Recolonisation

Despite the fact that C. biunguiculatus is known to live inside the shells of the Black-Lipped Pearl Oyster (Pinctada margaritifera), the specific nature of their relationship with the oysters is unknown. For example, whether or not the pair remain within the same shell for their entire lives is uncertain, as is the point in their lifecycle that colonisation occurs. This experiment aimed to test the capacity of C. biunguiculatus to recolonise oysters upon being removed from their original host. It was hypothesised that, based on the fact that each shell contains a male-female pair and the relative sparsity of Black-Lipped Pearl Oysters, C. biunguiculatus were specific to one host oyster and would therefore be unable to recolonise any oyster other than their own.
 

Methods:

Oysters were gathered from the Southern reef crest of the coast of Heron Island. A total of 6 oysters were used in this experiment. One mussel believed to be Atrina vexillum was also gathered as a comparison. Each oyster was placed in a tray empty of water and allowed to tire. That is, upon leaving the water the oysters were forced to hold their shells closed permanently, this caused their adductor muscles to slowly tire. After approximately 24 hours the oysters were unable to keep their shells closed. At this point, any C. biunguiculatus living within the oysters were removed, with special care taken to not damage the oysters (as they were to be used later in the experimentation). Of the 6 oysters gathered only 3 contained adult male-female pairs, with 1 containing a single juvenile. The mussel was also opened in a similar manner and the organisms contained within (of a different species to C. biunguiculatus) were removed. The gathered pairs were then separated and left for 24 hours in sea water. Meanwhile, the oysters from which they were taken were allowed to recover by placing them in tubs of seawater. One pair was originally placed in a tub containing the mussel,however this part of the experiment was concluded after 24 hours as the pair did not colonise the mussel at all during this time.  The remaining 2 pairs were then released into tubs containing oysters other than the ones from which they originally came. The juvenile was placed in a tub containing the empty mussel. The behaviour of the individuals within each trial was then monitored.

Image 4: A tub containing an adult C. biunguiculatus pairand a foreign oyster (left) and a tub containing the foreign species of mussel(thought to be Atrina vexillum) and an adult pair of C. biunguiculatus (right). Both pairs introduced to new host oysters recolonised, however the pair introduced to the mussel could not be induced to colonise.

Results:

As has already been stated, the adult pair placed in the tub with the mussel had no interest in colonising the foreign species. Upon releasing the two other pairs into the tubs containing the foreign oysters, however, both females immediately recolonised the oysters. Indeed, these female recolonisations both occurred in under 5 seconds. Both males also immediately swam for the oysters but, instead of entering the shells straight away, perched themselves on the lips of the shells. The males remained like this for a matter of minutes before eventually entering the shells as well. The juvenile, which was released into a tub containing the mussel (a species which is occupied by an entirely different species of crustacean)displayed similar behaviour to that of the adult male C. biunguiculatus in that it perched itself on the lip of the mussel. The juvenile did, however,eventually enter the mussel. After their original recolonisation none of the animals were observed outside of their new hosts again.

Discussion:

The fact that both pairs of adults entered the foreign oysters without delay firstly demonstrates that male-female pairs are not specific to an individual oyster. This is evidence against the original hypothesis that male-female pairs of C. biunguiculatus were specific to individual oysters. Of course, it must be said that the conditions in which this experiment were conducted are not in any way representative of the species'natural environment. That is, the pairs were only given one oyster to colonise. It may be that, given a choice between their original host and a new oyster a male-female pair would have chosen their original host. Furthermore, given that C. biunguiculatus rely on the oysters for protection it is unsurprising that the pairs, especially the females, wasted no time in reaching safety once it was made available.

Far more unusual, however, is the observation that the juvenile C. biunguiculatus colonised a species other than the Black-Lipped Pearl Oyster. If it weren't for the fact that the adult C. biunguiculatus showed no interest in colonising this mussel it would be reasonable to conclude that the juvenile colonised the foreign species for similar reasons as those speculated above. As it stands, however, this is not the case. Why the juvenile colonised a foreign species is, therefore, unknown.On a purely speculative basis, it may be that the specificity of C. biunguiculatus to the Black-Lipped Pearl Oyster is not innate but instead develops throughout the juveniles' growth. Once again, however, with a lack of sufficient experimentation such statements must remain firmly in the realm of speculation.