Fan worms are some of the most important presences in a range of marine communities, mainly assisting in nutrient recycling of their surrounding areas. These animals possess a fairly evolved photo-sensory system which allows them to quickly retract their branchial crowns into their tubes with a change in the light conditions (i.e shadow). This study tested the effect that different qualities/wavelengths of visible light had on the time it takes for fan worms, specifically Branchiomma sp. to re-emerge from their tubes after being stimulated. It was found that while a decrease in wavelength caused a significant increase in average re-emergence time, the only significant difference between the means was present between the blue light and all three other groups, while there was no significant difference between the yellow, red and green wavelengths comparatively. This presents many possibilities for further investigation regarding the effect that different colours of light have on the re-emerging behaviour of fan worms.

K: Animalia
P: Annelida
C: Polychaeta
O: Sabellida
F: Sabellidae
G: Branchiomma

Fan worms are sessile annelids (segmented worms) that dwell on the surface of both hard and soft substrates in all depths of the ocean (World Register of Marine Species, 2017; Lezzi et al., 2016). Focusing specifically on the structure and features of Branchiomma sp., these fan worms can range in size, reaching up to approximately 10cm in length. Specific to this genus, the tubes are a green/grey colour and are composed mainly of excreted mucus from the worm itself (seen in Fig. 1). This tube structure allows for the physical protection of the worm as well as keeping the organism attached the substrate fairly strongly.

The branchial/tentacular crown structure that these organisms possess (as seen in Fig. 1) allow for their filter feeding strategy. This crown usually remains outside of the tube, allowing for the organism to feed freely. However, as a defense/protective mechanism, the branchial crown is able to be retracted into the worm's tube if it is alerted of threats in a close proximity (Bok et al., 2016; Bok & Nilsson, 2016; Brusca et al., 2016).

In an evolutionary sense, the origin of the specialised eye structure is thought to have likely occurred from the transition from a mobile to sedentary lifestyle. These organisms used to be mobile which allowed them to evade predators using whole body movement, however with the adoption of a sedentary lifestyle, selective pressures were applied in order to allow for the evolution of the photoreceptor and eye structure (Vodopyanov & Purschke, 2017; Bok & Nilsson, 2016; Purschke et al., 2006). In summary, natural selective pressures were applied to the fan worms which forced an evolutionary change in the morphology of their optical mechanisms.
If individuals are alerted of danger (e.g. change in light), then they are able to contract their longitudinal muscles in a synergentic fashion, resulting in the complete retraction of the worm's entire body (Nicol, 1950). This retraction response is mediated by the photoreceptors found on the radioles of the worm's branchial crown (Nilsson, 1994; Nicol, 1950). In the case of Branchiomma sp., paired compound eyes are the structure that can be seen (as seen in Fig. 2). The stimulation of these eyes results in a quick withdrawal response, which is mediated by the giant axons, when light intensity decreases, i.e. a shadow is cast over the top of the organism (Bok et al., 2016; Purschke et al., 2006). These mechanisms act similarly to an alarm system to alert the organisms of possible impending threats (Nilsson, 1994).

In this study, the aim is to investigate the effect that different wavelengths of light (nm) have on the time it takes for fan worms (Branchiomma sp.) to re-emerge from their tubes after a physical disturbance. It is hypothesised that as the wavelength of light is decreased, then the time it takes for the fan worms to re-emerge from their tubes will increase as this level of light is closer to that of a shadow.

The results shown depict a clear difference present between the re-emergence times of individuals exposed to the blue light (475nm) compared to all other colours (yellow=700nm, red=650nm, green=510nm). As seen in Fig. 4, the average time corresponding to the blue wavelength (nm=475) is 60.42sec (SE=+/- 3.563) compared to the green wavelength (nm=510), which is the next highest re-emergence time, which had an average value of 40.74sec (SE=+/- 3.141); indicating a difference of 19.68sec between the two groups (see Fig. 7 for additional differences). In relation to Fig. 4, a negative relationship between wavelength (nm) and re-emergence time (seconds) is seen.

Fig. 5 depicts a boxplot representation of the data, which includes the total range of data within each group as well as the median and upper and lower quartile values. This allows for a more representative view of the data as opposed to the mean re-emergence time alone. The boxplot also shows a negative relationship between wavelength and re-emergence time.

The difference between the blue wavelengths and all others is seen to be significant, as displayed in the t-test results table in Fig. 7. However, there was no statistically significant difference seen between the other three wavelengths (yellow, red and green), also displayed by the p-values shown in Fig. 7. 

Through the use of the ANOVA table in Fig. 6, it can be seen that wavelength does have a significant effect on the re-emergence time of fan worm (Branchiomma sp.) re-emergence time (F_1,43=11.91046, p<0.0001). Overall, as seen in Fig. 4 and Fig. 5, there is negative relationship between wavelength and re-emergence time, however not all differences between the means are significant (seen in Fig. 6 and Fig. 7).

In relation to the difference in average re-emergence time, there is a statistically significant difference present between colours. For instance, the blue light (nm=475, average time=60.417s, sd=11.817) and the yellow light (nm=700, average time=39.759s, sd=7.660) are seen to have significantly different means; t=4.86513, p<0.0001. This difference is also present between the blue and green wavelengths, as well as the blue and red wavelengths. However, there is not a statistically significant difference present amongst the other three wavelengths (green, red and yellow) as indicated by the t-test results in Fig. 7. 

The statistical significance of wavelength in relation to its effect on re-emergence time is also communicated via the ANOVA table (Fig. 6). It can be seen that light wavelength is a significant contributor to changes in re-emergence time between wavelengths (F_1,43=11.91046, p<0.0001). However, as the difference between the blue wavelength and all other levels was so substantial, this may have impacted the analysis as a whole.

This difference in re-emergence time between the different wavelengths of light is a result of different reactions from the fan worms' photoreceptors. These receptors are found in the compound eyes of Branchiomma sp. which are located on the radioles of the branchial crown (Fig. 2). As the blue light (475nm) was the darkest light that was used in this study, it was expected that it would be perceived as a shadow to the individuals' eyes. As the photoreceptors cause fan worms to retract under shadows, it is probable that the lower light caused this delayed re-emergence response. However, there was an insignificant difference between the yellow, red and green levels of light (700nm, 650nm and 510nm respectively). There is also a larger range of values seen in both the green and red wavelengths (as seen in Fig. 5), indicating that the time was more variable under these conditions. As the response time did not significantly reduce with increasing wavelength, the original hypothesis is not supported.

A study completed by Bok et al. (2016) investigated the different types of radiolar eyes present throughout the Sabellidae family (fan worms). These radiolar eyes exist within all of the genera of Sabellidae and have similar reactions to changes in light levels. As discussed within this publication, it was seen that the withdrawal, or rapid retraction, is mediated by the giant axons within the worm. This stimulation of the giant axons occurs in the presence of a decreased light intensity (i.e. shadow). However, it was also found that the response occurs much faster in the presence of moving stimuli such as the moving shadow of a predator (Bok et al. 2016).

This conclusion made reference to a Nilsson (1994) study which researched the photoreceptors of fan worms as an "optical alarm system". In this study, it was seen that the detection of light directly stimulated the retraction response within many worms within the Sabellidae family, including Branchiomma sp. (Nilsson 1994). Both of these studies support the conclusions made within this study, in relation to retraction response being impacted by the light quality and intensity. 

Overall, it can be seen that the difference between the re-emergence time of individuals under the blue light is significantly longer than any other wavelength, however the difference between the remaining wavelengths is not significant. This may indicate that while the lowest wavelength increases the time it takes for the fan worms to re-emerge, that perhaps after a certain wavelength increase (i.e a threshold) the time is not impacted. Due to logistical limitations (both time and equipment), only four different wavelengths could be tested on a fixed amount of individuals and only one desk lamp was available for all tests. By continuously using the same light source, the heat emitted as testing was carried out may have affected the behaviour of the fan worm, specifically their re-emergence response. The conclusion drawn in this study has also been supported by past literature published within this field of a similar focus. In future, to possibly gain more representative data, an increased amount of wavelengths at more frequent increments, with an increased sample size could be tested. Also, only the visible range of light was utilised in this study, therefore allowing future research to possibly implement wavelengths that extend beyond the visible range. 

I would like to thank Bernie and Sandie Degnan at the University of Queensland (St Lucia) for the assistance and encouragement throughout this process. Also, thank you to the lab and tutoring staff for their guidance and help in making this process as smooth as possible.

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Bok, M.J., Capa, M., Nilsson, D. (2016). 'Here, There and Everywhere: The Radiolar Eyes of Fan Worms (Annelida, Sabellidae)'. Integrative and Comparative Biology, 56 (5), 784-795. doi:10.1093/icb/icw089

Lezzi, M., Del Pasqua, M., Pierri, C., Giangrande, A. (2016). 'Settlement and population dynamics of the alien invasive species Branchiomma bairdi (Annelida: Sabellidae) in the Mediterranean Sea: two years of observations in the Gulf of Taranto (Italy)'. Marine Biology Research, 12 (8), 830-841.

Moore, J.D., Marshman, B.C., Robbins, T.T., Juhasz, C.I. (2013). 'Continues absence of sabellid fan worms (Terrebrasabella heterouncinata) among intertidal gastropods at a site of eradication in California, USA'. California Fish and Game, 99(3), 115-121.

Nicol, J.A. (1950). 'Giant axons and synergetic contractions in Branchiomma vesiculosum'.  Journal of Experimental Biology, 28(1), 22-31.
Nilsson, D. (1994). 'Eyes as optical alarm systems in fan worms and ark clams'. Philosophical Transactions of the Royal Society B: Biological Sciences, 346 (1316), 195-212.

Purschke, G., Aredt, D., Hausen, H., Muller, M.C.M. (2006). 'Photoreceptor cells and eyes of Annelida'. Arthropod Structure and Development, 35(4), 211-230.

Vodopyanov, S., Purschke, G. (2017). 'Fine structure of the cerebral eyes in Flabelligera affinis (Annelida, Sedentaria, Cirratuliformia): new data prove the existence of typical converse annelid multicellular eyes in a sedentary polychaete'. Zoomorphology, 136(1), 1-19.

World Register of Marine Species (WoRMS). (2017). Branchiomma Kolliker, 1858. Retrieved fromhttp://www.marinespecies.org/aphia.php?p=taxdetails&id=129524, accessed on: 23-05-2017.