Local Distribution
The reef habitat is comprised of multiple microhabitats, each featuring unique and diverse communities. The cryptic invertebrate community can be particularly diverse, with a large proportion of species which we know very little about. One such group of organisms on which little information is known is Scintilla, a genus of clams within the Galeommatidae family. This group of bivalves is extremely diverse both morphologically and ecologically, with one species recorded as living commensally with a thallasinidean crustacean (Lutzen & Nielsen 2005). On a recent trip to Heron Island in Queensland Australia, a species of Scintilla was identified on the reef crest with the unusual feature of the mantle reflecting completely over the shell. A study was performed regarding the distribution of Scintilla spp. over the reef crest in order to gain some baseline data regarding its ecology and habitat preference.
A study was conducted on Heron Island in order to identify the distribution of Scintilla spp. and its abundance across different reef zones.
Methods:
Three organism searches were conducted on the reef flat over three consecutive days during the low tide, which occurred at 1650, 1747 and 0539 respectively. The reef flat was divided into four zones based on distinct changes in habitat;
1. Shallow intertidal:0-5m
2. Medium density coral rubble: ~5-150m
3. Sparse density coral rubble: ~150-300m
4. High density coral rubble ~300-500
A transect was run out from the Southern beach on Heron Island. Four 2x2m quadrats were randomly placed within each zone on either side of the transect line and ten minute searches were conducted in each quadrat. Searches involved flipping over boulders that lay within the quadrat and searching for Scintilla spp. before replacing the boulder as it was found. The total number of boulders flipped and the number of organisms found were recorded. The length (cm) of each boulder on which Scintilla spp. was found was recorded.
This was repeated at two more locations along the shoreline as demonstrated in Image 1. The second and third transects consisted of two 2x2m quadrat searches in each zone following the above procedure. The number of searches was restricted to 2 per zone at each location due to the late low-tide and loss of natural light and due to time constraints. A total of 8 searches were conducted in each zone on the reef flat.
Results:
A total of 45 Scintilla spp. individuals were found in the three transects; 38 in Zone 1 (0-5m), 4 in Zone 2 (5-150m), 1 in Zone 3(150-300m) and 2 in Zone 4 (300-500m). Analysis revealed that there were significantly more individuals in Zone 1 than each of the other zones (p<0.001). The mean size of coral rubble on which they were located was 53cm.
Discussion:
The results of this study indicate a definite preference of Scintilla spp. for shallow-water intertidal habitats. Given the difficult conditions that organisms are faced with in the intertidal region this was an unexpected result. The extremely variable temperatures and high levels of desiccation make this a difficult environment for organisms to persist in (Harley 2008). Conditions experienced in the shallow intertidal, such as increased temperature fluctuation, increased risk of desiccation and increased oxygen content within the water have been shown to reduce respiration rates and energy status in molluscan bivalves (Melatunan et al. 2012). The hypothesis that Scintilla spp. would display an even distribution across the reef crest can be rejected. Valuable information regarding the ecology of this species was acquired through this study. The average size of the coral rubble on which they were found indicates that they may have a preference for larger boulders. Furthermore, the majority of boulders on which they were found supported diverse and complex communities. This information indicates that Scintilla spp. may be a late coloniser, relying on environmental cues from the coral rubble community in order to settle.
Isolated Scintilla spp. individuals were seldom found within the first reef zone, with most specimens identified in groups of 2-5 on the single coral boulder. Those organisms found in zones 2, 3 and 4 were solitary and all were at the higher end of the size range. Large individuals of similar size were also found in the shallow intertidal indicating that preference for that habitat is most likely not related to size. As such the reason behind the preference for shallow intertidal habitats remains unclear and further research is required regarding this area in the future.
 
Image 1: Transects laid out on Heron Island Research Station. Image 2: Natural habitat of Scintilla spp.
Image obtained from Google Maps
Figure 3: Intertidal zone and reef flat of Southern side of Heron Island
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