Righting is a behaviour commonly performed by the Phylum Echinodermata, where an inverted organism will turn itself over to regain its normal position. Previous studies have shown that an increase in stress, caused by environmental factors, have attributed to slower righting times in Echinodermata. However, the effect of a predator on righting behaviour is unknown. Although a predator could be stressful to the organism, slowing the righting time, the organism may want to escape or defend itself, and would therefore turn over faster. In this study, two species of sea urchins (Echinoidea) were used to determine whether different morphologies or the presence or absence of a predator affected righting times. Body measurements of both species were recorded and organisms righting behaviour were timed both in light (predator absence) and dark (predator presence) conditions. The results showed a significant difference in righting times between species, in both light and dark conditions, showing that differences in morphology contributed to different righting times. The results showed that the species that had longer spines and tube-feet righted faster than the other. Both species also demonstrated significant differences in righting times between light and dark conditions, with organisms righting slower in darkness. Therefore, assuming that the darkness was perceived as a predator, it can be shown that predators are stressful to an organism and causes slower righting times. If the darkness was not perceived as a predator, the darkness would likely be seen as a non-stressful environment due to their preferred habitat consisting in dark environments. Therefore, the sea urchin may have felt more comfortable and did not see the necessity to right quickly. Due to consistency with previous studies, the assumption that darkness is a stressful environment is believed to be correct, however, future studies should still aim to separate these two conclusions. This could be completed by testing other predator stimuli, such as chemosensory cues, or measuring righting time in the preferred habitat of a sea urchin. Future research should also include more organisms and species as this was a major limitation in the study. 

Righting is a common behaviour performed by the Phylum Echinodermata, where an inverted organism will turn itself over to regain its natural position (Lawrence & Cowell 1996). One class of the Echinodermata is the sea urchins (Echinoidea). Like most Echinodermata, sea urchins have a mouth on their oral pole, which is against the bottom of the substrate and an anus at the top of the organism at the aboral pole (Ruppert, Fox & Barnes 2004). Therefore, when sea urchins are placed on their aboral side, they right themselves to move the oral side to the bottom; their normal position. Sea urchins use their tube-feet and spines for locomotion and therefore these structures are also used in the righting procedure. The tube-feet function with the help of the water vascular system, where they move forward, stick to the substrate and then move backwards (Ruppert, Fox & Barnes 2004). The spines allow the organism to push and raise the oral surface off the substrate, due to a ball and socket joint at the base and muscle fibres in-between (Ruppert, Fox & Barnes 2004).

Previous studies have used righting behaviour as an indicator of wellbeing and stress due to changes in the environment (Böttger, McClintock & Klinger 2001). This is because righting behaviour requires a high amount of neuromuscular coordination that would reflect its general wellbeing and the functioning of the neuromuscular system involved with locomotion (Böttger, McClintock & Klinger 2001; Himmelman et al. 1984; Watts & Lawrence 1990). Studies have shown that an increase in stress, related to salinity and temperature, slowed the righting time of Echinoderms (Himmelman et al. 1984; Kleitman 1941; Stickle, Liu & Foltz 1990; Watts & Lawrence 1990). Furthermore, one study by Lawrence & Cowell (1996) showed that longer periods of emersion also resulted in slower righting times in sea stars (Asteroidea). Another study by Böttger, McClintock & Klinger (2001) found that exposure to both inorganic and organic phosphate also increased righting times in sea urchins. Overall, previous research has shown that increasing the organism’s stress slows its righting behaviour.  

An interesting paradox to this theory is the idea of predation. Predation on sea urchins in coral reefs are mainly by fish, however, invertebrates including other Echinoderms such as sea stars are also known to predate on sea urchins (McClanahan 1995; Young & Belwood 2012). It can be assumed that the presence of a predator would cause stress to an organism, and therefore would slow righting behaviour, consistent with results of previous literature. However, it could also be theorised that an organism should want to escape or defend itself against a predator, therefore suggesting that the righting behaviour in Echinoidea should actually be faster while a predator is present. One defence against predation from sea urchins are their spines, which can be poisonous. Another defence that sea urchins use are called pedicellariae. Pedicellariae protrude from the sea urchin’s test and are long movable stalks with usually three jaws on the top (Ruppert, Fox & Barnes 2004). Some types of these are poisonous and induce paralysis in small predators and discourage large organisms, however non-poisonous pedicellariae are also used in defence (Ruppert, Fox & Barnes 2004).

This study aimed to determine whether different morphologies of two species of sea urchins had an effect on righting time. Additionally, this study also aimed to determine if the presence of a predator had an effect on the righting times in either of the two species of sea urchins.  

Table 1. Body measurements (cm) of Species A and Echinometra mathaei (n=1).

Table 1 illustrates that both organisms have approximately the same test diameter, however differ in spine and tube-feet length, thus impacting on their overall body diameter. E. mathaei has longer spines and tube-feet, resulting in a larger body diameter, compared to Species A. The spine length:body diameter is slightly larger in E. mathaei, however the tube-feet length:body diameter is much higher in E. mathaei than Species A.

Figure 2 and 3 both indicate that E. mathaei had a higher mean activity coefficient, meaning faster righting times, for both light and dark conditions respectively, compared to Species A. The unpaired t-test for the light condition showed a significant result (t(34) = 2.9089, p = 0.0064) as well as for the dark condition (t(34) = 3.2051, p = 0.0029). 
Figure 4 shows that in Species A, the light condition had a higher mean activity coefficient (faster righting times) than the dark condition. The paired t-test shows that this difference was statistically significant (t(17) = 2.5833, p = 0.0193). Figure 5 shows that the light condition produced higher mean activity coefficients (faster righting times) than dark conditions in E. mathaei. The paired t-test showed a statistically significant result (t(17) = 2.4679, p =0.0245). 

Examples of the righting behaviour are shown in Video 1, for Species A and Video 2, for Echinometra mathaei. 

Due to the significant difference between species, in both light and dark conditions, it can be assumed that the morphology of a sea urchin does play a role in righting behaviour. Although both species have approximately the same test diameter, E. mathaei had longer tube-feet and spines. It can be inferred from this information that the significant difference in righting behaviours between the species is attributable to the difference in tube-feet and spine length, or more importantly, the ratio of tube-feet and spine length to body diameter. This is consistent with the knowledge that both tube-feet and spines are important in the righting behaviour (Ruppert, Fox & Barnes 2004). These results indicate that longer tube-feet and spines compared to body diameter are beneficial for righting behaviour, thus resulting in faster righting times. However, due to the limited sample size, no causal relationship can be inferred. Further studies should include a larger sample size and different morphologies of sea urchins to determine if there is a causal relationship between these measurements and righting behaviour.

The results from the predation experiment show that righting time was significantly lower in a dark environment. Previous studies have shown that stressful environments are attributable to slower righting times (Himmelman et al. 1984; Kleitman 1941; Stickle, Liu & Foltz 1990; Watts & Lawrence 1990). Therefore, it can be inferred that the dark environment was stressful for the sea urchins, resulting in the slower righting times. It can only be assumed that the sea urchins actually perceived the dark environment as the presence of a predator and consequently that predators are stressful for the sea urchins. A possibility for why sea urchins actually exhibit slower righting times in the presence of a predator, rather than faster times, may be because there is actually no advantage in faster righting times. Future research could test this theory by determining whether sea urchins positioned normally or in an inverted position, exhibit different susceptibility to predation, possibly by analysing survival rates under both conditions.

A possibility for this study that needs to be considered is if the sea urchins did not actually perceive the darkness as a predator. Sea urchins prefer to live under rocks, as well as within crevices and are phototactic, where they move away from light into darkness (Ruppert, Fox & Barnes 2004). Therefore, should the darkness not be perceived as a predator, this condition would not be stressful for a sea urchin. The darkness would instead be perceived as a safe habitat and the slower righting time could perhaps be explained by the organisms feeling more comfortable and not seeing any danger in taking a longer time to right themselves. This is inconsistent with previous literature, which is why the former is more likely to be correct. However, further research should endeavour to distinguish between these two possible perceptions. Past literature has used chemosensory cues as predators in sea urchins studies (Hagen, Anderson & Stabell 2002; Parker & Shulman 1986). Therefore, future research could use these cues in relation to righting behaviour, to determine whether these stimuli also lower righting times. Consequently, should this result in lower righting times, it would provide further evidence concluding that the presence of a predator is indeed stressful to sea urchins. Possible research into an organism’s righting behaviour within their preferred habitats, such as rocks and crevices would determine whether righting behaviours are also slower in these safe habitats.

A major limitation in this study was the number of sea urchins used. Due to a limitation in available organisms, only one organism from each species could be used. Therefore no major conclusions or causality could be drawn, as this one organism may had been an outlier in the normal behaviour displayed by that certain species. Further research requires the use of more organisms and increased variability amongst species in order to determine a causal relationship between righting time and morphology, and also the presence or absence of predators.

Overall, despite its limitations, this study furthers the scientific understanding of righting behaviour in Echinoderms, especially Echinoidea. The results indicate a significant result in righting behaviour in different species, assumed to be due to the different tube-feet and spine lengths. Furthermore, light and dark conditions were shown to produce significantly different righting times in the organisms. However, future research, as explained throughout, should be completed to further understand both relationships. 

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