The polychaete, Branchiomma sp. (Polychaeta: Sabellidae) exhibits an anti-predator behaviour by retracting into its calcareous tube when startled by mechanical stimuli that is assumed to be the presence of predators. This anti-predator, behaviour however, restricts these polychaetes from feeding. Thus, these polychaetes will have to assess its risk of predation with its environment to recover the cost of lost-opportunity of feeding. In this study, we presented data from our experiment to investigate how the body size of Branchiomma sp. would affect the hiding time spent in tubes when exposed to predation risks. With body size affecting their filter feeding activity, it relates to their feeding rate, thus we would hypothesised that with a larger body size would lead to a longer period of hiding. However, with 37 Branchiomma sp. collected for the experiment, we found no significant effect of body size on the anti-predator behaviour of Branchiomma sp. Thus, this indicated that irrespective of body sizes, these polychaetes all exhibited anti-predator behaviour by retracting into their tubes when exposed to the presence of predator.

Keywords
Sabellidae, Tube worms, Mechanical stimuli, Filter feeding, Crown filament

An animal will exhibit its anti-predator behaviour by evading a predator’s attack through various actions such as seeking refuge, diverting behaviour or aggressive retaliation (Edmunds, 1974). Hiding in refuge by many marine invertebrates is often an extremely common behaviour that involves hiding of prey into structures to avoid predators (Dill and Gillett, 1991). However, along with all these anti-predator behaviour, there is a major cost-benefit trade off present faced by these animals while minimising the risk of predation at an expense of feeding and reproduction opportunity (Lima and Dill, 1990). Lima and Dill (1990) suggested that animals could assess the associated risks that accompanied its behavioural decisions while utilising these predation risk as a selective force to come out with survival strategies.

An animal that hides in refuge often faces a major problem; Re-emergence. The longer an animal remained in its hiding, the risk of being predated on declined as predators often divert their attentions to other prey items that would complement their efforts (Sih, 1992). However, remained hidden to avoid predation do come with a cost to its fitness. Often, studies have shown that many refuge-dwelling animals were not able to forage when exposed to continued predation risk outside their refuge (Martin and Lopez, 1999, Dill and Gillett, 1991). Thus, it is important for animals to optimise and balance their anti-predatory behaviour with other requirements. In fact, many animals could adjust their behaviour based on the levels of predation risks (Lima and Dill, 1990). However, all these actions would also depend on the environment which the animal is living in. Studies done by Dill and Fraser (1997) has shown that the optimal hiding time will depend on the food abundance at the proximity of the animal tested as the hiding time will often reduce when there’s more opportunity to feed during the high abundance of food.

In corresponding to anti-predator behaviour, studies have shown that body size does play a significant role in affecting the duration of hiding behaviour. Some polychaetes, of which Sabellidae and Serpulidae fan worms are tubicolous, with individual branchial crown protruding out from the operculum of their tubes for feeding (Nicol, 1950). These polychaetes are suspension feeders which use filter-feeding mechanism via their ciliary crown-filaments (Riisgaard and Ivarsson, 1990). Riisgaard and Ivarsson shows that the filtration rate may be an indirect measure through the total length of the crown-filament. This shows that the size of the crown-filament would be directly linked to the foraging behaviour (filtering rate) of these polychaetes. Natural predators of these polychaetes include a wide variety of small fishes and crabs, often reported nibbling off these fan lobes of these polychaetes during grazing activities (Yoshiyama and Darling, 1982, Kuwamura, 1983). Thus, the adaptive anti-predator behaviour of these animals to retract in its calcareous tube would place them in a decision between cost-benefit behaviour.

In the present study, we address the following question on Branchiomma sp. (Polychaeta: Sabellidae): How would the different sizes of Branchiomma sp. differ in their anti-predator behaviour in response to mechanical stimuli simulating the presence of predator?

In this study, Branchiomma sp. were collected from Manly Harbour and used for our experiments (Capa et al., 2013). We observed a total number of 45 Branchiomma sp. individuals that were located on three sediment plates that were left to be colonised. Observations were conducted under a normal light condition with plates placed one meter apart to prevent any disturbances caused by shadow or movements of the tester. We also rotated the tests performed on individual polychaetes between plates to allow sufficient recovery time of other sabellidae worms in the previous plates. This also prevents any significant influence of tests done on previous individual polychaete on the response of the next polychaete in subsequent tests.

In every plate, each polychaete was randomly selected to be tested as an independent observation as studies have shown that the response of an individual polychaete is unaffected by its neighbour (Dill and Fraser, 1997). Only Branchiomma sp. worms with its tentacle crown filaments fully extended and body hidden in their calcareous tubes were chosen to be tested by the experiment. Every worm will then be exposed to a mechanical stimulus via a sharp object. This will cause the polychaete to withdraw into their calcareous tube in response to mechanical stimulus. We then measured the time taken for the polychaete to re-emerge within 300 seconds. Polychaete that remained hidden after 300 seconds will be rejected. To standardise the re-emergence of Branchiomma sp. worms, the fan of the polychaete will have to be fully extended before the measurements can be stopped.

After each observation, the individual Branchiomma sp. were extracted out with a metal tweezer. To measure the size of the Branchiomma sp., we measured the total length of the extended crown filaments. There was a total of 45 replicates with each replicate being treated as independent setup.

In our statistical analysis, the results of the time spent by Branchiomma sp. hiding and their size were plotted and tested with linear regression. ANOVA test was also conducted to analyse the relationship between the body size and the time spent hiding by Branchiomma sp. worms.

Overall, 45 individuals of Branchiomma sp. were tested. 8 data points were rejected as the polychaetes spent more than 300 seconds hiding in the tubes. The remaining 37 individual Branchiomma sp. have crown filament size that ranges from 6 mm to 25mm with a mean size of 14.6 mm. Within 300 seconds of time spent hiding in their calcareous tube, the shortest time spent hiding was 18 secs and the longest time spent hiding was 299 seconds. A linear regression was done between body size and the time spent hiding by these polychaetes. There’s no significant relationship found between the body size and the time spent hiding by these polychaetes (ANOVA, F_1,35= 0.5665, P = 0.4567). Only 1.6 % of the variation were explained by the linear regression with a R² of 0.01593 (Figure 1). The linear regression has a slope of 2.118 which explained that with a 1 mm increase in body size will lead to an increase of 2.12 seconds spent hiding. With increasing body size, there’s a trend of increasing hiding time but it was not significant.

In this study, we do not have a significant result to prove the significant effect of predator presence on the duration of hiding time by different body size of Branchiomma sp. However, we have shown that regardless of body sizes, all Branchiomma sp. exhibit the same pattern of change in their response to mechanical stimuli (Figure 1, ANOVA, F_1,35= 0.5665, P =0.4567). This would suggest that for all size classes, these polychaetes perceived the risk of feeding similarly by regulating their hiding time. It is utmost important for animals to maintain their sensitivity towards a stimulus of a predator’s approach to maintain their survivability. Studies have shown that these tubicolous polychaetes will respond to not only mechanical stimuli but also to changes in photic intensity which they deem as an approach of a predator (Nicol, 1950). While some species of sabellid worms do exhibit chemical defenses and structural defenses to deter predators, it is unknown if Branchiomma sp. do exhibit any chemical defenses that would affect the responses of these polychaetes in this study (Kicklighter and Hay, 2007).  However, mechanical stimuli would always have greater significance biologically (above levels of fatigue) with greater urgency in terms of survival, thus measuring Branchiomma sp.’s response to mechanical stimuli would be appropriate (Nicol, 1950).

Our result clearly shows that Branchiomma sp. can alter their hiding time spent in their calcareous tube in response to the presence of predator. They exhibited a whole range of time spent hiding when stimulated regardless of body sizes. This show that these polychaetes could track slight changes in their environment such as predation and food availability to adjust their hiding time (Dill and Fraser, 1997). These polychaetes have developed ocelli and eyes on the radioles on their crown filaments to help protect their appendages from grazing by predators through the assistance of some forms of motion detection with spatial correlation (Bok et al., 2016). However, hiding in their tubes restricted them from feeding intakes, respiration as well as reproductive outputs. Studies have shown that sabellid worms adjust its hiding time in response to food availability in its environment when placed under potential predation risks (Dill and Fraser, 1997).  Thus, the responses of different hiding time from this study differ for every individual polychaete as different individual perceived the risk of predation differently by altering their duration of hiding. The lost-opportunity cost of remained hidden should then depend on the environment the polychaetes were in at the time of hiding and the body size of Branchiomma sp. did not place a significant role in determining the hiding time in this study.

Despite this consistent pattern of anti-predator behaviour by Branchiomma sp., we observed a trend where smaller polychaetes spent lesser time hiding in their calcareous tubes compared to polychaetes with larger size (Figure 1). This would be caused by few factors but it is obvious that we might have too little sample size to represent the different size classes of Branchiomma sp. in this study. There were studies done that would support the trend of the results of this study. Studies done on optimal foraging theory suggested that predators often select the most profitable prey (John, 1977). In the absence of handling preys, larger bodied individuals were known to represent a better choice of prey and often predators would make an active choice for larger prey items (Manicom and Schwarzkopf, 2011, Holzman and Genin, 2005). Thus, in the context of this study, larger Branchiomma sp. individuals would spend longer time hiding from predators before re-emergence compared to smaller polychaetes as they would be more visible in size and have higher predation risk. However, there were studies that would support the contrary of this trend. A study done on barnacles showed that smaller individuals showed longer hiding time as smaller individuals often have relatively high beating frequency of cirral net in relative to its size for feeding to compensate the loss of feeding opportunities. It was also shown that smaller barnacles were benefitting from hiding as they were at greater risk of being predated on (Newell and Northcroft, 1965, Dill and Gillett, 1991).

To conclude, this study has shown that body size of Branchiomma sp. does not play a significant role as predicted in affecting the time spent hiding from predation. However, it has shown that these polychaetes can demonstrate different intensities of anti-predator behaviors in general based on its requirements and the environment it is found (Nicol, 1950).

Limitation & Future studies

​ The limitation of this study was the sample size of Branchiomma sp. that were accessible for tests. Although the individuals were randomly selected, the size range of < 10 mm and > 20 mm has a very limited number to represent the trend appropriately. Thus, a larger sample size would be recommended to detect whether there’s a significance effect of body size on the hiding time of Branchiomma sp. Besides, care has been taken in this experiment and any influences have been minimised towards another polychaete when a test is conducted. Studies have shown that adaptation would not occur to tactile stimuli above levels of muscular fatigue, it would be a good recommendation to conduct this experiment on individuals that would have no influence on each other at any possible ways (Nicol, 1950).

Further studies can be conducted on the effect of anti-predator behaviour on the feeding rate and reproductive outputs to have a better understanding of the consequences of expressing anti-predator behaviour by Branchiomma sp. Perhaps there would be other biological factors such as mating behaviour and food availability that would affect polychaetes’ decision on assessing their environment for re-emergence.

I would like to thank Prof. Bernard Degnan and A/Prof. Sandie Degnan for their help and guidance in experimental design, their enthusiasm in teaching have been great inspiration to me. I also would like to thank the tutors for their guidance and assistance with the aquaria and laboratory work with the specimens.

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