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Caprella californica (Stimpson, 1857)

Jessie Hildebrand 2015


Introductory summary

Caprella californica, commonly referred to as the skeleton shrimp or the 'praying mantis of the sea' due to its elongated body structure and modified front appendages, is a Caprellid amphipod from the class Malacostraca. The genus Caprella that C. californica resides in, currently encompasses approximately 170 different species of skeleton shrimp. Originally identified along the Californian coastline, C.californica has recently undergone a large geographic range expansion and is now found in locations including North America, Chile, Japan, Hong Kong,South Africa and Australia. The specimens identified here were located in western Moreton Bay, Queensland, Australia. Caprellid amphipods (C. californica included) are characterised as having reduced pleopods and reduced abdomen. The wide geographic range of C. californica indicates its ability to adapt to a wide range of abiotic conditions and environments with its expansion being, in part, due to its existence as part of the biofouling community.

NB: There is some dissension between researchers as to what year this species was identified in. Some list it as Stimpson, 1856 and some as Stimpson, 1857.

Full scientific classification

  • Domain: Eukarya
  • Kingdom: Animalia
  • Phylum: Arthropoda
  • Subphylum: Crustacea
  • Class: Malacostraca
  • Subclass: Eumalacostraca
  • Superorder: Peracarida
  • Order: Amphipoda
  • Suborder: Senticaudata
  • Infraorder: Corophiidea
  • Parvorder: Caprellidira
  • Superfamily: Caprelloidea
  • Family: Caprellidae
  • Subfamily: Caprellinae
  • Genus: Caprella
  • Species:C. californica

Physical Description

Lab specimen description

C. californica is a marine invertebrate in the Phylum Arthropoda. The specimens observed in the lab were male and female, 15.5mm and 17.1mm in length respectively (indicating that they are adult individuals) and opaque white in colour with large clusters or red-brown dots covering the length of their body. They had an elongated circular body shape with four sets of paired jointed appendages, one at the anterior end of the body (2nd pereiopods), below the head and three at the posterior end (pereiopods 5, 6 & 7). The first set of paired appendages were claw-like in shape, as opposed to pairs two, three and four. The cephalon (head) had a small horn-like projection on the dorsal side, two sets of elongated antennae (not included in specimen length), two complex eyes, a pair of modified projections (gnathopod pair 1) on the ventral side and a pair of maxillipeds (feeding structures). Pereonites (thorax segments) three and four each had a pair of elliptically shaped gills protruding on the ventral side.



C. californica is an omnivore, with a broad diet range. They have been recorded consuming things such as detritus, protozoans, diatoms, smaller amphipods, copepods, polychaetes, sponges (demospongiae), crustacean larvae, dinoflagellates and other attached or floating food items. Studies have shown that detritus is most commonly consumed by this species with anywhere between 80% and 86% of stomach contents analysed being detritus from the specimens immediate environment. They collect this detritus by attaching themselves to the substrate with their posterior pereiopods (paired appendages two,three and four) and standing upright in the water column, allowing water to flow over their two pairs of antennae and first gnathopods and catching whatever food particles floats past. They have also been observed scraping the detritus off the substrate to which they are attached. This has led many researchers to label this species as a detritivore (modification on an omnivore). C californica are also occasionaly active hunters, meaning they can seek out their prey if necessary (consuming smaller amphipods or crustacean larvae), however there is a limit to the success of this behaviour as it is impacted by their inability to move swiftly in currents.

Interactions with endemic plant/algae species

C. californica has been recorded interacting with extant algal and attached plant species (e.g. Kelp species Macrocystis pyrifera) within its habitat. They are known to use these species as ‘rafts’, enabling them to travel larger geographic distances with resources on hand (minimising mortality rates). This behaviour has been noted on many occasions, one of which occurred in the Southern Californian Bite, C. californica’s original native region (prior to global distribution). The Australian coastline also has large kelp forests in temperate regions, and I predict that this rafting behaviour is likely to occur in these regions also, facilitating geographic distribution of this species even further.

Effects of anthropogenic pollutants

Effects of anthropogenic pollutants in the environment have short and long term effects on macrofaunal communities (of which C. californicais a member). These pollutants enter the environment through agricultural runoff, exhaust fumes and liquids from boats/ships, rubbish and many other sources and as such, coastal environments are most at risk of exposure to these substances. One such substance (tribulytin or TBT) has been studied in relation to its effects on survivorship and recolonisation ability of C. californica when present in high concentrations in the coastal waters. The study concluded that compared to other caprellid species, C. californica had a high survivorship rate when exposed to higher than average levels of this substance. This shows an increased rate of survival of C. californica under adverse conditions that are unfamiliar to it, as may be the case in other regions around the world. This could possibly contribute to its colonisation ability within the biofouling community.

Another study has shown that heavy metal copper has an impact on mobile invertebrate species assemblages (including caprellid amphipods) by modifying the habitat in which they reside as well as the extant sessile species in this area. Copper is another example of a common anthropogenic pollutant found in coastal waters and so effects on C. californica would be high.

Life History and Behaviour


C. californica is a dioecious gonochoristic species (meaning there are separate sexes). Adult males of this species are larger than females, often with modified first gnathopods. Sexual dimorphism is pronounced in many caprellid species. Adult females have oostegites forming a brood pouch on pereonites three and four, are smaller in total body length than adult males, and have second gnathopods that are similar to those of juveniles. Physical copulation of a male and a female must occur for fertilisation to take place with males grasping females prior to mating, often for several hours, with several copulation events occurring during this time. Males have tubular gonads at their posterior end, allowing sperm to be ejaculated from there, into a females gonopore, where fertilisation of her oocytes (eggs) will occur. After fertilisation has occurred, fertilised eggs are transported by females into the marsupium (brood pouch) on pereiopods three and four. Female meternal care has not been studied in great detail however juveniles are thought to be released from the brood pouch in a haphazard fashion after a certain period of time has passed. It has also been hypothesised that juveniles that hatch earlier inside the marsupium, may consume some of the other unhatched eggs prior to emergence from the pouch.


Caprellid species (including C. californica)move like inchworms, in a looping motion. They grasp the substrate with their first and second gnathopods, arch the pereon (central body region, thoracic region) to bring the posterior end forward, grasp the substratum with their posterior pereiopods, then straighten their pereon to move their anterior end forward. In this way they achieve locomotion through the water column and across the substrate. Their posterior pereiopods are also used for attachment to the substrate during feeding. Large scale movements are difficult as this species is very small and easily gets taken away by currents.

Attack, predation and defense

C. californica, as a macrofaunal species in coastal environments, comes into association with many other types of species. Some of these species (like the fishes) prey on this species as well as other small invertebrates. C. californica, as a competitor within its genus, has a morphological advantage over other native species of caprellids in Australia and around the world. It has gained this advantage due to a ‘horn’ like protrusion on the dorsal side of the cephalon. This horn is used in attack and defense against other caprellid individuals, often meaning it leaves a challenging encounter better off than its opponent.

Anatomy and Physiology

External anatomy

C. californica has a hard exoskeleton with no carapace (the hard upper shell crustacean). It has an elongated circular body shape that is not laterally compressed. The main locomotory appendages are four pairs of jointed appendages (prehensile legs). The cephalon and thoracic segments one and two have been merged to form the cephalothorax (also called pereonite I). This region supports two pairs of biramous antennae on the anterior dorsal side, one pair of maxillipeds just below the oral opening and gnathopod pair one. The cephalon region also has two sessile complex eyes. The oral opening also has a distinguishable mandible. Pereonite II follows the cephalothorax and has one pair of jointed biramous appendages extending from it (first paired locomotory appendages). These appendages have been modified into ‘claw-like’ arms and have setae lined along the ventral surface. Pereonite III and IV together form the marsupium (in females) and support the two pairs of ventrally protruding gills (in both sexes), also known as pereiopods three and four. Pereonite V supports the fifth pereiopod pair (second paired locomotory appendages). From there, pereonite VI has the sixth pereiopod pair and pereonite VII has the seventh (and last) pereiopod pair (third and fourth paired locomotory appendages respectively). The abdomen is vestigial in this species and the telson is highly reduced. All paired appendages (pereiopods) after pereonite II are uniramous.
Figure 1
Figure 2

Internal anatomy

The internalanatomy of C. californica is representative of the Amphipoda order in which it resides. It is highly muscularised, with circular and longitudinal muscles used in locomotion, feeding and digestion, sexual reproduction and defensive movements. It has a double nerve cord that runs the length of its body, with regular ganglion along its length. It has a complete gut with oral and aboral structures, pharynx, stomach, intestinal region, antennal glands and internal ceca. The following diagrams of sectioned regions of the animal detail all of these regions and their location in relation to other structures.

Figure 3
Figure 4
Figure 5
Figure 6



The chief gas exchange surfaces and organs for this species are lamellar or sac-like gills on the coxae of pereiopods four and five. The gills are on the medial side of the coxa, with the coxa themselves also having respiratory functionality.Water currents within the water column facilitate the gas exchange mechanism with the gills, as caprellid species lack pleopods that would usually create this current. Oxygen is carried throughout the body via blood in blood vessels and sinuses, facilitated by the human hemogolbin-like molecule called hemocyanin. This blood is pumped around the body by the heart (a dorsal tube) that lies in the thorax above the gills.


Excretory organs in caprellids are known as antennal glands and are (usually) located at the anterior end of the body and associated with the aboral opening.


Caprellids have a number of ganglionated nerve cords that run around their body, being associated with both internal gas exchange and transport organs and external sensory setae and organs. C. californica eyes are sessile (not stalked) and located on the anterior end of the cephalon.


C. californica adult males are larger in size than adult females. They have tubular gonads. Fertilisation occurs after copulation of male and female before fertilised eggs are moved to marsupium. In females, the ovaries are connected to gonopores by oviducts that open into the marsupium (also known as the brood pouch).

Evolution and Systematics

Evolutionary classification

Protostomia – (proto,“first”; stoma, “mouth”is a major subdivision of the coelomate animal kingdom (Bilateria) that are classified together based on their method of embryonic development. Protostome animals undergo spiral cleavage and develop their mouth directly from the blastopore, with their coelom being derived from embryonic mesoderm.

Ecdysozoa – is a superphylum classified together due to the formation of an external cuticle covering the body which functions as the animals (exo)skeleton. This exoskeleton is shed at least once during a lifetime and this process is known as ecdysis.

Arthropoda – (arthro, “jointed”;poda, “having feet”) are a phylum of animals characterised by segmented animals with paired, jointed appendages. Molting of their chitin exoskeleton is necessary for continued growth. Specialisation of the circulatory, digestive, excretory and nervous systems occurs in these animals. This phylum is the most species diverse of any other in the animal kingdom. There are five subphyla in the Phylum Arthropoda (Crustacea, Hexapoda, Myriapoda, Chelicerata and Trilobitomorpha (extinct)).

Crustacea – (crustaceus,"having a crust or shell"are a subphylum of animals characterised by biramous appendages, complex eyes (and a special type of eye during larval development), varying larval forms and a five-segmented head (cephalon). Tagmosis (division of the anthropod body into tagmata, or segments) occurs and is standardised across most species. They also encompass all features previously mentioned. The Crustacea subphylum has seven classes within it (Malacostraca, Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Ostracoda and Thylacocephala (extinct)). There are approximately 23,000 identified species in this subphylum.

Malacostraca – (malakos, “soft”;ostracon, “shell”) are a class of animals within the subphylum Crustacea characterised by a three-part segmented body (head, thorax and abdomen), a two-chambered stomach, a centralised nervous system, four sets of paired biramous appendages with the first pair often modified into pincers and appendages called pleopods that protrude from the abdomen. The Malacostracan class encompasses all crabs, crayfish, shrimp and invertebrates similar to this. There are three orders within Malacostraca (Amphipoda, Decapoda and Isopoda).

Amphipoda – (amphi, “different/both”; poda, “having feet”) are an order of animals characterised by a (mostly) upright body plan (meaning they are slender and laterally compressed), with the head (cephalon) fused to the thorax forming the cephalothorax, sessile compound eyes (not stalked) and uniramous thoracic appendages (loss of biramous appendages on thorax). The first pair of thoracic appendages are usually modified into maxillipeds (feeding appendages) while the next seven pairs (pereiopods) vary in their functions with pereiopods one and two (the first pair) forming claw-like appendages for feeding and defense. Mature females have a brood pouch (marsupium) constructed from plates at the base of the thoracic appendages. There are four suborders within the order Amphipoda (Senticaudata, Gammaridea, Hyperiidea and Ingolfiellidea) with over 6,000 species encompassed.

Phylogenetic tree

Figure 7

Biogeographic Distribution

C. californica was originally identified by Stimpson in 1856(7) off the coast of California, U.S.A. It is found in shallow sub-tidal coastal waters ranging from temperate to subtropical and even tropical, where it is often seen anchored to the substrate, hydroids, bryozoans and to algae species such as kelp. It has also been identified as a common species found in the biofouling community, meaning it can easily attach to ship hulls, pilings,ropes and other anthropogenic objects, making it easy for this species to be accidentally transported across large distances into geographic regions it may not have expanded to yet. This species has since been identified in regions such as North America, Chile, Japan, Hong Kong, South Africa and Australia, giving evidence for this large scale geographic expansion. Within Australia alone, it has been identified separately in Sydney Harbour, Cockburn sound in Western Australia, Hobsons Bay in Victoria and the Port Of Cairns in Queensland. This indicates the colonisation by C. californica of the entire east coast of Australia and possibly the west coast as well. The eastern coast of Australia ranges from temperate waters in the south to sub-tropical waters around Brisbane and tropical waters in the north, all of which C. californica has been shown to exist in.

Figure 8

Conservation and Threats

There are currently no conservation efforts for this species in Australia as it is not a native species. On the eastern coast of America where this species is native (as well as in Japan), efforts have been made to reduce pollutants in coastal waters to reduce possibility of increased mortality rates of coastal marine invertebrates (including C. californica). As this species has a high survival rate in adverse unfamiliar conditions, it has no current major threats to its survival or population sizes. It is adaptable to a broad range of water temperatures (from temperate to tropical and everything in between) and abiotic conditions. It is currently listed as not-endangered.


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