Skeleton shrimps or caprellids have very slender bodies and a cylindrical shape. The head is bulbous with antenna 1 generally longer than 2. The appendages are reduced in number. The animal can be divided into a head, thorax or pereon and an abdomen. They tend to be slow-moving. Caprella mutica is a large caprellid amphipod. Males can be up to 5 cm long and females 1.5 cm. They are orange to red in colour and the brood pouch of the female is covered with dark red spots.The first record of Caprella mutica for the UK was in July 2000, from a fish farm in Scotland (Willis et al., 2004). The species' natural distribution is the coastal waters of the sub-boreal areas of north-east Asia (Willis et al., 2004). Little is known about the biology or ecology of this species. Each female can produce up to 150 hatchlings every 45-50 days. It was introduced to North America (Pacific coast) accidentally through shipments of Japanese oysters. The method of introduction to the UK is unknown, but likely to be via shipping and aquaculture. Caprella mutica has been found in high concentrations in marine Special Areas of Conservation (SACs) designated for their biogenic reefs. The impacts on these habitats, if any, is not known. Found in high densities especially during May to September. On the west coast of Scotland their abundance can reach 300,000 individuals per m² (Cook et al., 2007).