Big discoveries in small shrimp

Strange and new species of opossum shrimp (Crustacea: Mysida/Lophogastrida) revealed in the Australian Museum collection.

Mysids are small crustaceans commonly referred to as opossum shrimp because the females carry fertilised eggs in a pouch, formed from plates usually attached to the rear pair of legs, from which the juveniles emerge directly as small versions of the adult. In this respect they differ from prawns and shrimp, that they superficially resemble, which have a planktonic stage that is released into the water column and gradually transforms into the adult form through a series of moults. Other distinguishing features between shrimp, prawns and mysids require close examination but include differences in fusion of the body and head, the sequence in which the body segments overlap and the number of swimming appendages along the body (6 to 7 pairs in mysids versus 5 in prawns and shrimp). Mysids generally lack a well developed serrated and extended point (rostrum) at the front of the head characteristic of most prawns and shrimp. Many mysids also have a noticeable fluid filled cavity at the base of each side of the tailfan, known as a statocyst, movement of the fluid provides a signal aiding orientation and balance during swimming.

There are approximately 1,200 mysid species currently recognised. These are primarily marine although some occur in freshwater. The majority are less than 25 mm long and translucent so they are not regularly observed. They occupy nearly all possible aquatic habitats, including the bottom and the water column, and some are commensals of other invertebrates. Mysids can be abundant and commonly form swarms, particularly near the sea floor. Because of their abundance they are an important link in foodwebs. Many species feed on decomposing algae and seagrass. In turn they are a significant food source for other crustaceans, fishes, birds, seals and whales, and also a traditional part of the human diet in East and South-East Asia. There are examples of mysids as invasive species in North America, South America and Australia. Along with other crustaceans which require calcium carbonate to form their outer body covering, or shell, mysids may be under threat from increased ocean acidity associated with global warming.

Mysid species diversity increases towards the equator, with peaks in the Caribbean and West Indo-Pacific. Systematic studies of mysids have been conducted in the Arctic, North-West Pacific and to a certain extent in Mediterranean and Antarctic regions. Even in well studied areas undescribed species are still regularly recorded, emphasising the ability of mysids to utilise cryptic habitats. Other regions have received limited study based on ad hoc collections and occasional expeditions. Attempts to revise genera are still rare, largely because of this lack of regional study for broad areas. From Australia approximately 160 species and 45 genera have been recorded, and despite only fragmentary study this accounts for ~12.5% and 25%, respectively, of the world fauna.

The Australian Museum holds significant unidentified collections of mysids from around Australia and through the Indo-Pacific. Many of the specimens have been obtained over the last 50 years from samples made by SCUBA diving and examining substrates that mysids occupy along the New South Wales coast. During the last two months Dr Mikhail Daneliya, a mysid expert from Finland, has studied approximately 260 samples from this collection as a Visiting Australian Museum Research Institute (AMRI) Collection Fellow. Mikhail comments on what he has uncovered so far, “almost everything I find is interesting, if not a new species, then rare, sometimes the first record after the original description and almost everything is a new distribution documented for New South Wales. It is not like routine identification of known things so it takes a lot of time”. To date Mikhail has discovered in the collection approximately 20 new species, mostly from south eastern Australia. In the subfamily Heteromysinae for example he has recorded 11 species, 9 of which are new records for New South Wales waters (including 7 new species he is describing), an increase of more than 80%.

A highlight has been finding, in samples obtained from iconic Bondi Beach in 1977, specimens of the unusual genus Rostromysis. This is otherwise known just from the original description of the only species from south western Australia which was recognised in 1987. Unlike many mysids it has a relatively long rostrum (point) at the front of the head, from which the genus name is derived, females differ from males in having leaf-like swimming appendages which is a characteristic unknown in other mysids. While previously identified material in the collection has been useful for Mikhail to put his studies in context, he notes the specimens of Rostromysis he recognised date from ten years before the original description, causing him to comment the “gems of Australian Museum collection lie in the specimens that nobody else has looked at because that is where the exciting discoveries are to be made”. Rostromysis is an example of one of the many mysids only known from Australia. Recognising such endemic species is crucial in establishing areas to conserve biodiversity.

Another endemic which Mikhail has extended the known distribution for is Siriella longidactyla, which has not been recorded since the original description in 1940 based on specimens from Port Stephens, approximately 200 kilometres north of Sydney. It has now also been found in material collected at Garden Island in Sydney Harbour in 2001. From his desk at the museum Mikhail could almost see this new location and wryly notes “no need to go far to realise how little is known about the fauna”. It also seems that this is the same as a species reported from the Sahul shelf in north western Australia but more checks need to be made to confirm this.

A further important find is the recognition of the introduced species Neomysis japonica, native to Japan, in the Newcastle region from samples obtained in 2010 confirming it’s persistence in this area since being first reported from Australia in 1977. Mikhail notes this species "can live in brackish and fresh water, and can probably affect native estuarine Rhopalophthalmus species (another mysid)".

Mikhail’s work will leave the Australian Museum with the first comprehensively identified collection of Australian mysid specimens. His plans include describing the new species he has recognised, and publishing new distribution records and identification keys. The Australian species can then be better placed in wider studies to understand the diversity and evolution of mysids and the origin of the West-Pacific fauna. This increased understanding will provide a firmer basis to include mysids in biodiversity studies, ecology and conservation management, and the detection of invasive species.

Stephen Keable, Collection Manager, Marine Invertebrates