The most prominent association Alveopora, as with most corals, has is its mutualistic symbiosis with the dinoflagellate zooxanthellae. This unicellular algae lives in the cells of the coral’s gastrodermis and produces energy that is transferred into the coral tissue. The coral, in return provides habitat for the algae to thrive in [see Feeding] (Pearse 1971).  

In the nature of true cnidarians, Alveopora will obtain much of its nutrients by using their tentacles to capture phytoplankton, zooplankton, dissolved organic and inorganic matter, particulate organic matter and bacteria floating through the water column (Goreau 1971). Their long tentacles contain nematocysts to stun and kill their prey before transferring it to its mouth.They also secrete large amounts of mucus that capture food particles which are transferred to the mouth via nematocysts and currents driven by cilia (Ruppert 2004) [see video below].

Alveopora is a sessile coral, however the individual polyps are highly flexible and active, constantly moving around and feeding. These polyps can extend to impressive distances and are sometimes mildly aggressive towards other corals, if too close (Thamrin 2001).

The mechanisms by which Alveopora sp. are able to tolerate and expel sediment are vital to the organisms, as a majority are found in habitats that are consistently exposed to moderate to high levels of turbidity, commonly a result of high amounts of total suspended solids (Kemker 2014), reflecting the sunlight so crucial to the corals’ zooxanthellae. Therefore, the more particles (total suspended solids) in the water column, the less light the coral is likely to receive. This can impose an energetic cost onto the corals, are less light means less photosynthesis and thus nutrient production. Riegl et. atl. (1995) found that this decrease in photosynthesis led to a decrease in respiration and an increase in mucus production, significantly interfering with the coral’s energy balance.

The size of the suspended sediment particles is also important. Smaller particles are less likely to settle and thus may linger in the water column, preventing light penetration for longer, whereas larger particles are more likely to settle sooner (Kemker 2014). General observed mechanisms of sediment removal include retraction of the colony of polyps, mucus production, or actual ingestion of sediment (Rosenfeld 1999 & Stafford-Smith 1992).

This experiment aims to observe any differing responses of each colony to the different sizes in particles and see if there is a significant difference in time taken to expel the sediment between each size treatment. To observe these mechanisms and examine how they may deal with these fluctuating levels of suspended solids present, a simple experiment was conducted testing Alveopora’s response to different particle sizes. Over three weeks, a colony of Alveopora and a nearby colony of the similar genus Goniopora were subjected to three small plumes of sand/silt, each differing in particle size. The plume sand grain sizes measured approximately 2mm, 0.5mm or 0.1mm. Once exposed to these “plumes”, the time it took for the colonies to rid themselves of the majority of the sand was recorded.

In the first week, each colony of Alveopora and Goniopora - which were permanently located in a small aquarium - were exposed to sediment plumes with 2mm particles. Using small test tubes, 10 mL of the sediment was measured for each of the colonies, and then simultaneously poured onto the polyps. The behaviour and response of the polyps was then observed until most or all of the sediment had fallen off the colonies or been ingested and the time taken to do so was recorded. This was repeated for the following two weeks with the 0.5mm and 0.1mm particle plumes.

The results observed showed no significance between times [see figure 11]. When graphed, there appears to be a slight trend in the times taken to expel sediment, with the 2mm particles taking on average (between the two colonies) 47 minutes versus 44 minutes for 0.5mm and 32 minutes for 0.1mm. Ingestion of the sediment by either colony was not observed, but may explain this slight difference in times as smaller particles are easier to uptake. When treatments were applied, colonies were also observed for behavioural responses. Alveopora showed immediate responses by retracting some polyps back, whilst extended ones became slightly more aggressive and began waving around. This response, whether or not a mechanism to assist in sediment removal, initially accelerated the process. Most of the polyps remained retracted for the whole 30-40 minutes, even after the sediment had been expelled. Goniopora’s only initial response was increased movement of the polyps, no retraction was observed. Throughout the three treatments Goniopora removed the sediment at a faster rate than Alveopora, however the colony was larger and thus more polyps were present to assist in the removal.