Skeleton shrimp jump out of the closet

Every day is Halloween for these tiny shrimp and visiting researcher José Guerra Garcia loves to celebrate with those in the AMRI collection 

Sixteen years after his first visit to the Australian Museum, Dr José Guerra Garcia, from the Universidad de Sevilla in Spain, is back making more discoveries. These include three new species and a new genus of his favourite animals, the skeleton shrimp. These small crustaceans have been implicated as invasive in marine environments, useful bioindicators of environmental health, models for interpreting morphological evolution and a resource for aquaculture and aquarium industries, so it is important to understand their diversity, distribution and identification.

Skeleton shrimp are classified in the order of crustaceans known as the Amphipoda, which number nearly 10,000 species (80% marine, 17% freshwater and 3% terrestrial), characterised by a range of features including unique segmentation, absence of a carapace and bodies that are generally laterally compressed. Within this larger grouping the skeleton shrimp are categorised as a sub-order known as the Caprellidea (or caprellids).

There are more than 400 described species of caprellids, 50% of these are in the genus Caprella. Most are a few millimetres to centimetres in length and all are marine. The general characteristics of the Caprellidea, compared to other amphipods, include a slender and cylindrical body (the reason for the common name – skeleton shrimp), fusion of the head and the first body segment, reduction in the number of pairs of legs and leg segments, two or three pairs of gills, females with brood plates for holding eggs on only a few body segments, and a degenerated abdomen and abdominal appendages. In short, they are highly modified. This divergence from the body plan of other crustaceans makes them significant in evolutionary theory, with multiple losses or reductions of various morphological parts apparent in different lineages.

The unique appearance of caprellids is undoubtedly related to their specialization of clinging to a wide variety of substrates including other invertebrates (such as hydroids, sea squirts, lace corals and sponges), marine plants and sediment. They feed on suspended materials, and prey or graze on other organisms. In general terms they can be considered as detritivores. Caprellids can be locally abundant, are important prey for many coastal fish species 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 behavior make them attractive to aquarium hobbyists.

Many caprellids have a wide distribution and there are numerous examples of species that are considered cosmopolitan. Swimming appendages used in other amphipods are reduced in caprellids and they lack a planktonic larval stage. This suggests they are distributed passively by clinging to floating materials such as algae but may also be distributed by artificial means such as on fouling growth on boats or buoys. Although the abundance and species richness of caprellids in many areas is still poorly known, surface water temperature is an important factor determining the distribution of the littoral species. A combination of factors including global warming seems to have led to several species spreading to new areas and potentially affecting the ecology of the native species.

The AM’s skeleton shrimp collection is one of the most diverse and important available, including 34 genera, 74 species and over 160 specimens used in the original description of many species. During his three-month visit, since December 2017, José has been referencing this material to write a comprehensive identification key for caprellids. While his research centres on ecology and biogeography of these amphipods he notes there can be little progress in these areas without a clear taxonomic framework and that well maintained museum collections are vital in making this possible.

Stephen Keable, Marine Invertebrates Collection Manager, Australian Museum, and

José Guerra Garcia, Professor, Facultad de Biología, Universidad de Sevilla, Spain.

More information:

There are a number of publications from José's earlier studies of the Australian Museum caprellid collection:

• Guerra-García, J.M. 2004. The Caprellidea (Crustacea, Amphipoda) from Western Australia and Northern Territory, Australia. Hydrobiologia 522: (1-3) 1–74.
• Guerra-García, J.M. & Takeuchi. 2004. The Caprellidea (Crustacea: Amphipoda) from Tasmania. Journal of Natural History 38: 967–1044.
• Guerra-García, J.M. 2006. Caprellidae (Crustacea: Amphipoda) from the Great Barrier Reef and adjacent localities. Records of the Australian Museum 58 (3): 417–458.