The hairy mussel is a common in marine habitats on the east coast of Australia. The animal lives within sessile communities in clumps attached to rocks, reefs, or other artificial structures (Boyd, 2011; Laseron, 1956; Robinson & Gibbs, 1982) in inter- and sub-tidal areas (NSW Department of Primary Industries, n.d.). Trichomya hirsute prefers entirely marine habitats, but they can persist in low salinity environments (Jenkins, 1976). The mussel can live to a depth of 15 m. 

Within the colonial clumps of Trichomya hirsute lives a myriad of other marine invertebrate species. To really analyse what type of community these clumps create and what types of animals can be found within this micro-community, three separate mussel clumps (as seen in figure 5) of varying sizes were broken up and examined under the microscope. The following subheadings refer to these findings in order of abundance from most abundant to least abundant.

Limitations of this experiment were that the animals that were catalogued tended to be easier to spot and easier to catch. It’s recommended that further, more thorough research be conducted in this area really catalogue the amazing and great diversity seen in the micro-communities that T. hirsute creates. 

Generally, similar species were found within the three clumps. The most common find were sedentary polychaetes, with great variety in sizes. Most of the time, the animal was not seen, but the calcareous tube was present. Several of these were found on each individual mussel. Examples of these can be seen in the images below.

The next most group of animals was probably the bivalves, with several different types present on all clumps. Some were beginning to create a clump of their own, like the oysters; others were attached to the hairy mussel’s shell, and were living with the spaces between the mussels, like clams. Examples can be seen in the images below.

Also extremely abundant in the clumps were gastropods, including several species of whelks, periwinkles, and nudibranchs. All of these animals were found on the outer shell of the hairy mussel, caught in the ‘hair’, or found inside and empty shell. Examples of these are shown in the images below. 

Errant polychaetes were also a very common find, with wide variety of different species being collected within each clump. Among the three clumps, there were many wildcards, but a couple of the same species tended to pop up more than the others. These more abundance species are indicated on the images below with an *.

Another common find were Sipunculans, with several being found within each clump, with one clump containing 21 individuals. These animals tended to live in the tight space between two mussels, often near the attachment site. Below is a video of two individuals collected from one of the clumps. 

Several small crabs were also found within each clump, often hiding under and between mussels.

Also common were several types of other tiny crustaceans. It’s likely that these animals were much more common than recorded, due to the fact that they are really hard to see and catch as they are quite small and move very quickly. Also, the mussels were transported between buckets of sea water, meaning there were probably many crustaceans swimming around in the original bucket of seawater that were not examined. Examples of those animals that were found can be seen in images below. 

Occasionally, solitary ascidians were present on the outside of mussel shells or on the inside of vacated shells. Several different types were found and can be seen in the images below. 

Growing on the outside on a few mussels was a spongy mass, usually yellow in colour, was found. While exactly what it was could not be determined, it was assumed that it was some type of sponge. An image of this organism can be seen below.

There were also some animals found that were only seen once or very rarely. This may be because they don’t usually occupy this niche, or they were difficult to find or see. One such animal was a single tiny brittle star. These echinoderms are known predators of the hairy mussel, so it’s likely that there were more that were difficult to find. Also, a single small sea cucumber was found, but a photo was not taken. Also occasionally living on the shell were small ectoprocts, it was also likely that these are more common than recorded, because these were quite small and sometimes difficult to differentiate from hairs on the mussel, at least at a quick glance.

These species can be seen in images below. 

Consistent with the majority of bivalves, Trichomya hirsute is a sexually reproducing gonochoristic bivalve with fertilisation occurring via broadcast spawning (Goggin, 1994; Hickman et al., 2011). From the limited research conducted, it seems that the hairy mussel, at least in Lake Macquarie, spawns year round, with three peaks in reproductive activity in early winter, early summer, and early autumn (Goggin, 1994). In other Australian mollusc species, it seems that reproduction in temperature dependant, with molluscs usually spawning in cooler months, with the southern ranges on the species typically having longer periods due to the reduced temperature (Wilson & Hodgkin, 1967). From this, it is hypothesised that it’s possible that T. hirsute populations in northern Australia, around Townsville, may have more seasonal spawning than their southern counterparts, but more research is required on this subject before we can be sure (Goggin, 1994). 

Not much is known about the life cycle of Trichomya hirsute, so the following information will discuss the life cycle of animals in the family Mytilidae. It’s important to keep in mind that this may or may illustrate T. hirsute life cycle, it’s simply the most educated assumption we can make with the given information. Clearly, more research needs to be conducted in this area on T. hirsute, specifically.

Like in the majority of molluscs, once the egg is fertilised, the embryo will undergo spiral cleavage eventually resulting in the emergence of a trophophore larvae. This stage is followed by the uniquely molluscian stage of veliger, where the foot, shell, and mantel begin to develop (Hickman, 2011). In the family Mitilidae, once the larvae grows to between 208 and 350 μm, the animal will settle on the preferred substrate (Semenikhina et al., 2008) and develop into the adult form. A figure illustrating the basic bivalve life cycle can be seen in figure 43. 

While the predators of Trichomya hirsute have not been studied or recorded, it’s likely that their predators are similar to those of other bivalve molluscs in the area. The most important of which include predatory gastropods, starfish, crabs, and shore birds (Bedman, et al., 2003). To combat this, mussels in the Mytilidae family have developed several defences. One of the most obvious is the hard, outer, bivalve shell that protects the animal; however, most predators have developed ways to either open or crack the shell. This has led to the development of harder and thicker shells, especially in areas with an elevated level of predation. Other examples of defence mechanisms include stronger adductor muscles to make it more difficult to open the shell, the use of byssus to create stronger attachment to the substrate (Bedman, et al., 2003).

See physical description for a description of the shell and external anatomy of Trichomya hirsute. The shell is held together via the hinge ligament, and held closed through the aid of the abductor muscles (figure 44) (Harris, 1990), and within the shell the mantel and visceral mass of the animal is held (Hickman et. al, 2011). Thinly lining the internal walls of the shell is the mantel, which contains blood vessels and nerves, and has the job of secreting the shell and the hinge ligament. The mantel is ciliated and is able to carry waste covered in mucus out of the shell (Harris, 1990). Within the mantel cavity the gills are found, along with labial palps which aid in feeding, and the modified foot with byssus fibres, which aid in attachment. The main body of the animal contains the gut, glands, heart, kidneys, and likely the gonads (Harris, 1990). Figure 44 shows a cross section of Mytilus edulis planulatus, a species from the same order as T. hirsute (Harris, 1990).

In Trichomya hirsute, and the majority of molluscs, gas exchange occurs through the mantel, but primarily via the gills. Gas exchange via the gills occurs when specialised cilia on either side of the gill filaments create a current, causing water carrying oxygen and nutrients to pump past the filaments and to diffuse across the cells. The filtered water is then pumped out the cavity via the exhalent siphon. Therefore, gills perform two important jobs in filter feeding animals, such as T. hirsute – gas exchange, and feeding (Harris, 1990).

Feeding occurs when cilia around the gills collects food and moves it to the mouth. There is much debate about how this happens specifically, with the view that they are mucociliary feeders being held by many, but this creates problems when thinking about how particles are sorted according to size (Harris, 1990). More research needs to be conducted in this area to understand how mussels feed.

Mussels, such as T. hirsute, are able to control the speed of the water through the mantel, the rate of extraction, and the rate of ingestion of food via the size of the shell opening, rate of cilia beating, and muscular movements of the gill filaments and the velum (Harris, 1990). 

Like the majority of molluscs, Trichomya hirsute, has an open circulatory system (Jones, 1983) with a three chambered heart consisting of a ventricle and two atria that beats very slowly (Hickman et. al, 2011). Some of the blood is oxygenated in the mantel, returning to the heart; the other part circulating though the sinuses to the kidneys, onto the gills to become oxygenated and then back to the heart (Hickman et. al, 2011). 

As is the case for all animals, mussels create waste products, mainly in the form of varying concentrations of ammonia and amino acids that they must excrete. Excretory organs in mussels, including Trichomya hirsute, are the kidneys and pericardial glands (Harris, 1990)

While not much is known about nervous systems in mussels, generally bivalves have a fairly limited nervous system. Specifically, they have three pairs of ganglia connected via a system of nerves and commissures. Along with this, they have a few underdeveloped sense organs including statocysts in the foot, a couple of osphradia (olfactory) in the mantel, and tactile cells (Hickman et. al, 2011). More research is needed to determine what sense organs Trichomya hirsute may possess. 

It’s extremely difficult to determine where a phylum, let alone a species, sits in the phylogenetic tree of the animal kingdom, due to convergent and lost traits, and more recently the introduction of molecular data. For this reason, the discussion of the evolution of this animal will occur at the class level.

Molluscs first arose in the early Cambrian, with bivalves evolving not long after (shown in figure 46) (Miller & Sepkoski, 1988). The most recent ancestor of bivalves is believed to be a now extinct class of molluscs called monoplacophora that possessed a single laterally compressed shell (Hutchins et al., 2007). From here, they then radiated steadily from there, thriving despite several mass extinction events, and exploding during the Mesozoic period (Miller & Sepkoski, 1988). This information can be inferred fur to the extremely comprehensive fossil record present for bivalves (Hutchins et al., 2007). 

·         Kingdom: Animalia

o   Phylum: Mollusca

§  Class: Bivalvia

·         Subclass: Pteriomorphia

o   Order: Mytiloida

§  Family: Mytilidae

·         Genus: Trichomya

o   Species: T. hirsute

·         Common name: Hairy Mussel

·         Synonyms

o   Mytilus hirsutus

o   Trichomya hirsutus

The hairy mussel is common throughout the east coast of Australia, from Townsville to Tuross, through South Australia and the Great Australian Bight (Iredale, 1939; Cotton 1961; Middelfart et al., 2010), and northern Tasmania (Middelfart et al., 2010). Sightings for the bivalve have been recorded in map shown in figure 48. An interactive map can be found here.

Because there hasn’t been much research into the population dynamics of Trichomya hirsute, it’s hard to determine what the future of the populations will be. One study on the population dynamics of marine organisms on the Curtis coast, however, suggests that the hairy mussel is increasing in abundance (Ulm, 2006). More research needs to be conducted in this area to determine the current and potential future abundance of this animal. 

The hairy mussel has been exploited by humans for centuries, going back all the way to pre-western aboriginal societies, as they are a one of the dominant species in the Morton Bay area (Ulm, 2000). Studies suggest that traditional Aboriginal harvesting practices are conducted in a way that will be sustainable for the species (Catterall & Poiner, 1987). 

Our oceans and other bodies of water are becoming more and more polluted with heavy metals, and this is having varying effects on a wide range of marine and freshwater organisms. In general, bivalves are known to be able to endure a wide array of different salinities, temperatures, suspended sediments, and dissolved oxygen levels (Anderson, 2001). Due to the fact that Trichomya hirsute is a filter feeding animal, it is directly exposed to the levels of heavy metals in its environment as they are taken into the animal directly via the gills and bioaccumulate them to levels above what is available in the water (Lopez et al., 2014). While the effects of these heavy metals to the biology of this species has not been quantified, in freshwater mussels, exposure to heavy metals has been known to alter growth, filtration efficiency, enzyme activity, behaviour, and can cause death to the animal (Naimo, 1995). However, it has been found that some bivalves are able to detoxify and store metals for some time without dying (Lopez et al., 2014).

There has been much research surrounding the use of mussels, and T. hirsute specifically, as bio-indicators in the presence and increase in the levels of heavy metals in marine environments where they are present (Lopez et al., 2014; Klumpp & Burdon-Jones, 1982). This is due to the fact that, as mentioned previously, filter feeding mussels take in and are able to store heavy metals from the environment. This can give an indication about the type of pollutants and long the environment has been polluted for. It’s been found that T. hirsute is the most effective bioindicator for heavy metal, excluding zinc, and could be used to effectively monitor pollution (Lopez et al., 2014; Klumpp & Burdon-Jones, 1982).