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Tasselled Crab Pilumnus fissifrons

Declan Smith 2021


Decapods represent a large portion of the arthropods known as crustaceans. Among these are the Brachyura – the infraorder that encompasses what are considered “true crabs”. The beaded hairy crab, Pilumnus fissifrons, is a representative of Brachyuran crabs from the family Pilumnidae; many of which are referred to as “hairy crabs” owing the hairy growths apparent on the carapace.  Pilumnids are commonly found in coastal waters dominated by tropical and rubble reefs where sedentary fauna provide shelter. In these environments P. fissifrons is notably common along the south-eastern coasts of Australia, including Moreton Bay (from where the photographed specimen was collected).

There are many species of Pilumnid crabs that have been identified yet there is little information on both the group as a whole and the individuals that are comprised by it. The following information will explore the specimen in question (P. fissifrons) as well as the genus it belongs to, Pilumnus, in order to develop a greater understanding of their bioecology.

Physical Description

P. fissifrons is a small species, approximately 2cm wide when fully matured. The carapace is hexagonal in shape and, along with the legs, are distinctly covered in clubbed setae. Setae are dense and remarkably long giving the impression that the crab is “hairy”. The dorsal surface of the carapace is convex and smooth with velvet-like texture. Regions of the carapace are distinctly identified by the density of setae or whether setae are present/absent. Lateral margins are lined with at least 3 teeth. The posterior margin is slightly shorter than the anterior margin. Orbits sit within frontal margin; eyestalks are short in length and exposed by orbit. Antennae are medium in length with visible flagellum, antennules folded transversely (Naderloo, 2017). Legs are curved in shaped and setae are less dense along this region. Cheliped are club-shaped, 3 teeth on the dactyl, 4 teeth on propodus. Right-side chela are larger and rounder in males. As observed: dominant colour appears to be greyish-green generated by the density of setae. Speckled white, pinks and oranges on carapace – somewhat resembles granite cluster. 

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Pilumnid crabs are typically found along coasts from intertidal zones to depths of approximately 70m (Taylor and Poore, 2014). As with most brachyuran crabs they are benthic when fully mature and can be seen on soft substrates as well as coral or rubble reefs. Pilumnids are small crabs that prefer dense coverage of rocks and crevices where they may comfortably live. Their cryptic nature means that they are not initially visible without disturbing the surrounding habitat. Distribution of P. fissifrons among these habitats has shown to follow to presence of other organisms. It is commonly found in reef environments that contain sponges, bryozoans, or corals – sedentary fauna with which they have been known to have a commensal relationship (Patton, 1966). This species is often observed using these other organisms for shelter, protection from predators or assisted modes of feeding. It has also been hypothesised that these relationships may be more mutualistic than previously thought, though further research is yet to be conducted on how closely linked this connection may be (Ward and Rainer, 1988).

Pilumnids are deposit feeders that generally thrive on organic debris or detritus found on the benthos. P. fissifrons has been recorded as a known detritovore but has also been found to feed on living matter of sedentary organisms (bryozoans, sponges, corals etc.) (Taylor and Poore, 2014). It is unknown whether P. fissifrons is a major source of food for any wide number of predators, but in general Pilumnids are predated upon by species of shorebirds and predatory fish local to that particular region. There is evidence to suggest that the Eastern Curlew feeds on small crustaceans of which includes P. fissifrons (Dann, 2014), as well as the Whimbrel bird Numenius phaeopus variegatu which has taken a particular liking to it (Dann, 1993).

Life History and Behaviour


Pilumnids are gonochoristic and require a partnership of parent crabs to reproduce. A male pilumnid crab will attract a mate using pheromones to trigger olfactory response in a viable female (Weis, 2012). Once the male has transferred spermatophores via gonopods to the gonophores, the female will then carry an egg mass under the pleopods. As in the case of P. fissifrons (and with most tropical brachyuran crabs) reproductive activity is more stable throughout the year due to the stability of temperature and environmental factors. Temperate pilumnids are more susceptible to change and reproduction will be highest during warmer months (Litulo, 2005).

Larval development follows the same plan as all members of the monophyletic suborder Reptantia (which includes lobsters, brachyuran crabs, and anomuran crabs). Hatched larvae begin life in a planktonic stage referred to as zoea. Throughout development the larvae will undergo four distinct embryonic forms until it metamorphoses into the secondary megalops stage. Another metamorphosis will occur from megalops to the juvenile crab form wherein in begins life as a benthic organism (Wolff and Gerberding, 2015). From this point the crab will undergo ecdysis until it is fully matured wherein it can generate its own progeny. In extreme cases the megalopa phase can be skipped entirely and the last zoeal stage can moult directly into the juvenile crab stage (Hertzler, 2015).

The exact life history of P. fissifrons are yet to be thoroughly researched, but the process is similar between many previously recorded members of Pilumnus.

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Common Behaviours

A commonly observed facet across the pilumnids is their cryptic nature. Most pilumnid species utilise small spaces unoccupied by surrounding sedentary organisms. Some have been observed making homes within coral skeletons (Johnson et al, 2008). Some species use bryozoans as cover and will burrow underneath a colony, acting as a concealment defence. Some even live between the ostia of calcareous sponges using spicules to protect them from predators. The local distribution of pilumnid crabs has a close correlation with the availability of protected space among other organisms. A high population density is mostly due to the increased presence of colonies of bryozoans, corals or sponges. Small size combined with textured camouflage (provided by dense setae) ensure that these crabs are difficult to spot among reefs and stony shores.

With this desire for well-defended space comes the competition to occupy this space. Territorial behaviour can also be observed in pilumnids as protected space may be viewed as a valuable resource. Many pilumnids will show aggression towards other individuals that enter their territory. Many individuals will also attempt to encroach on territory that provides good defence against predators (Lindberg and Frydenborg, 1980). In some cases, sex has no bearing on territorial behaviour, being evident among both male and female crabs.

Anatomy and Physiology

The anatomy of P. fissifrons follows the basic body plan of most brachyuran crabs. The carapace is typically wider than they are long, reaching no larger than 2cm in width and covers the cephalothorax. Similar to most pilumnids the carapace is covered in clubbed setae – small sensory hairs that can be used by many brachyurans as mechanoreceptors or in grooming (Wortham and Pascual, 2019). The primary use of specialised setae in pilumnid crabs are for concealment, using the density of them to blend into the surrounding algal-rocky environment.

As with all decapods P. fissifrons has 5 distinct pereopods; a forward pair chelipeds used in feeding, defence, and display; and four pairs of legs used for locomotion. Each pereopod consists of 4 segments; carpus, merus, propodus and dactylus (modified to a hinge claw on the chelipeds). Modified legs at the mouth, termed maxillipeds, assist in feeding (Guinot et al., 2013).

Pilumnids are sexually dimorphic where males tend to have larger chelae than females of the same species (Naderloo, 2017). The abdomen is one particular way to sex an individual. Compared to other members of Decapoda, brachyuran crabs have a reduced abdomen that is tightly concealed under the thorax, as is the case for P. fissifrons, meaning it is visible only when the organism has been turned upside down (Guinot et al. 2013). Pilumnids typically have seven somites on the abdomen. In males the first and second pleopods are modified into gonopods for sexual reproduction. The first gonopod is elongate and hooked while the second is shorter and sigmoid in comparison (Poore, 2004). In females the pleopods are used for holding egg broods suspended ventrally to the abdomen.

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Biogeographic Distribution

Members of the genus Pilumnus are invariably widespread across multiple geographic locations with primary representatives present in Southeast Asia, eastern coasts of North and South America, and the British Isles. Australia hosts a large number of Pilumnus species along its eastern and southern shores, including P. fissifrons, which is native to south-east Australia. Here it is found on reef beds and intertidal zones of more temperate waters, though it has been recorded in tropical regions. The specimen in question was found in Moreton Bay, but P. fissifrons is also found as far south as the Bass Strait where it is one of the most common pilumnid species present in the region (Taylor and Poore, 2014).

Evolution and Systematics


Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura
Section: Eubrachyura
Subsection: Heterotremata
Family: Pilumnidae
Subfamily: Pilumninae
Genus: Pilumnus
Species: Pilumnus fissifrons

Wider Groups

P. fissifrons belongs to the phylum Arthropoda, known for including a diverse and wide array of invertebrate organisms. The subphylum Crustacea is inclusive of lobsters, crayfish, shrimps, krill, crabs, and other aquatic arthropods – many of which are part of Malacostraca, the largest of the crustacean classes. The defining characteristic of Malacostraca is the composition of 20 segmented body parts, including a head, thorax, and abdomen. The five pairs of limbs are representative of the order Decapoda.  Brooding methods incorporated by P.fissifrons is also shared by the members of Pleomyemata (wherein the female incubates eggs under her pleopods) (Ng et al., 2008).

As discussed previously, Brachyura is the infraorder that refers to the true crabs, while the subsection of Heterotremata refers to the location of the genital opening of sexes (legs in males, sternum in females) (Ng et al., 2008). Pilumnids are a resident family of crabs in this clade but had only been identified in recent decades. Previously it had been included as a part of the Xanthidae due to the multitudinous similarities between the two families (Huys, 2009). A key difference between the two is the common presence of clubbed setae in pilumnids. A less identifiable difference is that male pilumnids have seven abdominal somites while male xanthids only have five (Poore, 2004). Eriphiidae is another family of brachyuran crabs that has contended with the classification of Pilumnidae were it not for the slight distinction that the male gonopods of eriphiids are the same length (Poore, 2004). Pilumnid gonopods, on the other hand, are distinctly different in length and shape. The identification of pilumnid crabs can be difficult if species do not have well defined features. In these extreme cases, examining the male pleopods can assist in placing into which clade the individual will fall (Guinot et al., 2009).

Genus Pilumnus

P. fissifrons has the advantage of clubbed setae that quickly identifies it as a pilumnid crab, as well as the visible seven abdominal somites. Identifying it apart from other members of genus Pilumnus can be difficult and requires thorough observation of the gonopods and a record of where the specimen was recovered (Poore, 2004). Almost all members of this genus are observed with clubbed setae, though a way of differentiating individuals can be by identifying the differences in how densely and how long the setae grows in parts, as well as carapace colouration.

Conservation and Threats

It is not known if there are any issues that threaten the existence of P. fissifrons, nor if they are a threat to their own surroundings. In general, pilumnid crabs are too small to be favourable nutrients for human consumption and are assumedly not profitable for aquaculture. However, it can be assumed that, like all organisms in a threatened environment, anthropogenic impact and climate change may have some influence on population dynamics.

Considering the knowledge that P. fissifrons is dependent on the existence of other organisms that comprise a reef environment, negative impacts on tropical and temperate reef systems may eventually lead to the reduced success of the species.


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