Related species of skeleton shrimp from Australia and Far East Asia

An unusual find of thousands of skeleton shrimp on commercial fishing nets in the Gippsland Lakes, south-eastern Australia has led to the revision of a species from Far East Asia, review of previous records, and recognition of a new species.

Skeleton shrimp are small distinctive invertebrates that are classified in the superfamily Caprelloidea within the order of crustaceans known as the Amphipoda. While nearly 10,000 species (80% marine, 17% freshwater and 3% terrestrial) of amphipods are known, only around 400 of these are caprellids and these only occur in marine or estuarine environments. Caprellids have been given the common name skeleton shrimp because they have narrow cylindrical bodies and reduced appendages that show extreme modification associated with a mode of life that is adapted to clinging to substrates.

This divergence from the body plan of other crustaceans makes caprellids significant in evolutionary theory. They may feed on suspended materials including phytoplankton, prey or graze on other organisms, or forage on detritus. Caprellids can be locally abundant, are important prey for many coastal fishes and have also been found to be sensitive to marine pollution. They can be used as food for other organisms in aquaculture operations, and their unusual appearance and behaviour make them attractive to aquarium hobbyists and divers. Skeleton shrimp are thought to be highly endemic (restricted in distribution) as they lack the planktonic juvenile stage that enables other crustaceans to be dispersed widely by ocean currents. However, they can be transported long distances naturally, by rafting on floating seaweed, or artificially by clinging to biofouling on ships or through aquaculture activities.

Nearly half of the described species of caprellids are classified in the genus Caprella. While Australian skeleton shrimp are incompletely known, with numerous new species described regularly in the last 20 years, approximately 10 species of Caprella have been recorded here.

In 2018, professional fisher Matt Jenkins was surprised to find thousands of skeleton shrimp swarming his nets in Lake Tambo within the Gippsland Lakes, Victoria, a situation he and his colleagues had not encountered previously.

Video of Caprella tamboensis on fishing nets courtesy of Matt Jenkins, used with permission:

Matt was able to preserve some specimens and send them to the Australian Museum Research Institute for identification. Close inspection showed they were clearly different to most other Australian species of Caprella but very similar to a species known as Caprella acanthogaster. This species was originally found as early as 1890 in Far East Asia and has a distribution in that region including Japan. Caprella acanthogaster had also been reported in Mercury Bay, Tasmania, from samples collected in 1993 and 1996, but the occurrence in Tasmania was attributed to an artificial translocation associated with the introduction of scallop spat from the North Pacific as part of aquaculture activities.

This situation led to a reconsideration of the species Caprella acanthogaster based on specimens from Japan, close to the original reported locality, with comparison to the Australian material from Tasmania and the Gippsland Lakes. Results from the study found consistent morphological differences between the Japanese and Australian samples. Crucially, specimens from Japan which were twice the size of those from Australia had fewer segments in the first pair of antenna – this is unusual, because if they were the same species it would be expected that they would have more segments, as these appendages in caprellids have been shown to increase by one or two segments every time the individuals moult their exoskeleton as they mature. Additional differences also include the shape of the grasping limbs the caprellids use to feed and groom with, and in the relative lengths of comparable segments in the antenna.

As a result, a new species with the name Caprella tamboensis has been proposed for the Australian specimens. It is still possible this species from the Gippsland Lakes and Tasmania may have been introduced to those areas but alternatively it could be a native species, widely distributed but as yet largely unrecorded in southern Australia. It has been highlighted in other recent publications that increased analysis of the caprellid amphipods from southern areas is needed to fill gaps in knowledge of the biodiversity and biogeographical patterns of the Australian fauna.

Observations made in the redescription of C. acanthogaster and recognition of C. tamboensis also highlight that further detailed morphological study, possibly combined with genetic analysis, is necessary. This is important for reconstructing evolutionary relationships and for our understanding of the distribution pattern for C. acanthogaster and other species from temperate to cold waters of the northeast Asia – there are several in that region which are also very similar to C. acanthogaster and can be easily confused with it, so clarification is key!