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​Cerapus nudus

Abigail Shaughnessy 2019


Cerapus nudus belongs to the class Amphipoda in the sub-phylum Crustacea. Defining characteristics that identify amphipods include their lack of carapace, sessile compound eyes and laterally compressed bodies (Duffy and Thiel,2007). A key characteristic of Cerapus nudus is their lack of setae. The term "nudus" is Latin for naked, which refers to the near lack of setae on their legs and body (Just, 2009). They occupy a habitat of sand and rubble, in subtropical marine environments (Just, 2009).  C. nudus creates its own microhabitat by constructing a tube from silk and fine sediment.  They have a sedentary lifestyle and rely on their antennae to filter sediment, detritus and food from the water column. 

This species has currently only been found along Queensland coast, Australia. Previously, the holotype was found at Lizard Island. The specimens described were found at Manly harbour on settlement plates. Notably, this is the first record of this species south of Lizard Island. 

Physical Description


Despite all Crustacea originally sharing a common body plan, there is considerable morphological variation between groups. The group amphipods have a distinct morphology compared to other crustraceans. 
 Amphipods are unique as they are laterally compressed, in which their bodies are flattened from side to side. Their body axis has a strong convex shape (Glazier, 2009). They are segmented animals and their body is divided into three main functional segments (tagmata): Cephalon (head), pereon(thorax) and pleon (abdomen) (See figure 1). It is a defining characteristic of amphipods for their pereon and pleon not to be distinctly separated (Glazier, 2009). The three specimens that were observed averaged 3mm in size. 
C. nudus differs from individuals in the same genus by its elongate, slender antennae and by its distinctive near lack of setae along their coxae and pereopods 3-7 (Just, 2009).


The head of C. nudus has two pairs of antennae of equal length (Just, 2009). The antennae are primarily used as chemo-receptors in both mate detection and food. Moreover, setae are found along the antennae to aid the filtering of sediment, detritus and food from the water column (Thiel and Norenburg, 2009). The mouth parts of amphipods are well-developed and on the ventral side of the cephalon. The mouth parts consist of multiple appendages including mandibles, maxillae and maxilliped. Each appendage of the mouth plays a role in the suspension-feeding mechanism (Duffy and Thiel, 2007).


Despite, a carapace being important in crustacean identification, it is absent in amphipods (Duffy and Thiel, 2007). Instead, amphipods have broad coxal plates which make up the coaxae. These plates are functionally equivalent to a carapace found in other crustaceans. The plates create a protected space for the gills, branchial chamber and marsupium. Along the pereon, the species bares seven pairs of pereopods (i.e. Walking legs) (Covich,2010).


The pleon is equivalent to the abdomen and includes the telson and uropods.  Similar groups of crustaceans have one pair of uropods, while amphipods have three pairs. In addition, unlike other amphidpods, the uropods are not fanned shape to form a tail fin. Instead, for C. nudus they act as anchoring appendage for their tube (Barnard, 1965).

Figure 1


All the thoracic appendages in C. nudus are uniramous, and the various appendages are adapted for different functions. The first two pereopods are known as Gnathopods. The two pairs of limbs are important in grooming, mating, feeding and aggressive inter- and intraspecific behaviour (Holmquist, 1982). In free-swimming amphipods, the pleopods are paddled-shaped. However, in tube-building amphipods such as C. nudus these pleopods are important for pumping water through the tube (Castellani and Edwards,2017).


Most amphipods show strong sexual dimorphism, where males are often larger than females. Males have strongly developed gnathopods and gnathopod 2 is much larger. Females have a marsupium, for the development of embryos. (Thieland Norenburg, 2009). For C. nudus, the female’s gnathopod 2 is similar to gnathopod 1 but only slightly larger. In females the pereonites 3-5 and coxae are more elongate than the males.  In addition, the antennae of the female are more slender and possess fewer aesthetascs than the male (Just, 2009)

Figure 2



Like other tube-dwelling amphipods, C. nudus has a sedentary lifestyle in a marine benthic environment. They are found to occupy habitats in sand and rubble. The holotype was found at a depth of 17m, yet it is hypothesised that they inhabit the littoral zone (Just, 2009).

Constructing a tube serves a s form of protection from predation. A study showed that when amphipods could construct a tube, fewer amphipods were eaten by fish and other invertebrates (Nelson,1979). They reside in their tubes for long periods of times and emerge to extend their antennae into the water column (Thiel and Norenburg, 2009). C. nudus constructs its own microhabitat by forming tubes from uniformly fine sediment grains. Their tubes are stationary. Usually, within the range of their tube openings, established filter-feeders have their distinct feeding territories. Although, direct aggressive interactions between individuals have shown to be rare (Connell, 1963). This could be achieved by simple avoidance mechanisms. For example, during earlier phases of colonization, aggressive interactions could occur prior to individuals establishing their tubes (Connell, 1963).

It is common for amphipods to have associations with other invertebrates (Hughes and Ahyong, 2016). The specimens found at Manly Yacht Club used bryozoans as vantage-points for the construction of their tubes. The bryozoans had erect stems are were also filter feeders.

Figure 3


Suspension is a form of feeding, where small suspended particles are removed from the water column (Ruppert et al. 2004). The beating of appendages produces a current for filtering (Thiel and Norenburg, 2009). C. nudus filters out organic suspended matter from the water column for both feeding and further construction of their tubes. Their antennae are modified for suspension feeding as they are very long and setose (i.e. many setae). Their mouthparts are important in the manipulation of food particles. The paired mandibles of C. nudus are blade-like shaped appendages (Just, 2009), likely employed for feeding (Pavesi and Olesen, 2017). The maxillipeds aid in removing suspended matter from the antennae, while the maxillae sort the particles and transport them to the mouth or to the pereopods for tube building (Thiel and Norenburg, 2009).

The specimen demonstrates the filter feeding behaviour, where the sediment is grasped by two pairs of antennae by rapid whipping motions. The specimen's maxilliped quickly remove the particles off the antennae and pass them on to the other appendages of the mouth (See video link below)

C. nudus using its antennae to capture particles in the water. The maxillipeds are removing suspended matter from the antennae

Life History and Behaviour


The dispersal of amphipods is limited as their life history lacks a larval stage. In addition, C. nudus has a sedentary lifestyle and their appendages are modified to live inside a tube instead of swimming. As highlighted in the physical description, their pleopods are not adapted for swimming but are responsible for pumping water through their tube. Their pleon is responsible for anchoring themselves in the tube (Castellani and Edwards, 2017). Because of their tube-building behaviour, short-distance dispersal may occur. The selection of a site for building a tube is a critical process as their fitness is dependent on tube construction (Nelson, 1979). Therefore, this behaviour further constrains their dispersal.

Tube construction

Tube-building behaviour has been observed throughout various species of amphipods. . To initiate the tube-building, the individual rolls around to coat themselves in sediment and detritus. After a few minutes a rough tube is built by binding the materials together with silk (Barnard et al. 1991). Amphipods attach a small amount of silk to the sediment and pull the silk from their legs to spin it into fibres. (Kronenberger et al. 2012). The fine sediment is collected by filter feeding with their antennae in the water column.  A study from Barnard et al. (1991) has shown that amphipods work continuously to build a tube and approximately half an hour is required for an individual to build a tube as long as the body. After the primary tube is built, the amphipod continues to either enlarge the tube or starts an extension with a second tube (Barnard et al., 1991).  C. nudus builds a stiff circular tube from uniformly fine sediment and detritus (Just, 2009). The specimen's tubes were anchored onto an erect form of bryozoans.

Amphipod silk is produced by glands found along the third and fourth periopods (Moore 1988). The silk is a fibrous material, that is both flexible to build a tube and water-resistant. The morphology of the silk glands allows for silk to be produced from a cement- like substance. The proteins in silk have three important types of amino acids: glycine, lysine and aspartic acid. These amino acids are responsible for the silk being stretchy and able to form fibres.

The structure of amphipod silk is similar to the threads produced by some spiders. Both have a fibrous filament for strength and are enclosed by layers of other material for adhesion. Likewise, they both deliver the silk with a similar pulling mechanism, where the silk is pulled from their legs. Despite the chemical processing of silk from amphipods differing from terrestrial spiders, the similar internal spinning mechanism, indicates a generic underlying process to produce silk (Kronenberger et al. 2012) (Vollrath and Knight, 1999).  

The specimen’s tubes were placed under an electron microscope, to see how their tubes are held together with silk. The sticky nature of the silk was seen, as many sediment particles were clumped together along the threads. In addition, the silk was placed irregularly around the tube, where each thread was attached in a different direction. The silk appeared make a mesh-like structure across the sediment to hold the tube together (See figure 4 and 5).

Figure 4
Figure 5


Peracardia means 'pouch' shrimp, thus the females in the superorder have a marsupium. The marsupium is a brood pouch found ventrally on the amphipod. It is constructed from the coxal plates at the base of pereopods. It plays a role in incubating the embryos until early juvenile stages. Before a female ovulates, the male injects his sperm into the marsupium (Duffy and Thiel, 2007). The egg cluster is supplied by oxygenated water through beating by the pleopods.

During courtship, the pereopods can be used by males for precopular clasping of females (Hessler, 1982). It is common for amphipods that live in tubes for the pair to be heterosexual, where they mate together for multiple reproductive cycles (Thiel and Norenburg, 2009).

Development and Parental Care

In contrast to other crustaceans, amphipod embryos experience direct development. This means amphipods do not experience a larval stage.  The embryos are rich in yolk (Scholtz, 2000) and thus allow direct development. When the juveniles emerge from the marsupium, they resemble miniature adults (Hughes and Ahyong, 2016).

On the settlement plates it was observed that more than more individual resided in the tubes. Parental care of offspring has been observed in the family Ischyroceridae (Berents and Lowry, 2010). In C. nudus, the parental care could have taken the form of providing a microhabitat for juveniles. Currently the duration of parental care in tube-dwelling amphipods is unknown. However, it is hypothesised that it may be long lasting, such as exceeding 20 days (Thiel,1999).

Anatomy and Physiology


Crustaceans have a hard exoskeleton composed of chitin and hardened with calcium carbonate (Covich, 2010). Moulting is the shedding of the exoskeleton and is a process amphipods go through to grow and reach their adult stage (Trevisan et al. 2014)


The gills are flat epipods, which are small projections arising from the pereopods. The gills of an amphipod are relatively unsophisticated as they are simple plates (Ruppert et al. 2004 ). Through the course of evolution, the gills have moved to the inner side of the pereopods and are unprotected. The pleopods create a current providing rich oxygenated water to the gills. In the blood, oxygen binds to a kind of respiratory pigment, called hemocyanin (Ruppert et al. 2004). Amphipods have a dorsal heart which pumps the blood from the oxygenated gills to other organs of the body. Due to the amphipod’s small size, they lack arteries (Bousfield and Conlan, 2017).

Sensory system

The antennae are the most important part of C. nudus sensory system. They have different types of sensory organs located along the antennae to receive information about their environment. Rheoreceptors are important in detecting the direction and velocity of currents (Duffy and Thiel, 2007). The antennae also have many aesthetascs (Just, 2009), which is a chemo-sensory organ. Waterborne chemicals such as pheromones stimulate the aesthetascs, which aids in food detection (Helen et al. 1968), recognize other individuals and their sexual status (Bousfield and Conlan, 2017).

Unlike other crustaceans, who have their eyes on stalks, C. nudus eyes are sessile (Hallberg et al., 1980). C. nudus has one pair of ruby-red compound eyes. Compound eyes are able to discriminate colour and detect movement (Duffy and Thiel, 2007).

Biogeographic Distribution

During a 2009 study of benthic amphipods off the coast of Lizard Island, Australia, C. nudus was discovered as a new species of the amphipod genus Cerapus (Just, 2009). Currently this new species is only known from the locality in Australia. The specimens were found on settlement plates in Moreton Bay at Manly Harbour. The eastern coastal waters of Australia range from temperate to tropical.

Figure 6

Evolution and Systematics

There is a limited understanding of the amphipod’s relationship between other Peracarida. Currently there is no fossil record to demonstrate how amphipods evolved.  The delicate cuticle from small marine forms seem to have left no traces (Dahl 1977). Therefore, studies on the current structure and function of various groups can help provide more information. For example, it is hypothesised that barnacles and amphipods could share a common ancestor, since the silk in both crustaceans is extremely similar (Kronenberger, 2012).

Currently, there are 20 known species of the genus Cerapus. In 2017, the tribe (i.e. a rank below family) Cerapodini Smith was established within the Ischyrocerine family. In Australia, the tribe is represented by five species of the genus Cerapus, including Cerapus nudus. Members of this tribe share unique character states. For instance, in comparison to antennae 1, antenna 2 is slender and in mature females the coxa is elongate and enlarged (Just, 2017).

Taxonomic Classification of C. nudus:

Kingdom: Animalia

Phylum: Arthropoda

Subphylum: Crustacea

Class: Malacostraca

Superorder: Peracarida

Order: Amphipoda

Suborder: Senticaudata

Family: Ischyroceridae

Tribe:  Cerapodini

Genus: Cerapus

Species: Cerapus nudus


Conservation and Threats

 C.nudus  like other amphipods, is an ectotherm, which means their body temperature is dependent on the surrounding environment. Studies have shown that the temperature of the water can affect rates of growth, metabolism and development in marine invertebrates. Frequent observations have been made, where increasing the temperature can cause a reduction in amphipod size (Aktinson, 1994). Increase in water temperatures due to climate change is expected to influence the life-history characteristics of amphipods. However, the magnitude of these effects is still uncertain.

Amphipods are also sensitive to pollutants in their environment and are recurrently the first organisms to disappear from a disturbed site. Marine coastal sediments are vulnerable to anthropogenic inputs such as the accumulation of pollutants. Other tube-dwelling amphipods have experienced impacts from the accumulation of trace metal copper. These include females no longer brooding embryos, lower survival rate of juveniles and delayed female sexual maturation (Marsden 2002).

There are currently no studies investigating the direct threats of Cerpus nudus.  However, the threats highlighted above have shown to impact other amphipod species and are likely to be a potential threat to C. nudus.


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I would like to thank Jean Just for his help and guidance in identfying my specimen. 

I would also to thank Bernie Degnan, Sandie Degnan, Davide and Eunice for their support, suggestions and motivation throughout this project.