Aposematic Coloration of Sea Urchins Howell 1

Evidence for Aposematic coloration of the Sea Urchin Astropyga pulvinata

Nathan Howell Department of Physics and Astronomy University of California Los Angeles EAP Tropical Biology and Conservation Program, Fall 2017 15 December 2017

Abstract

Astropyga pulvinata is a brightly colored urchin found in some tropical pacific waters including the coast of Costa Rica. The aim of this study was to test a possible explanation for the coloration of A. pulvinata. I tested the hypothesis that the coloration of A. pulvinata is aposematic. I tested this by poking A. pulvinata and another species of urchin, the uniformly black Diadema mexicanum, then comparing their reactions. The results indicated that A. pulvinata were more likely to move as a response to the poking stimulus than D. mexicanum, which was contrary to the predicted hypothesis. 100% of A. pulvinata urchins moved after being poked, but only 60% of D. mexicanum urchins moved. However, after further investigation and observations, this behavior does not disprove the warning coloration hypothesis. Another test involved placing both species of urchin in the feeding garden of the territorial giant damselfish. The results showed that the damselfish bit D. mexicanum significantly more than A. pulvinata. After 9 trials with 120 minutes of video, damselfish bit D. mexicanum a total of 314 times and A. pulvinata only twice. The best explanation for such behavior is that A. pulvinata exhibits aposematic coloration.

Evidencia para coloración aposemático del erizo del mar Astropyga pulvinata Resumen

Astroypyga pulvinata es un erizo de mar de coloración brillante que se encuentra en el océano pacifico, incluyendo la costa de Costa Rica. El objetivo de este estudio fue probar una posible explicación de la coloración de A. pulvinata. Probé la hipótesis que la coloración de A. pulvinata es aposemático. Para probarlo, empujé el erizo con un palo y comparé la reacción de este erizo contra otra especie, el erizo negro, Diadema mexicanum. Los resultados indican que A. pulvinata se movió más que D. mexicanum después del estímulo, que fue el opuesto de la hipótesis. Cien porciento de los A. pulvinata se movieron, pero solamente 60% de los D. mexicanum se movieron. Sin embargo, después de más investigación y observación, este comportamiento no refuta la hipótesis que la coloración es aposemático. Para otra prueba, puse las dos especies dentro del territorio de un pez, la jaqueta gigante, y observé la reacción del pez. Los resultados mostraron que la jaqueta gigante mordió las espinas de D. mexicanum mucho más que las espinas de A. pulvinata. El pez mordió las espinas de D. mexicanum 314 veces y solamente mordió las espinas de A. pulvinata dos veces. La mejor explicación mejor por este comportamiento es que la coloración de A. pulvinata es aposemático.

Introduction Sea Urchins are small animals in the phylum Echinodermata that live in the ocean. There are a wide variety of Sea Urchins with around 200 known species. Sea Urchins have round shells, called tests, that enclose all of their internal organs (Follo & Fautin, 2001). Their mouths are usually centered on the lower half of their body, called the oral surface, and the anus is centered on the top of their body. Urchins are not sessile, and can move using hundreds of tiny tube feet that are powered by a water vascular system in their body (Yoshimura, 2012). Main predators of sea urchins are sea otters, crabs, wrasse, and trigger fish, and their primary form of defense is their long spines that often contain toxins. They are sensitive to light, touch, and chemicals (Barnes, 1982). Aposematic Coloration of Sea Urchins Howell 2

Near the fishing village of Cuajiniquil, Costa Rica, two common species of urchin are Astropyga pulvinata, and Diadema mexicanum. Both species are aggregating urchins, which means that they are found in groups of two or more closely together touching at least by their spines (Alvarado 2008). They are both in the family Diadematidae. D. mexicanum are the most prevalent species of urchin on the small islands near Cuajiniquil, and are most often found on rocky substrate. A. pulvinata are less common and seem to prefer sandy substrate over rocky substrate. They have iridescent blue spots, long white spines, and 5 large, bright white spots on their test. These contrast with the more abundant D. mexicanum which are completely black, occasionally with some green at the base of their spines. I began to wonder if the striking colors on A. pulvinata had an adaptive significance to the urchin. The spines of A. pulvinata are known to cause localized paralysis which may indicate that their coloration is aposematic (Cleveland & Hickman, 1998). Aposematism is an antipredator adaptation organisms use to warn potential predators that they dangerous (Santos & Coloma, 2003). This warning signal may be colors, sounds, or odors (Eisner, & Grant, 1981). These warning signals are usually honest indications of noxiousness because conspicuousness often evolves in tandem with noxiousness (Maan & Cummings, 2011). Aposematic coloration in the marine world has been very controversial. This is because there are many brightly colored organisms in the ocean that either have no physical or chemical defenses, or are hunted at night. In many of these cases, the bright colors of these organisms cannot be explained by aposematism (Pawlik, 2012). Through my study, I attempt to test if aposematism may be a viable explanation for A. pulvinata’s coloration. The main question I attempt to answer is: what is the adaptive significance of the coloration of the urchin Astropyga pulvinata? My hypothesis is that the coloration of A. pulvinata is aposematic, or warning coloration, and evolved as a predator defense mechanism. I offer my hypothesis as well as two predictions from the hypothesis in Table 1.

Table 1: Hypothesis and predictions for the coloration of A. pulvinata.

Hypothesis Predictions 1. A. pulvinata move less than than D. The Coloration of Astropyga pulvinata is mexicanum Aposematic 2. Damselfish will will bite D. mexicanum more than A. pulvinata.

As previously mentioned, urchins are not sessile, and can move using their tube feet. However, according to my observations, urchins are relatively stationary and do not move often. Therefore, I predicted that A. pulvianta would not move as a response being agitated as much as D. mexicanum. This is because organisms that are aposematic rely on their warning signals as their primary defense, and are less likely to resort to other defense mechanisms such as fleeing (Ruxton et al., 2004). My second prediction involves the giant damselfish. Giant damselfish are known to be very territorial, and in some cases actively exclude urchins from their feeding territories known as gardens, or lawns (Sammarco & Williams, 1982). According to a study by Siebeck et al. 2008, damselfish can also see and perceive color extremely well. For my second prediction, I expected both species of urchin to agitate damselfish when placed in Aposematic Coloration of Sea Urchins Howell 3

their gardens, but I expected damselfish to not bite A. pulvinata because they can see the its coloration.

Materials and Methods This study took place in four different locations. Three of the locations were small islands off of the coast of Cuajiniquil, Costa Rica. They were Bajo Rojo, Muñeco, and Isla David. The fourth location is called the muelle which is the main dock of Cuajiniquil. I tested my hypothesis in 2 ways following my two predictions (Table 1). The first way I tested my hypothesis was by poking each species of urchin with a small stick. The two different species are shown in Figure 1. I used the same sized stick for all trials, and the branch was small enough to fit in between the urchins’ spine to reach the test. Since many D. mexicanum urchins stay in small holes, I only poked urchins that were aggregating on either a rock or the sea floor. I poked an urchin’s test between the spines 3 times, and if it did not move, I poked the urchin another 5 times. If after 5 pokes the urchin still did not move I would poke it 10 more times. I waited 5 seconds in between each set of pokes before poking the urchin again. I compared the reactions of D. mexicanum to the reactions of A. pulvinata. The only two reactions I recorded was if the urchin moved, or if it remained stationary. I also only poked urchins that were not moving before I approached them. The second test involved the giant damselfish. First, I had to get the two species of urchin in the same location. A. pulvinata urchins were only found at the muelle, so I took them in a bucket from the sea floor in the muelle and brought them to Bajo Rojo where there were many D. mexicanum urchins. I then snorkeled until I found a damselfish that was protecting its garden. I used a GoPro to record the location, and then I placed both species of urchin in the fish’s garden. I recorded the fish’s reaction for 15 minutes on the GoPro. I later watched the footage to determine if the fish bit one species more than the other.

Figure 1: Astropyga pulvinata (left), and Diadema mexicanum (Right)

Aposematic Coloration of Sea Urchins Howell 4

Results My first test showed that A. pulvinata were more likely to move as a response to being poked than D. mexicanum. Of 59 total A. pulvinata urchins, 100% moved as a response to the poking stimulus compared to 60% of D. Mexicanum urchins. (X2=33.5, df = 2, p=0.0001).

Figure 2: A. pulvinata urchins were much more likely to respond to stimulus by movement. 65 D. mexicanum urchins moved after being poked, and 49 did not move. This contrasts with the A. pulvinata urchins of which 59 moved after being poked and no urchins remained still. Aposematic Coloration of Sea Urchins Howell 5

Of the urchins that moved, A. pulvinata (59) were more likely to move after the first three pokes than D. mexicanum (65). D. mexicanum urchins required more stimulus to move whereas A. pulvinata urchins moved almost immediately. (X2=28.7, df = 3, p=0.0001).

Figure 3 Not only were A. pulvinata urchins more likely to move after being poked, but they required less stimulus to be prompted to movement. A. pulvinata urchins were much more likely to move after 3 pokes than D. mexicanum urchins, and D. mexicanum urchins required more pokes in order for them to move. Aposematic Coloration of Sea Urchins Howell 6

The second test, using the damselfish garden, clearly showed that the damselfish are more likely to bite D. mexicanum urchins than A. pulvinata. Of 120 minutes of video and 9 trials, the damselfish bit D. mexicanum urchins 314 times, and A. pulvinata 2 times.

Figure 4: Combining the data from all of the trials, damselfish bit D. mexicanum urchins a total of 314 separate times, and only bit A. pulvinata urchins twice.

To illustrate this with more accuracy, figure 4 shows each individual trial when the urchins were placed in the damselfish garden. (Wilcoxon Signed Rank Test, S=22.5, P=.002)

Aposematic Coloration of Sea Urchins Howell 7

Figure 5: Number of times each individual fish bit D. mexicanum and A. pulvinata.

Discussion The results of the first test, poking the urchins, yielded results contrary to what I expected. As seen in Figures 2 and 3, A. pulvinata were more likely move when poked compared to D. mexicanum. If their coloration is aposematic, then it is expected that they would be less likely to move away because their coloration is their primary defense. However, these results do not completely refute the hypothesis that the urchins are warningly colored. One explanation for A. pulvinata’s movement could be that, in general, they are more likely to move on their own without any stimulus. Based on my observations, A. pulvinata seemed to move more often than D. mexicanum. As I was diving down to A. pulvinata, I noticed that before I even got close to the urchins, some of them were already moving. This contrasts with D. mexicanum urchins which were all stationary. A separate study on another species of urchin in the Astropyga genus, Astropyga magnifica, found that A. magnifica were very mobile and moved at speeds of at least 1 m/min (Hendler et al., 1995). Another possible explanation for this result is that the D. mexicanum urchins that were poked were almost always in aggregations that were larger than the aggregations of A. pulvinata. I noticed that A. pulvinata urchins were often in very small aggregations of only 2 or 3, or were often solitary. The principal of the selfish herd may apply in this situation. D. mexicanum urchins may be less likely to move because they have more safety in numbers. D. mexicanum also aggregated in crevices Aposematic Coloration of Sea Urchins Howell 8

and beneath overhangs. The A. pulvinata urchins were only on sandy substrate and were not near sheltered overhangs. They were more exposed, which may also explain why they fled as a response to being poked. Data from the second test with the damselfish, however, closely followed my prediction. The damselfish rarely bit A. pulvinata, and I present three possible explanations for this behavior in Table 2.

Table 2: Possible explanations for why damselfish only bit D. mexicanum.

Hypothesis to explain why Damselfish only bit D. Explanation Support? mexicanum. Damselfish would have no Reject. The damselfish were A. pulvinata do not affect the reason to risk harm and bite clearly agitated by the damselfish A. pulvinata presence of A. pulvinata. A. pulvinata were not Reject. A. pulvinata were Damselfish were unfamiliar present at the test sites and seen in the area on a with A. pulvinata damselfish did not want to recreational dive near the touch something unfamiliar. test site. The damselfish do not bite A. Support. The damselfish Warning coloration of A. pulvinata because their color repeatedly approached A. pulvinata deters fish from warns the fish that they are pulvinata but almost never biting. dangerous. bit at them.

The first possible explanation for why the damselfish did not bite A. pulvinata is that the urchins did not negatively affect the fish in any way. This hypothesis is rejected for two main reasons. The first is that urchins are scavengers, and the presence of the urchin in the garden decreases the fish’s food supply. Not much is known on about the diet of A. pulvinata, but like all other urchins they are grazers and feed off of the substrate they are on. If they are on a rock in the garden of a damselfish, they could certainly be eating the fish’s food. Even if they are not consuming the same resources as the fish, they are at least reducing the area of rock on which the fish can feed. This also increases the likelihood the fish could get poked by the spines. This hypothesis is also rejected is because it was clear to see that the fish was agitated by the urchin’s presence after the fish repeatedly approached the urchin in the same manner as D. mexicanum, and only at the last second turned away instead of biting its spines. There was at least one case of the damselfish directly biting the spines of A. pulvinata. For these reasons, it is clear that the urchin had a negative effect on the fish. Another explanation as to why damselfish did not bite A. pulvinata is that the fish may have never encountered the urchin before and simply didn’t want to bite something with which they were unfamiliar. However, A. pulvinata were seen when doing a recreational dive to a depth of 50 feet very close to the test site. This shows that A. pulvinata are in the area, but are just too deep to see while snorkeling. This increases the chance that the damselfish may have encountered an individual of A. pulvinata in its lifetime. Even if none of the damselfish had previously interacted with an individual of A. pulvinata before, damselfish are likely familiar Aposematic Coloration of Sea Urchins Howell 9

with A. pulvinata in their evolutionary history. As a result, they could instinctively know to avoid A. pulvinata. (This is similar to the multipredator hypothesis detailed in the study by Blumstein et al 2009. In this study, yellow-bellied marmots reacted to an extinct predator which they would only have known in their evolutionary history.) With the current available data, this hypothesis is rejected. The final hypothesis is that the damselfish did not bite A. pulvinata because the urchin is warningly colored. This hypothesis seems to be clearly supported by the videos. The fish were clearly bothered by the presence of A. pulvinata, yet rarely bit at their spines. There seem to be no other factors that would cause this clear phenomenon besides the coloration of the urchin. Even though the first test gave evidence contrary to the aposematic coloration hypothesis, further observations and evidence suggested that A. pulvinata urchins moved as a predator defense due to other environmental and evolutionary factors despite their warning coloration. According to the second test with the damselfish, the data and observations showed that the most likely explanation as to why the damselfish did not bite A. pulvinata is the urchin’s coloration. Therefore, according to my study, the most likely adaptive significance of the coloration of A. pulvinata is aposematism.

Acknowledgements I would like to thank Frank Joyce for his advice and guidance, as well as for putting up with my general obnoxiousness. I would also like to thank Eliot Headley, Rebecca Ash, Zac Durall, Eric Medina, and Brooke Hawkins for their (forced coercion) help to find A. pulvinata every day with me in the muelle in 1 foot of visibility. Thanks to Stephanie Li, John La Bonte, Jennay Argiris, Andrés Camacho, and Sunshine Lopez. Also to Jackie and Juliana for their help in the boat and great pictures.

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