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Australomysis sp.

Perry Bennion 2016


Australomysis is a genus of small (approx. 8mm) mysid shrimp (Crustacea: Mysida) found predominately along the South East coast of Australia in Eelgrass (Zostera muelleri) communities. Mysidacea are most distinguishable by the presence of a marsupium or brood pouch in females, where larvae are held until they moult into juveniles.  While there is currently little work other than taxonomic descriptions done on species in this genus, the importance of mysid shrimp as a whole in macrophyte turnover, pelagic-benthic connectivity, and key components of food webs has been well established. To combat the lack of species-specific information available, what is known about Australomysis sp. is used alongside work done in related taxa to infer or predict life history and ecological traits as well as potential threats to the species.

Physical Description

Australomysis sp. are small crustaceans with adults approximately 8mm and carrying 'Caridoid facies' characteristics (Meland et al., 2015). A rostrum is present and acutely triangular and not extending past the eyes. Eyes are present and stalked with no separate regions or divisions. The second antennal scale is longer than the peduncle of the first antenna. The carapace and body is smooth and transparent, with the green ovaries of females clearly seen through the cuticle. Adult female Australomysis sp. have a brood pouch formed by the posterior thoracopods coxae and exopods in which larvae develop (Fenton, 1985). Thoracopods on the anterior two segments are adapted into maxillipeds, with the remaining thoracopods either biramous and natatory with plumose setae present in males, or natatory reduced and plate-like where not comprising the brood pouch in females (Fenton, 1985). A statocyst is present in the uropods and the telson is longer than it is wide with a medial cleft. Telson has robust setae along entire margin and is lacking plumose setae (Fenton, 1985). 
Figure 1


Mysid shrimp have been identified as a key link in many food webs and have been suggested to be responsible for structuring freshwater,pelagic and brackish communities through predation on zooplankton (Mauchline, 1980; Fenton, 1985; Albertsson, 2004). Diel vertical migration found in some species of Mysida is suggested to also plays an important role in benthic-pelagic connection of food webs (Albertsson, 2004).  Although little to no work has been done on Australomysis sp. outside of taxonomic descriptions, some ecological parallels can be drawn from work done with other species within the Mysida.

Stomach contents combined with stable isotope analysis by Fenton (1985) along with investigation into trophic interactions by Albertsson (2004) suggests that while specific diet varies between species, availabilityand season, mysids are omnivorous, feeding on algae, diatoms, dinoflagellates, copepods, smaller crustaceans, and detritus (Fenton, 1985).  Australomysis sp., being found in Eelgrass (Zostera sp). meadows, are likely to play a large role in the turnover of macrophyte biomass (Mauchline, 1980; Fenton, 1985). This is further supported by findings by Foulds and Mann (1978) that, whether by gut micro-flora or self-produced cellulases, the related mysid, Mysis stenolepsis, is capable of digesting cellulose. While no work has yet investigated the species level ecology of any species in Australomysis, it is plausible to suspect that turnover of macrophyte biomass and consumption of detritus, combined with the occurrence of Australomysis sp. on blades of Eelgrass (Tattersall, 1927; Fenton, 1985), likely has a positive effect on the growth of Eelgrass (by decreasing macrophyte activity and cleaning detritus), and thus a cascading effect on those animals that rely on the Eelgrass meadows.

Mysids in turn are predated on by a range of predators including amphipods, birds, cephalopods, ctenophores, decapods, fishes, isopods, gastropods, ostracods, seals, and whales (Mauchline, 1980). Predation by juvenile commercial fishes in shallow waters has been observed while  Eastern Australian salmon (Arripis trutta) have been found to consume large quantities of mysids in Eelgrass of coastal Victoria, Australia (Fenton,1985). These observations, together with the feeding habits described above,suggest that Australomysis sp. likelyplay important roles in top-down regulation of macrophyte levels in eelgrass as well as providing an important link within trophic pathways of seagrass communities.

Life History and Behaviour

Mysidacea are unique in the crustacea in that larval development occurs in a marsupium (or brood pouch)(Tattersall, 1927). Studies across four subfamilies, comprising 12 plus specieshas suggested that within Mysidae, developmental pathways are uniform, with alteration in gestation time and brood size being the main areas of variance (Wortham-neal and Price, 2002). The larval development of mysids is comprised of three periods comprising 5 stages (Mauchline, 1980; Wittman, 1981). In the embryonic stagethe larvae is still within the egg membrane, thus is often referred to as the 'egg stage'. From this, the larvae hatches into the eyeless naupiloid form(period two), for two stages. In the first of these stages (Stage two), the larvae elongates, beginning to resemble a 'comma-shape' and thoracic appendages begin to form. In stage three, pigmentation begins to form in the eyes and the second larval period ends with a moult of the naupicular cuticle, beginning the final, eyed, post-naupiloid period. This period is comprised of two stages,during the first of which, a yolk protuberance is present anterodorsal to the carapace, whilst in the final stage, the yolk is eclosed into the carapace.This final stage ends with release from the marsupium, followed immediately by a moult into a juvenile (Wortham-neal and Price, 2002).

Female mysids moult shortly after brood release, while releasing strong attractants that induce search behaviour in males as females receptive period is quite short (Wittmann, 1981). Egg andlarval development is synchronised in mysids, with mating stimulating eggmovement into the brood pouch, resulting in fertilisation. The process of brood release to fertilisation normally occurs over one or two nights (Wittmann,1981). Developmental and generational time is related to environmental temperature, with Arctic/ Antarctic species reproducing at around 0.5generations/year, while temperate littoral/ estuarine species (such as Australomysis sp.) achieve rates closerto 3 generations/year (Mauchline, 1980; Wittmann, 1981). Brood number is affected by female body length, season, latitude, and egg size (Mauchline,1980)

While no study has investigated intra-specific resource partitioning within Australomysis, the Japanese estuarine mysid Hyperacanthomysis longirostris displays an age gradient with adult H. longirostris occurring further upstream and juveniles downstream(Suzuki et al., 2009). At present,this is the only available work done on species-specific population dynamics and structure within Mysidae, thus no conclusions can be drawn regarding population structure or resource partitioning within the Australomysis.

Figure 2
Figure 3

Anatomy and Physiology

All Mysida display to differing degrees the morphological traits of the 'Caridoid Facies' (Meland et al., 2015). These characteristics are considered plesiomorphic within the Malacostraca and include; enveloping of throax by the carapace, moveable stalked eyes, biramous antennules, plate/scale-like exopods on antennae, thoracopods with natatory exopods, elongate abdomen with internal organs excluded and strong ventral flexibility, Fan-like tail formed by telson and uropods, and pleopods 1-5biramous and natatory (Meland et al.,2015). Within Mysidae, as with Malacostraca, tagmata consist of the cephalon, thorax and abdomen, with apparent fusion of the cephalon and thorax, resulting in the cephalothorax. Segmentation comprises five somites in the cephalon (with a sixth putative ocular region), eight somites in the thorax, and seven somites in the abdomen, with the seventh somite reduced and fused into the sixth (Meland et al., 2015). Mysida can be distinguish from other Mysidacea orders by  fusion of no more than the first four thoracic somites anteriorly to create the carapace and adaptation of the first and second pair of thoracic limbs to maxillipeds(Meland et al., 2015). Sexual dimorphism in mature mysids is present with males having appendix masculina, an anterioventral process, covered in long sensory setae, at the distal end of the antennal sympod's third segment (Melandet al., 2015). Male genitals  are located on papillae arising from the eighth thoracopod coxae (Meland et al.,2015). Female mysid have a marsupium formed by concave, thin-walled oostegites comprised of the coxae and exopods of the posterior two or three thoracic appendages(Meland et al., 2015). The anterior five abdominal pairs of pleopods are natatory with plumose, multi-articulate exo- and endo- pods in males and reduced, unjointed plates in females (Meland et al., 2015). The endo- and exo-pods ofthe uropods on the distal somite of the abdomen are fringed by setae, with aproximal statocyst, exclusive to the Mysidae, present created by an invagination of the integument with a statolith within that aids with balance and visual stability (Meland et al.,2015).

Australomysis sp. can be identified by; adult length approximately8mm, rostrum present and acutely triangular, eyes not divided, antennae two scale longer than antenna one peduncle,  telson is longer than it is wide with a medial notch at the distal end and robust spinesalong the entire margin, robust setae present in inner margin of endopod, pleopodone is uniramous while two to four are biramous. (Tattersall, 1927; Fenton,1985)

Mysidae have no brachiae bearing appendages and all respiration occurs on the inner surface of the carapace(Meland et al., 2015). Mysids in general display low tolerance to variation in salinity, turbidity and waterflow, thus are restricted to discrete zones (Roast et al., 1998). They are particularly sensitive to many contaminants, making them prime test species for toxicity analyses by groups such as the EPA (Roast et al., 1998)

Figure 4

Biogeographic Distribution

Australomysis sp. have been describe predominately from South Australian waters occuring in eelgrass to depths of 20m (Fenton, 1985). Thisrange has expanded outside of Australian waters to Japan with the description of Australomysis hispida (Fukuoka and Murano, 1994). As no work has yet been done to examine the range of Australomysis sp., analysis using the Atlas of Living Australia was used to draw some predictions. First, reports of Australomysis were plotted, showing a distribution along the South East Australian coastline to just North of Brisbane. A report is also present for just South of Perth on the South West coast. Australomysis sp. were widespread throughout Port Philip Bay.

In Tattersall's (1927) first description of the genus, along with Fenton (1985), Australomysis sp. were described as being found on leaf blades of Eelgrass, Zostera muelleri. Assuming this is the habitat in  which Australomysis sp. are to be found, reports of Zostera sp. were plotted and compared against reports of the mysid. Comparison of plots suggests similar ranges, with Zostera sp. ranging further North on both the East and West coasts.Given the lack of published work and reporting on the genus, and the presence of a Northern Hemisphere Australomysis sp. , it is not possible to infer the full range. Alongside this, the complete lack of molecular data on the genus inhibits the ability to predict when the Northern and Southern species diverged and as such what environmental or geographical gradients were being followed.
Figure 5
Figure 6

Evolution and Systematics

The Eumalocostraca date back to the Devonian period, with radiation into the majority of groups occurring during the Carboniferous (Meland et al.,2015). During this radiation, the Lophogastrida and the Pygocephalomorpha first appeared, with the latter group comprising the majority of fossils and comprising no extant species (Meland etal., 2015). Fossil evidence of the Mysidae is rare due to their softbodies, however the calcareous statoliths found in the statocysts have been found mineralised in deposits from the Vienna Basin (Austria) to Lake Aral (Kazakstan) dating to the Miocene (Meland etal., 2015). These statoliths, resembling those found in the genus Paramysis, have now been attributed to the fossil genus Sarmysis (Meland et al., 2015).

The Mysidacea, comprised of the three Orders Mysida, Stygiomysida and Lophogastrida comprise approximately 1200 species across 187 genera, with extrapolation predicting closer to 4000 species (Meland et al., 2015). The taxonomic position of Mysidacea is currently unresolved, but has been placed within Peracarida (current), Decapoda, Stomatopoda, Nebaliacea, and Euphausiacea during different revisions (Meland et al.,2015). Because of strong disagreement between molecular and morphological studies on the group, revision of phylogenetic and taxonomic structures has been called for several times (Remerie etal., 2004; Meland and Willassen, 2007; Meland et al., 2015).

Within Order Mysida, two families, Mysidae and Petalophthalmidae are present and distinguishable by the presence of a statocyst in the uropods of Mysidae species (Meland et al., 2015). Tribes Mysini and Leptomysinae were upgraded to subfamilies Mysinae and Leptomysinae due to evidence from both taxonomical and molecular data (Remerie et al., 2004). Australomysisis currently classified within Leptomysinae, one of ten subfamilies within Mysidae.

The type specimen for Australomysis was originally described from Port Phillip Bay, Victoria, Australia as Mysidopsis incisa Sars (1885), however was reclassified into the new genus by Tattersall (1927). There are currently five accepted species within Australomysis (Mees, 2015).

Conservation and Threats

Australomysis has not been assessed by the IUNC redlist and no conservation work has yet examined any species within the genus. Some general threats can however be discerned by examining threats to its habitat (Zostera sp.) or by extrapolating from data available from other Mysidae.

It is predicted that since the1990's we have lost more than half the world's seagrass area and approximately14% of seagrass species are at risk of extinction (Sherman et al., 2012). This contrasts with work performed by Cuttriss et al. (2013) that found that seagrass coverage in Moreton Bay, Queensland Australia, have trended upwards since 1987as well as a classification as "Least Concerned" for Zostera muelleri by the IUCN redlist.Fragmentation however, has increased, which may pose a threat to Australomysis sp. as the larval stage,commonly responsible for dispersal, is restricted to the brood pouch. No work has yet been done to investigate the dispersal ability of juvenile or mature Australomysis sp..

Mysids have been found to be particularly sensitive to chemical contaminants and are thus used to define limits for contamination in toxicity testing by the EPA (Roast et al., 1999). It has been found that the antifoulant Tributyltinchloride (TBTCl) reduces mysid energy consumption and lowered respiration at concentrations that are environmentally relevant(Verslyke et al., 2003). Alongside this, The common herbicide Atrazine has been found to reduce plant biomass and chlorophyll concentration in Zostera marina from levels as low as 10µg/L and reduce the effective yield from seedlings from concentrations as low as 2µg/L (Gao et al., 2011). Persistent high turbidity reduced the numbers of Mesopopsis slabberi, a key mysid species of the Guadalquivir estuary in Spain, along with some of its primary predators(Subida et al., 2010).

Lack of data is a large issue when considering the main threats to species within Australomysis. As so little is known in regards to its ecology,range and even taxonomy, we currently cannot state whether it is threatened or not. Inferences made from work done on related mysid taxa and species of Zostera, suggest that antifoulant and chemical use, together with habitat fragmentation and anthropogenic changesthat increase the persistence of turbidity events, such as dredging,  are the most likely threats to Australomysis sp.. Habitat loss due to contamination by herbicide runoff could also contribute to these threats,however, the stability of Z. muelleri,indicates that this is most likely not the case.


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