Australian Ghost Shrimps are known for their ability in bioturbation (Katrak and Bird, 2003; Bird, Ford and Hancock, 1999; Spilmont et al, 2009; Contessa and Bird, 2004), or the reworking of sediments by animals, in this case due to burrowing. Organisms that create burrows directly affect the transfer of dissolved chemical particulate matter between sediment and the water column (Bird, Ford and Hancock 1999), and thus create conditions facilitative to particular community establishment. 

T. australiensis have been found in many studies to have a notable influence on the biogeochemistry between the sediment strata and the overlying water column. Jordan et al (2009) found the presence of T. australiensis to significantly alter sediment-oxygen-demand, and the residence time/overturning of nitrogen-based nutrients. This finding is likely to be correlated with results obtained by Contessa and Bird (2004), who reported increased chlorophyll-a concentrations in the water column where yabbies were absent. This is likely due to the increased residence time of nutrients in the absence of T. australiensis, facilitating phytoplankton population establishment. The role of yabbies in these two processes suggests that T. australiensis contributes to the localised water quality pertinent to many species in their shallow, intertidal habitats.

Callianassids have been found to inhabit waters of up to 2,400 feet (Hailstone and Stephenson, 1961) however, T. australiensis is found almost chiefly in the intertidal zone, burrowing into soft-sand sediments. Fig. 4 shows the tidal flat sampled within this study, a typical habitat of the Australian Ghost Shrimp.
T. australiensis builds intricate networks of burrows into the substrate, to depths of >50cm (Katrak and Bird, 2003), the entrances to which are visible on the surface of the substrate (Fig. 3).

The facilitative properties of T. australiensis’ burrowing habits also create relationships with other organisms living sharing a common niche. Kerr and Corfield (1998) observed a commensal relationship whereby T. australiensis burrows provided optimal feeding locations for M. vitrea, a species of deposit-feeding bivalve. As a result, while it was shown that M. vitrea was able to survive in the absence of yabbies, the bivalve’s distribution vertically within the benthic strata was altered in the presence of T. australiensis. M. vitrea exhibited a shift to deeper within the sediment in all treatments with T. australiensis populations present, in order to exploit these superior feeding conditions.

T. australiensis is largely a deposit feeder, obtaining its nutrition below the surface of the benthos. It can either feed on food particles within the walls of its burrows, or resuspend particulate matter comprising the burrow walls, and feed on the newly suspended food items (Stapleton, Long and Bird, 2002; Spilmont et al, 2009; Katrak and Bird, 2003). Spilmont et al (2009) observed that T. australiensis exhibits a particular diet, feeding specifically on benthic diatoms, and also noted that the feeding activity of yabbies slowed during the summer months. Fig. 5 shows the anterior end of a collected male T. australiensis specimen, with the elongated rostrum and feeding structures such as the maxilla and mandibular palps visible.

While a distinct lack of information on the reproductive biology of T. australiensis is evident within the literature, it is known that Australian Ghost Shrimps exhibit the same reproductive traits and processes as most species within their family, the Callianassids. As in the majority of decapods, fertilization is external (Encyclopaedia Britannica Online, 2016) and eggs are carried by the female, adhered to the ventral side of the abdominal region (Fig. 8). The fertilized eggs then remain attached here until they have hatched. 

Whilst little information is available on the larval development of T. australiensis, six larval stages have been observed and described (Dakin and Colefax, 1940 as cited in Hailstone and Stephenson, 1961). The Hailstone and Stephenson study (1961) noted the presence of the earliest larval stages within the plankton around one month post-breeding season (late summer), and observed these larval stages to have a carapace length (CL) of approximately 1 millimetre. The latest of the six larval stages were observed in the Hailstone & Stephenson study during spring at a CL of approximately 3 millimetres, and the assumption was made that the post-larval forms would then settle to the benthos during early summer. 

Following maturation, Callianassids are known for displaying distinct sexual dimorphism, distinguished by significant enlargement of the major cheliped in male individuals (Shimoda et al, 2005).  


Sex Ratio Distribution:

While a number of works have investigated the temporal patterns of reproduction in callianassid shrimps, including T. australiensis, our knowledge of how their reproductive patterns vary spatially is lacking. Here, we conduct a small-scale, simple investigation to the sex ratios of yabbies across a singular tidal flat at Dunwich, North Stradbroke Island, Queensland, Australia. The purpose of this experiment was to identify possible spatial dynamics of reproductive activity within the intertidal zone. The objective was also to elucidate the spatial preferences of the shrimps with regards to copulation and sexual reproduction; whether more reproductive activity was likely to occur higher or lower in the intertidal zone. Knowledge of any such patterns may be useful in regulating the collection of T. australiensis as bait by recreational fishermen (see ‘Conservation & Threats), and may allow harvesting of this species at a lesser cost to the overall population due to the preservation of sexually active adults.
In this study, data was collected on the percentage of males and females respectively among the total individuals collected. These data were collected at 10 metre intervals along a 180 metre transect. The respective percentages of the genders were used here as a proxy for sexual interaction between individuals; it was assumed that the closer the sex ratio to 50% males and 50% females, the more reproduction would take place. Thus, assuming this to be the case, it can be inferred that regions within the tidal flat exhibiting the most indifferent proportions of males to females would be areas of highest reproductive activity. 

Methods:
A transect was marked out, running from the high intertidal (defined as the convergence of beach sands and tidal flat sediments, see Fig. 4) to the low tide waterline at 10:30am on Saturday the 14th of May, 2016. At intervals of 10 metres, T. australiensis specimens were collected using a yabby pump. In order to maintain commonality of sampling methods, 10 pumps were carried out at each interval, and could be from anywhere within a 1m² area surrounding the given 10 metre interval mark (e.g. 20m, 70m, 110m). The yabbies obtained from each sample interval were collected and examined; the total number of individuals, the number of males and number of females respectively were recorded before the yabbies were released to re-burrow. This process was repeated every 10m, moving away from the beach in a perpendicular fashion until reaching the waterline. The result was a 180m transect, comprising 18 intervals of 10m where specimens were collected and counted as mentioned above. 

A second transect was attempted at a different location 300m to the south on the same coast, however no specimens were obtained throughout the entirety of the transect despite the presence of burrow entrance holes. This was likely due to the apparent presence of a solid layer of strata approximately 30 centimetres below the surface of the sediment, which likely meant that the area was not ideal habitat for yabbies, which are known to construct burrows of >50cm (Katrak and Bird, 2003). The holes observed at the surface were likely caused by a different burrowing macro-invertebrate, which was not observed during this study. Due to the insufficient data obtained from the second transect, it was subsequently omitted from results. 

Results:

It can be observed that between 0 – 30m away from the high intertidal zone, females dominated comprehensively, with zero males obtained at the 0, 10 and 20 metre intervals (Fig. 6, 7). Barre the 40 metre interval, it can be seen that gender dominance remains female, and does not truly fluctuate between genders until 70 metres from the high intertidal. Between 70 and 140m, sex ratio was shown to fluctuate episodically and significantly, with 100% gender dominance occurring at 90, 100, 120 and 130m. From 140 metres onward however, the sex ratio was shown to rapidly approach 50% male, 50% female, and can be observed to fluctuate only slightly from equal proportions. These results suggest that as a general trend, the sex ratio of T. australiensis approaches 1:1 with distance seaward from the high intertidal zone. 

Discussion:

Following qualitative analysis of the results visualized in Fig. 7, it can be inferred that sex ratios of T. australiensis approach 1:1 with distance seaward from the high intertidal zone, and following the assumptions of our proxy, reproductive activity is likely to increase as the ratio approaches 1:1. Based on these findings, T. australiensis appears to exhibit a preference for reproduction in the lower regions of the intertidal zone. 
These results however, must be reinforced by further works before being used to inform management decisions regarding T. australiensis’ population dynamics. While Fig. 7 lends evidence to this relationship, due to a lack of resources, and with sampling opportunities limited due to weather and tides, only one transect was successfully completed. This transect may prove informative of the population dynamics of T. australiensis on the west coast of North Stradbroke Island, however the spatial dynamics of reproductive activity suggested here may not hold true in other locations within the yabby’s distribution. Further studies should seek to collate data from transects such as the one executed here from a multitude of locations along the east coast of Australia. Additionally, while the assumptions of our proxy were outlined, it must be acknowledged that many further factors contribute to the reproductive success of any two individuals and this proxy may not provide absolute accuracy. 
Despite the caveats inherent within this experiment, the results obtained here give rise to a number of future works. The results of these studies are now required, in order to confirm or dispel whether T. australiensis shows a preference for reproductive activity in the lower intertidal zone in all regions of its distribution. 

As part of the superphylum Ecdysozoa, Callianassid shrimps all inevitably undergo ecdysis, as confirmed in Thessalou-Legaki and Kiortsis (1997) where ecdysis is mentioned in Callianassa tyrrhena. Ecdysis is a form of moulting where the chitinous cuticle or exoskeleton is jettisoned and a new one created. This process is mandatory for the growth of the organism within the exoskeleton, and doubles as an opportunity to repair the exoskeleton following any damages sustained to the previous one (Daley and Drage, 2016). As noted in Hyžny & Klompmaker (2015), Callianassid moulting appears to occur within the burrow, however the discarded cuticle is removed from the burrow. The occurrence of moulting episodes is governed by concentrations of the hormone ecdysone, which is present in all arthropods (Daley and Drage, 2016). In the specific case of T. australiensis, Hailstone and Stephenson (1961) observed three peaks in ecdysis throughout the year, at February, June and October/November. 

T. australiensis has a tendency of building intricately branched, systems of burrows, that have been documented to reach depths of 50cm or more below the surface of the substrate (Katrak and Bird, 2003). It is their burrowing behaviour that renders them crucial in the role of bioturbation of the sediments they inhabit (see ‘Ecosystem Services’ above). Footage was taken of a female T. australiensis specimen re-burrowing, upon being released after specimen collection for this study at Dunwich, North Stradbroke Island (see video below).


Video 1: Female T. australiensis burrowing following release after collection. Video: Lucas Sumpter, 2016.