Eyesight in Phataria unifascialis Weed 1 Phataria unifascialis use eye sight to fi nd their preferred rock location Erica Weed University of Californ ia, Santa Barbara EAP Tropical Biology and Conservation Program, Fall 2016 16 December 2016 Abstract Sea stars living on the ocean substrate rely on multiple senses to detect touch, temperature, and chemicals in the water. They have rudimentary compound eyes similar to arthropod compound eyes on the tip of each arm These eye s allow them to detect light and dark images and see in a wide range of directions. However, it is unclear whether sea stars use eyes in their environments, or if the ability to see influences sea star behavior. One species, Linckia laevigata has been shown to rely on its eyes ight to navigate back to its preferred reef location after being displaced. This study aimed to see if anothe r species Phataria unifascialis also uses eyesight to find their preferred rock location Sea stars in three different experiments were displaced away from the initial rock where they were found and then their movements were observed. My data and analysis shows that m ost sea stars w ith ey es covered did not move back to wards their initial rock location, but sea stars with uncovered eyes d id return to wards their initial rock location Phataria unifascialis usa la visiÂ—n para encontrar su ubicaciÂ—n preferida en las rocas Resumen Las estrellas de mar que viven en el sustrato oceÂ‡nico se basan en sentidos mÂœltiples para detectar el tacto, la temperatura y los productos quÂ’micos en el agua. Tienen ojos compuestos rudimentarios, similares a los ojos compuestos d e artrÂ—podos, en la punta de cada brazo. Estos ojos les permiten percibir la luz y la oscuridad y ver en una amplia gama de direcciones. Sin embargo, no estÂ‡ claro si las estrellas de mar utilizan los ojos en su hÂ‡bitat o si la capacidad de ver influye en el comportamiento de las estrellas de mar. Se ha demostrado que una especie, Linckia laevigata depende de su vista para regresar a su hÂ‡bitat de arrecife preferido despuÂŽs de haber sido desplazada. Este estudio tuvo como objetivo ver si otra especie, Phat aria unifascialis tambiÂŽn usa la vista para encontrar su ubicaciÂ—n preferida en las rocas En tres experimentos diferentes, desplacÂŽ las estrellas de mar lejos de la roca inicial donde fueron encontradas y luego observÂŽ sus movimientos. Mis datos y anÂ‡lis is demuestran que la mayorÂ’a de las estrellas de mar con los ojos cubiertos no se devolvieron hacia su ubicaciÂ—n inicial de la roca pero las estrellas del mar con los ojos descubiertos volvieron hacia su roca inicial. Introduction
Eyesight in Phataria unifascialis Weed 2 Sea stars are slow moving predators that are usually found on rocky substrates and reefs. They have tube feet containing adhesive chemicals, w hich they use to attach to food, grab onto the substrate, and crawl ( Hennebert et al, 2012 ). They also use multiple senses to d etect touch, light, tempera ture, and chemicals in their environment For example, they use chemoreceptors to smell and find food like algae, mollusks, or other slow mov ing or sessile prey ( MESA 1999 ) In 2004, Drolet and Himmelman performed an experiment to observe chemo sensing abilities in the species Asterias vulgaris They showed that the sea star, which is a predator of mussels and other sessile creatures, could orient itself toward prey when there is a water current and prey o dors are present upstream of that current. Individuals of Asterias vulgaris were placed in either a tank with no current and a chemical stimulus, or a tank with a current and a chemical stimulus upstream of the sea star. A majority of the sea stars in the tank with the current and chemical stimulus could orient themselves and move towards the stimulus (Drolet and Himmelman, 2004). This study shows that Asterias vulgaris may rely on chemoreceptors when moving short distances to find food; however, to move g reater distances and navigate towards large objects, sea stars may rely on vision. All s ea stars have rudimentary compound ey es made up of groups of photoreceptor cells on the tip of each arm which are similar in structure to arthropod compound eyes. In arthropods, their compound eyes allow them to see dark and light, and as well as see in a wider range of directions This is in contrast to the cup like eyes of other animals, which cannot perceive as wide a range of direction, but have higher resolving power (Olsen, 2002). Even though sea stars have eyes similar to arthropods, it is unclear whet her all sea stars use these compound eyes to differentiate between light and dark in their environment or if the ability to see light and dark influences their behavior and movement within that environment (Garm and Nilsson, 2014). Drolet and Himmelman were able to show that one species of sea star may rely on chemoreceptors to search for prey. H owever other behaviors such as navigation and orientation towards a preferred rock location may depe nd on another sensory modality In 2014, Garm and Nilsson tested this homing mechanism towards a preferred reef within the species Linckia laevigata They concluded tha t these sea stars strongly rely on their eyes for orientation and navigation towards their reef In their experiment, observed sea stars were able to find their way back to the reef after being displace d one meter away, but sea stars that had their eyes re moved were not able to find their way after being displaced the same distance away from the reef A control manipulation was also performed by removing tube feet and ossicles near the eyes This w as an attempt to create the same level of disturbance on the sea star s without interfering with their vision and to determine if the movements of blind sea stars are comparable to non blind sea stars W hen these non blind control stars were displaced they were still able to fi nd their way back to the reef T he s tudy by Garm and Nilsson provides insight to the function of sea star eyes within one species, however there have been almost no similar studies with other species of sea stars. During preliminary observations in Cuajiniquil, Costa Rica, I observed an indi vidual of the species Phataria unifascialis The species Phataria unifascialis feeds on algae gr owing on rocks, and usually stay s close to one preferred rock for a period of time. I f a predator comes, removes a sea star from their rock and displaces it, the sea star will ha ve to navigate back towards protection. In addition, if the food supply is low on one rock, a sea star might have to leave a rock and search for another source Phataria unifascialis is within the same taxonomic famil y of sea stars as the Linckia laevigata s tudied by Garm and Nilsson, in the Family Ophidiasteridae
Eyesight in Phataria unifascialis Weed 3 and so they may exhibit similar behaviors The purpose of this study was to see if Phataria unifascialis do rely on eyesight in order to determine orientati on and relative location in a similar way to the species Linckia laevigata Materials and Methods I used a mask, snorkel, fins and a wetsuit while find ing and observing sea stars. I used a n underwater slate to take notes on movements and behavior of the sea stars I used a watch to time my observations of the sea stars during each trial and a measuring tap e to m easure how far the displaced sea stars moved For my second and third experiments I used 2cm x 2cm squares of black plastic and small elastic rubber bands I snorkeled off the coast of Cuajiniquil, Costa Rica, near Isla Mu Â–ecos and Isla David where I could observe the movements of Phataria unifascialis in an open substrate away from their initial rock location I found sea stars attached to rocks between one to seven meters deep. These rocks had algae growing on them which Phataria unifascialis uses as food. The rock also contained small holes and nooks where the sea stars could better grab onto the substrate. Th e open area around the rocks consisted of sand which was lightly colored in contrast to the dark rock substrate. I defined the sea stars' initial rock location as the rock to which they we re attached each time I observed them. I performed three different experiments "Natural", "Eye Treatment" and "Control Treatment", and performed 17 trials In each trial at Isla MuÂ–ecos, I place d a small piece of rock at the base of the large initial rock location from which the sea star s were taken to use as a marked reference point I also placed another small piece of rock on the open sand, a s hort distance away from the initial rock location to use as my second reference point I displaced the sea stars onto the sand next to this rock during each trial T he initial rock location at Isla David was much smaller and dome shaped than the large vertical initial rock at Isla MuÂ–ecos and so I was able to place a small re ference rock next to the exact s pot from which th e sea stars were taken At Isla David I also placed a second small piece of rock on the sand, a s hort distance away from the initial rock to use my other marked reference point o nce the sea stars were displaced In my first experiment "Natural", I removed sea stars from their initial rock and placed them on the sand next to the small reference rock but there was no manipulation with their eyes (Fig. 1 ) At Isla MuÂ–ecos I collected six "Natural" sea stars and placed them at different distances between 62cm 300cm away from the rock At Isla David, I col lected three "Natural" sea stars from one rock and placed them at different distances between 44cm 64cm I waited five to ten minutes to allow the sea stars to adjust and a ttach to the substrate, and began measurements. I measured how far the sea stars move d and noted which dir ection they traveled in relation to my reference rocks. In the second experiment "Eye Treatment", I removed sea stars from their initial rock and covered their eyes ( Fig. 1 ) I cut 2cm x 2cm squares from a black plastic trash ba g and wrapped them around the tips of the sea star arms where their eyes we re located ; elastic rubber bands were used to hold the black plastic in place. After their eyes were covered, I placed the sea stars back on the initial rock from which they were ta ken for at least one hour so they could adjust to the manipulation. All sea stars remained near the same location during this time period
Eyesight in Phataria unifascialis Weed 4 and I was easily able to find the sea s tars for the trials afterwards. I performed two "Eye Treatment" trials at Isla MuÂ–ecos and two at Isla David. T hese were placed between 48cm 87 cm away from their initial rock. I also collected one sea star at Isla MuÂ–ecos which had the tips of all five arms already chewed off by a predator, and so this sea star was naturally lacking eyes I categorized this sea star with the "Eye Treatment" sea stars and performed a trial. The third experiment wa s a control that was similar to the second experiment however the sea star eyes were not covered with the black plastic These we re the "Control T reatment sea stars (Fig. 1 ) I r emoved sea stars from their initial rock and placed only the elastic rubber bands around the tips of the sea star arms in the same p lace where the rubber bands had been wrapped in the second experiment The purpos e of this manipulation was so that the sea stars were still be able to see, but also experienc e a similar disturbance as the "Eye T reatment sea stars. I then placed the "Control T reatment sea stars back on their initial rock for at least one hour so they could adjust to the rubber bands. I collected two "Control T reatment sea stars from Isla MuÂ–ecos with this manipulation and one "Control T reatment sea star from Isla David whic h were placed between 57cm 75cm away from the initial rock. Figur e 1 "Natural" sea stars had no eye manipulation "Eye T reatment sea stars had black plastic and rubber bands placed around the tips of their arms to cover their eyes "Control T reatment sea stars had each arm wrapped with a rubber band but with eyes left uncovered. All sea stars were displaced from their initial rock location onto the sand In my data analysis I calculated the amount of time each sea star took to move 10cm. I separat ed these values into the three categories based on my three experiments and compared the three groups using One Way ANOVA in R Commander This test determines if the average rate of movement during each set of experiment s differ between each other. This su ggests whether the "Eye Treatment" and "Control Treatment" manipulations influenced the movements of the sea stars compared to the "Natural" I also created a circle diagram showing the direction each sea star was traveling at the end of each observation i n reference to their initial rock I used Rao's Spacing Test in R Commander to determine directionality or uniformity between the blind "Eye Treatment" sea stars and the non blind sea stars. The null hypothesis of Rao's Spacing Test states
Eyesight in Phataria unifascialis Weed 5 that the data points around a circle diagram are uniformly distributed; rejecting the null hypothesis suggests that the data points are more clustered in one direction. The "Eye T reatment sea stars were com pared to the "Control T reatment sea stars in a contingency tab le in order to display differences between the two treatment groups I used a Chi Square test in R Commander in order to determine if there was a correlation between sea stars that can see and what direction they move Rejecting the null hypothesis of this test means that the sea stars do use their eyes to navigate. Results The average time for each sea star to move 10 cm did not differ across all groups (Fig 2 .) My observations and findings supported the null hypothesis that sea stars in each group moved at a similar rate Therefore, each categ ory of sea stars are comparable. Figure 2 This graph shows the amo unt of time each sea star took to travel a total distance of 10cm. Each sea star is categorized by the type of ma nipulation during the study (F = 0.5374, p = 0.5967 accept ANOVA null hypothesis )
Eyesight in Phataria unifascialis Weed 6 I also observed whether the sea stars returned towards their initial ro ck location As shown in Figure 3 and 4 twelve of the seve nteen sea stars returned back Eleven of t hese were either "Natural" or "Control T reatment sea stars which all had their eyes uncovered O ne other sea star that returned towards home was the sea star that was naturall y blind O f t he six sea stars that did not move back towards their initial rock location five of them wer e the "Eye T reatment sea stars, and one was a "Natural" sea star which moved to another nearby rock Rao's Spacing Test determined that "Natural" and "Control Treatment" sea stars moved towards one direction, but that the "Eye Treatment" sea stars moved i n random directions Figure 3 Out of the 17 sea stars observed, most "Eye Treatment" stars did not return, but most "Natural" and "Control Treatment" sea stars did return towards their initial rock location. n=1 n=4 n=11 n=1 0 2 4 6 8 10 12 14 Returned Not Return Number of Sea Stars "Natural" and "Control Treatment" "Eye Treatment"
Eyesight in Phataria unifascialis Weed 7 Figure 4 Diagrams show the direction each sea star moved by the end of each trial. A) "Natural" and "Control Treatment" sea stars were able to see (Rao's statistic = 180.069, Level 0.10 critical value = 159.33, reject null hypot hesis of uniformity). B) "Eye Treatment" sea stars were blind (Rao's statistic = 117.832, Level 0.10 critical value = 168.66, do not reject null hypothesis) Five sea s tars in this study were in the "Eye T reatment category, and only one moved in the direction towards the initial rock location the other sea stars moved away from their initial rock Three other sea stars had rubber b ands placed around the eyes in the "Control T reatment but did not have their eyes covered. All of these sea stars returned back towards their initial rock location The contingency table sh ows that there is a relation between sea stars with uncovered eyes that orient themselves back towards their initial rock, but sea stars with covered eyes are less likely to return back towards their initial rock (Table 1) Table 1 Five "Eye T reatment sea stars were observed and compared to three "Control T reatment sea stars to determine if there is a correlation that sea stars with uncovered eyes m ove towards their initial rock. ( Chi Square = 4.8, p = 0.028 ) Eye Treatment Control Treatment Towards Initial Rock 1 3 Away From Initial Rock 4 0 Discussion T he results suggest that my three experiments were all comparable because the average time each group of sea stars took to move 10 cm did not differ from each other. The manipulations in my experiments did not interfere with the sea stars' ability to crawl because the elastic rubber bands and black plastic did not add too much drag to the s ea stars as they were moving, and most of their tube feet were left uncovered The black plastic used in the "Eye A) "Natural" and "Control Treatment" B) "Eye Treatment"
Eyesight in Phataria unifascialis Weed 8 Treatment" manipulation covered some tube feet on the sea star s however they have tube feet lining the rest of the underside of their body wh ich they were still able to use ( Nicholson, 2016) My data also shows that Phataria unifascialis do rely on their eyes as a homing mechanism to navigate from the sand back to their initial rock location These results are similar to Garm's and Nilsson's findings in 2014 with the species Linckia laevigata suggesting that other sea stars, at least in the Family Ophidiasteridae, do rely on eyesight as well. In my findings and observations, m ost of the "Natura l" sea stars and "C ontrol Treatment" sea stars, which d id not have their eyes covered, did move back towards their initial rock location whereas the blind "Eye T reatment sea stars did not r eturn towards their initial rock However, the one sea star which naturally had all eyes chewed off did move back to wards the initial starting rock. I t is unclear if this occurred by chance, or if the sea star was relying on other senses to navigate since it had already lost its eyes prior to my observations This sea star w ould have been able to regenerate the rest of its arms and eyes later but th is process takes time, and so, t his sea star may have been navigating with another sens e, suc h as smell using chemoreceptors located all over the body of sea stars (Nich olson, 2016) Although m y observations suggest that Phataria unifascialis rely on eyesight, the experiment performed by Drolet and Himmelman suggest that another species of sea star use s chemoreceptors when searching for food Therefore, it is also possible that sea stars use a combination of senses while navigating in their environments. In the future, I think it would be interesting to compare the navigation abilities of sea stars that only use their eyes to sea stars that only use their chemorecep tors. Performing these trials with sea stars at different distances from rock habitats or food could also determine if sea stars use one sensory modality depending on how far away they are from their target location Acknowledgements Thank you to Frank J oyce who gave useful feedback and guidance throughout the entire project. Thanks to Minor Lara who provided transportation to study sites and was helpful during treatments of the sea stars. Lauren Cech and Emily Miao were also extremely patient and helpful with searching for Phataria unifascialis and accompanying me during observations. Literature Cited Drolet D., Himmelman J. H. 2004. Role of current and prey odour in the displacement behaviour of the sea star Asterias vulgaris Canadia n Journal of Zoology http://cjz.nrc.ca Garm A., Nilsson D E. 2014. Visual navigation in starfish: first evidence for the use of vision and eyes in starfish. Proceedings of the Royal Society B. 281: 20133011. http://rspb.royalsocietypublishing.org/content/281/1777/20 133011 Hennebert, E. et al. 2012. Echinoderms don't suck: evidence against the involvement of suction in tube foot attachment Zoosymposia. http://www.mapress.com/zoosymposia/content/2012/v7/f/v007p025 032.pdf MESA. 1999. Echinoderms: Sea Stars. http://www.mesa.edu.au/echinoderms/echino01.asp Nicholson, F.C. 2016. Starfish. http://science.jrank.org/pages/6450/Starfish.html Olsen, Brian. 2002. Evoluti on of the Insect Eye. http://www.d.umn.edu/~olse0176/Evolution/insects.html