Aposematic Coloration of Sea Urchins Howell 1
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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.