P. serpulifera, while in P. keesingi, it is located below the suture and is smaller in size. The length of the legs is also notable, with the first pair being longer in males and shorter in females, according to H. Milne Edwards' description.
In summary, Paranaxia serpulifera is a large species of spider crab found in northern Australia, with a body length of up to 18 cm. It is characterized by its size, the presence of spines, and variations in the sculpture on its carapace. This species plays an important role in the ecosystem and contributes to the diverse marine life found in Australian waters.
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Crabs must moult to grow bigger
Crabs, including Australian spider crabs, must moult to grow bigger. Moulting is a process by which crabs shed their old shells, expand their soft flesh, and harden a new shell. This process is necessary because a crab's hard shell does not allow room for growth. Crabs must break free from their shells to expand their size in the brief window before the new skin hardens.
The moulting process leaves crabs vulnerable to predators as it renders them soft, stunned, clumsy, and uncoordinated. To protect themselves, Australian spider crabs aggregate in massive numbers, forming \"piles\" that can be as tall as a person. This safety-in-numbers strategy is thought to reduce the chances of an individual crab being eaten by predators such as rays, seals, dolphins, and birds. Aggregations of Australian spider crabs can be found in Port Phillip Bay and along the coasts of Victoria, Tasmania, and South Australia.
The process of moulting takes up to an hour, and all the crabs in an aggregation moult almost simultaneously. The aggregation itself can last a few weeks, with the crabs staying together until their new hard shells form, which probably takes a few days. After moulting, the spider crabs return to deeper waters and their solitary lives, leaving the seafloor littered with discarded shells.
The giant spider crab (Leptomithrax gaimardii), found in Port Phillip Bay, is one of the largest species of spider crabs in Australia. It has a body length of up to 18 cm, with claws and legs that can reach almost 30 cm. The annual congregation of these giant spider crabs in Port Phillip Bay ahead of their winter moult is a spectacular event that attracts tourists and has been featured in documentaries.
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Giant spider crabs congregate in Port Phillip Bay
Giant spider crabs, or Leptomithrax gaimardii, are a species of crab native to Port Phillip Bay. Every year, thousands of these crabs congregate in the bay ahead of their annual winter moult. This event, known as an "aggregation", sees the crabs form a massive group that can be easily spotted against the sandy sea floor. The purpose of these aggregations is still not fully understood by scientists, but it is believed that the crabs may be seeking safety in numbers during this vulnerable time.
Giant spider crabs are one of the most iconic and mysterious species of the Great Southern Reef, which spans the southern coast of Australia. They are easily recognisable by their long, thin legs and unusual body shapes. The largest species of spider crab found in Australia, Paranaxia serpulifera, has a body length of up to 18 cm, with claws and legs that can reach almost 30 cm. However, the giant spider crabs of Port Phillip Bay can grow even bigger, with some individuals reaching the size of an average adult's hand.
Aggregations of giant spider crabs in Port Phillip Bay can be an impressive sight, with hundreds of large, orange crabs marching together in shallow waters. These aggregations may serve as a form of protection for the crabs, as they are usually solitary and well-camouflaged by algae, sponges and other organisms that grow on their shells. By coming together in large numbers, the crabs may be better able to defend themselves against predators or strong tidal currents.
Despite their intimidating appearance, giant spider crabs are not considered dangerous to humans. In fact, their aggregations have become a popular attraction for locals and tourists alike, even featuring in international documentaries. However, despite their fame, these crabs remain largely mysterious, and scientists are still working to understand their behaviour and life cycle.
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Frequently asked questions
Australian spider crabs can grow as big as an adult human hand. The largest species of spider crab found in Australia, the Paranaxia serpulifera, has a body length of up to 18 cm, with claws and legs that can reach almost 30 cm.
Crabs need to shed their old shells, expand their soft flesh, and harden a new shell. This process is called moulting and leaves the crabs vulnerable to predators.
Spider crabs moult in shallow waters off the southern coast of Australia, in places like Port Phillip Bay, Tasmania, and South Australia.
