Habitat Association of Arbacia Lixula in the Ligurian Sea Abstract

Habitat Association of Arbacia Lixula in the Ligurian Sea Audra Barrios, Kimberly Powell, Melissa Nehmens Abstract The sea urchin Arbacia lixula is a common intertidal species found in the Mediterranean in association with crustose coralline algae. Previous studies on habitat association between A. lixula and crustose coralline algae reported that A. lixula creates mosaics of crustose coralline algae interwoven between erect foliose algae. Through surveys of urchin distribution and algal distribution, we were able to determine that the urchin distribution was not due to random chance, but based on preference with a p-value of >.00001. Because A. lixula is a mobile species driving the habitat association, a field study was done to determine whether the habitat association was due to the food preference of the urchin or because the coralline algae acts as a refuge from harm for the urchin. We tested the force required to remove the urchins from different substrates and found that a force of 2000g was required to dislodge the urchin from crustose coralline algae, while only 350 g were needed to dislodge the urchin from erect foliose algae. This showed that the urchin does benefit from living on crustose coralline for hydrodynamic purposes. Urchins dissected from the field and lab tests helped determine that the food preference of A. lixula was erect foliose algae which averaged 92.62% of their diet. Introduction Examining habitat associations allows for a better understanding of how a species interacts with its environment. The general theory of such relationships often results in a net benefit for the species driving the association where the habitat provides food resources, protection, and/or enhanced ability for reproduction. Habitat associations are seen in many systems such as parrot fish and corals, where the parrot fish uses the coral as an algal food source. In the case of Arbacia lixula, a sea urchin found in the Mediterranean sea, there is an association with crustose coralline algae. Coralline algae is a habitat that provides both food and protection, thus allowing them to successfully occupy the inter and subtidal regions in the Ligurian Sea. The black sea urchin, more commonly known as the male urchin, has a coastal distribution ranging in depths from 0-50 m, is herbivorous and grazes on a variety of substrates (Bulleri et al, 2002). A. lixula is more commonly found on vertical walls covered with coralline encrusting substrates, rather than walls covered in erect foliose algae (Bulleri et al 1999) and bare rock. A. lixula is also commonly found on regions of mosaics dominated by crustose coralline algae and erect foliose algae (Bulleri et al, 2002), which in this study we refer to as “patch”. A. lixula is located in shallower regions near and within the intertidal, and thus, must be able to tolerate harsh wave action (Bulleri et al, 2002). A study performed by Bulleri, Bertocci, and Micheli (2002) suggested that urchins are mainly found on coralline alga due to wave exposure which allows for easier attachment on coralline substrate. Through preliminary urchin counts and observations of distribution, we noticed a pattern in which urchins congregated on areas composed of coralline algae, and even more commonly on the mosaic coralline regions. From this observed pattern, our goal for this study is to determine the mechanisms which cause the observed habitat association between A. lixula and its environment, particularly coralline algae. To that end, we tested two hypotheses: (1): A. lixula is found on coralline algae because it has eaten the erect foliose algae allowing coralline algae to proliferate. (2) A lixula is found more often on crustose coralline patches because they can withstand wave action better on coralline as opposed to erect foliose algae. Materials and Methods Study Site Our experiment was conducted in Revellata bay at STARESO marine field station in Calvi, Corsica, France ( 42º34'48.85”N , 8º43'26.89” E). (Figures 1 and 2). The rocky walls of the intertidal are composed of granite, which is where the urchins in this study were found. We observed five sites to the North and four sites to the South of the harbor at STARESO for the duration of October, 2010. Our surveys took place within two meters in depth from the surface. Study Systems Arbacia lixula is a key component of the system in which it is found. It plays a large role in algal distribution through its active grazing. Its a mobile species that uses tube feet, or podia, for locomotion, feeding and attaching to substrate. The podia have suction pads at the extremities, which are a component of their water vascular system (Smith, 1989). They occur most abundantly on vertical surfaces of rock walls. For our experiments, A. lixula was easy to manipulate since they do not move quickly. However, removing them from rocks without damaging their podia was difficult due to the suction pads. They do not migrate over long distances however, generally staying in the same area throughout their life. (Guidetti, 2004) They reproduce via broadcast spawning, which is when the organism releases sperm and eggs into the water column. The habitat association with coralline algae begins once the larva settles, usually on encrusting coralline algae. (Pedrotti, 1993) Sea urchins graze at night to avoid predation (Guidetti, 2004) moving from their coralline covered areas to patches of erect foliose that surrounded the encrusting coralline algae. General methods Patterns of habitat association for A. lixula In order to determine if A. lixula has associations with particular habitat forming species, we did urchin surveys at five random study sites to the North of STARESO and four study sites to the South. Within each survey site we counted every urchin we found within two meters of the surface. The sites were picked randomly and varied in size. The size was also picked at random. For each urchin we notated its size, its depth and whether it was found on rock, crustose coralline algae, erect foliose algae or a patch of crustose coralline algae. All of these surveys were taken during the day. Once we established a general pattern of urchin distribution, we looked at survey data taken on the algal cover from the same area (Fields and Hubach 2010).With the algal survey we developed an expected distribution of urchins for various algal groups: erect foliose algae, crustose coralline algae and Posidonia. The algal data was compared to the urchin data using a chi square test to determine if the pattern of habitat use by urchins differed from that expected, based on the overall frequency of the algal groups. In order to test our hypothesis of whether A. lixula is found on coralline due to food preference, we first had to determine if we could discriminate algal species following consumption by urchins. We collected four urchins right before sunrise from the North and within the STARESO harbor, took them to the lab where they were promptly dissected so the digested material would not be expelled or processed further. The four urchins were collected from four different substrates; bare rock, crustose coralline algae, a patch of crustose coralline algae surrounded by macrophytic algae and erect foliose algae. This was to discern if their stomach contents differed depending on their location. From the urchin dissection we assessed the percent composition of the stomach contents by looking at three different 0.4 g samples from each urchin under a dissecting microscope. We placed each sample in a petri dish and broke up the urchin pellets from the gut so as to see the algae more clearly. In order to identify the different types of algae, we created a key based on visual categorization. To help the categorization we used control samples of Posidonia oceanica, various erect foliose algal species and crustose coralline algae that were taken from the bay. Each sample was cut and placed on a slide. We categorized the algal contents based on observation and separated them into different groups labeled A through I. We also categorized the portion of the stomach contents that is digested beyond recognition as “fluff”. For the purpose of this study, this categorization was more specific than necessary, so for data analysis and comparison they were lumped into three groups of Posidonia, erect foliose algae and crustose coralline algae. As a means for further comparison and categorization of stomach contents, we performed a series of feeding experiments on 16 urchins removed from various locations around STARESO. We separated the urchins equally into four tanks without substrate or food. For two days we did not allow the urchins eat as to empty their gastrointestinal tract so the previous stomach contents would not contaminate the experiment. After two days, we added Posidonia oceanica in the first tank, erect foliose algae to the second tank, Padina pavonica, an erect foliose algae in the third tank, and rocks covered with crustose coralline algae to the fourth tank. We allowed them to feed for one week, then dissected them to compare these known stomach contents to the stomach contents from the non- experimental urchins that were previously dissected. This was to give us a better understanding of what the stomach contents in the field look like digested. We did a brief survey of the stomach contents of urchins collected after a few days of heavy storms to see if stomach contents differed from urchins collected during calm days. These urchins were found on coralline algae during the night because of the high wave action. According to Bulleri's study, urchins can switch from active foraging to passive feeding on drift algae in barren areas (Bulleri et al, 2002). Although this study was done in areas that were not barren, we did look at urchins during abnormally high wave action from the storm.

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