Microhabitats

Where does Stichopus chloronotus like to live?

          Within the Great Barrier Reef, Stichopus chloronotus and Holothuria atra are the dominant holothurians (Harriott 1980). Stichopus chloronotus is generally found throughout all reef zones, although it may be discouraged from using certain areas due to environmental conditions, such as increased wave action (Kerr et al. 1993). It has been observed that S. chloronotus abundance varies within reefs (Baker 1929; Franklin 1980), but the reason for this variance is not known. Research conducted on the Great Barrier Reef found that S. chloronotus utilizes several microhabitats (Kerr et al. 1993), and some have found that they are most abundant at near shore locations (Baker 1929). Baker (1929) thought this might be due to the lack of live coral in the area, but since then others have suggested it is more likely due to increased asexual reproduction in near shore areas (Uthicke 2001a). Asexual reproduction rates may be increased in areas where there is more food available, and near shore areas often have higher nutrient inputs than off shore areas (Uthicke 2001a). It has also been hypothesised that mid shelf reef areas support fewer holothurians, because these areas can experience relatively large fluctuations in temperature and salinity (Uthicke 2001a). S. chloronotus has also been found to exhibit patch selectivity correlating with organic content in the sediment, but it has not been confirmed that organic content in the sediment influences abundance of S. chloronotus (Uthicke and Karez 1999; Uthicke 2001a). 

          During a recent (September 2011) marine invertebrates course at the University of Queensland, a short study was conducted to attempt to eliminate or confirm some of the proposed hypotheses relating to S. chloronotus abundance on coral reefs. Transects were conducted on the coral reef surrounding Heron Island, located on the southern Great Barrier Reef. The number of S. chloronotus individuals that were observed was recorded, and sediment samples were taken to determine carbon content as a measurement of organic content in the sediment. The second part of the study involved observing how the distance between sea cucumbers changed over time to see if the holothurians may be attracted to or repelled away from each other.

         Several transects were conducted from the water line at low tide to varying distances toward the reef crest. Due to the limited time available and poor weather conditions on days spent in the field, only four transects were completed. The lengths of these transects were 200 m, 340 m, 300 m, and 200 m. Each transect was broken up into 20 m sections and the number of Stichopus chloronotus individuals found in each section within 0.5 m of either side of the transect line was recorded. Every 20 m a sediment sample was taken to determine if any changes in organic content were occurring. The sediment samples were taken back to the lab and processed. Excess water was dumped from the containers holding the sediment, and the samples were placed into an oven at 60˚C to dry for three hours. Crucibles to be used in the oven at higher temperatures were weighed, and their weights were recorded. The sediment samples were removed from the oven, placed into the crucibles, and weighed. These weights were recorded, and the samples were placed into an oven at 125˚C for approximately six hours. After this time the samples were removed and weighed again. By subtracting the weight of the empty crucibles from that of the samples within the crucibles, the sediment weights were calculated. The final weight of the sediment was then subtracted from the initial weight of the sediment and carbon content was calculated as a percentage of the dry weight.

         The second part of this study involved attempting to determine if Stichopus chloronotus individuals attract or repel each other and whether this might have an effect on how they were distributed on the reef. To test this hypothesis a simple experiment was conducted in which four sea cucumbers were placed into a tank with a distance of 50 cm between each individual. The distance between each sea cucumber was measured once an hour between 6 p.m. and 12 a.m. and again between 7 a.m. and 11 a.m. At night the distance seemed to be reaching a plateau, so the distance was not recorded between 12 a.m. and 7 a.m. The tank in which the sea cucumbers were held also contained coral rubble and some other small invertebrates such as mantis shrimp, polychaetes, and ascidians. The average distance between the sea cucumbers at each hour was compared to determine if the distance increased or decreased.

        For both parts of this study, the number of samples obtained was too small to conduct statistical analyses that would be significant. For this reason it was only possible to observe general trends from the results and make assumptions based on those.

Figure 2. The carbon content as a percentage of total sediment weight is shown with the number of S. chloronotus specimens found at the sample site.

Figure 3. The average distance between four sea cucumbers (S. chloronotus) over a set period of time. The distance was not measured between the hours of 12 a.m. and 7 a.m.

         From the results shown in Figures 1 and 2 there does not seem to be any relationship between the number of sea cucumbers found and distance or carbon content. The carbon seems to vary little over most of the distance of the transects. Unfortunately no statistical analysis can be accurately conducted to see if this is true, because not enough samples were collected. Figure 3 shows that the distance between sea cucumbers increased greatly in the first four hours of the experiment, but it then decreased slightly and plateaued. It was also observed that as the experiment went on and it got later into the night, the sea cucumbers seemed to hide under objects in the tank. For example, one hid under a large bit of coral rubble, while another hid under a pipe. After reaching these positions, the sea cucumbers moved very little through to the end of the experiment.

         Based on this study, it seems that organic content does not increase the likelihood of finding a Stichopus chloronotus individual in a given location. Past experiments, however, have found that S. chloronotus exhibits patch selectivity correlating with organic content in the sediment (Uthicke and Karez 1999). Due to the low number of samples and transects that were able to be completed in the present study and the past evidence of patch selectivity, it is hard to conclude that there is no correlation between the number of sea cucumbers found and organic content. More samples need to be collected and more transects completed to give a complete picture of what the relationship is between organic content and S. chloronotus distribution. A second experiment was conducted to determine if the presence or absence of other individuals might increase or decrease the likelihood of finding S. chloronotus in a given place. Although the sea cucumbers appeared to move away from each other quickly, and they did not get close to each other again, this is most likely due to their lack of activity at night rather than some sort of avoidance behaviour. It was observed during the experiment that after moving away from each other, the sea cucumbers hid under large objects in the holding tank. Uthicke (1994) observed that S. chloronotus hid under rocks and coral at night and that they only fed during the day. These observations match the behaviour of the sea cucumbers used in the present experiment, so it is most likely that they were not avoiding each other, but exhibiting behaviour that is always present at night. To confirm this, the experiment would need to be repeated over a longer period of time, so multiple distances could be recorded for each hour of the day. Although many have observed that S. chloronotus abundance varies over the area of a reef, it is still not known why this occurs.  It is possible that organic content in the sediment influences where S. chloronotus gathers, but this still needs to be tested and confirmed.