The Ecological Niche of the Marine Iguana

Marine | 1

Amblyrhynchus cristatus

Corinne P. Sathoff

PVHS Science Research

Abstract

Marine iguanas, Amblyrhynchus cristatus, are remarkable organisms because they are so well-adapted to their environment. I studied the reptile’s ecological niche, “all the physical, chemical and biological factors the organism needs to survive, remain healthy, and reproduce” (Raven and Berg, 2006). Also, I conducted observed the evolutionary adaptations of the marine iguana in the Galapagos Islands and was able to learn about the iguana’s physical characteristics, habitat, and unique behaviors, such as swimming and sneezing saltwater. The trip to the archipelago took place August 17-23, 2009, the beginning of the colder garua season on the islands with typical overcast skies. The trip included visits to South Plaza, Santa Fe, Floreana, Española, Santa Cruz, Rabida, Sombrero Chino, Bartolome and North Seymour where we were able to see the marine iguanas interacting in their natural habitat.

Introduction

Isolated just over six-hundred miles off the coast of Ecuador lies the group of volcanic islands known as the Galapagos. The cluster of islands, made famous by Charles Darwin and his study of the origin of species through natural selection, is renowned for a diversity of organisms found nowhere else of the planet. Often referred to as Las Encantadas or Enchanted Isles, the Galapagos are the definition of an inhospitable environment with few sources of freshwater, strong currents, arid soil and isolation. This prompts the question: how did the islands come to support such an array of life? The answer lies in the unique adaptations specific to each organism. “These reptiles and insects, small mammals and birds, evolved unmolested on the various islands on which they were cast into unique species adapted to the boulder-wrecked shores, the cactus deserts of the lowlands, or the elevated jungles of the large islands’ interiors” (Dillard, 1988).

Over time, the species of the Galapagos have changed with their environment to exist within specialized ecological niches. The word niche comes from a French word meaning nest. The term was first described by a naturalist, Joseph Grinnell, in 1917. One can call a niche the sum total of all the ecological requirements that allow a species to mate and produce offspring, thus surviving and flourishing with any undue pressures (Chupp, personal communication, August 9, 2009). Unquestionable, one of the finest examples of adaptation on the islands is the marine iguana. I chose to study the niche of the marine iguana because it is the only species of iguana on the planet to adapt to an ocean environment and gain the ability to swim. Charles Darwin described the marine iguana as a “hideous-looking creature, of dirty black coulour, stupid and sluggish in its movements” when he saw them on the islands, but through by observational studies, I hoped to discover the true reasons for this reptile’s bizarre appearance and behavior (Constant, 2002). I was particularly interested in studying the physical characteristics, coloration, development and behaviors of the iguanas. The story of its existence is remarkable because it seems improbable that an exothermic creature like the marine iguana would be able to survive in the cold seas surrounding the Galapagos, so I was also interested in learning about their mechanisms for maintaining homeostasis. The marine iguana is an important species to the field of biology because of its adaptations and modifications that are clearly comparable with land iguanas in the archipelago as well as ancestors from the mainland. “The marine and land iguanas of the Galapagos are more closely related to each other than either is to mainland relatives; Dr. Sarich said, ‘They have evolved in very different ways.’” (Brown, 1992).

Method

Materials

Materials used to gather observational data include waterproof field notebook, field guides, course reader, snorkeling gear (fins, mask, snorkel, wetsuit), camera, tennis shoes, M/Y Yolita yacht, pangas, Level 3 Naturalist Guide Rommel Daniel Saa.

Methods

My journey through the Galapagos Islands to conduct observational studies allowed me to explore different islands each day and record data about the marine iguana in its natural environment (Figure 1.1). Each island in the Galapagos has different geographic, geological and topographical components. I want to highlight the various differences from island to island while tracking the behaviors of the marine iguana.

August 17, 2009: We began at South Plaza Island at 8:15am (Figure 1.2). It was a sunny morning (74°F) at low tide and we first walked along the shoreline, known as the coastal or littoral zone of the island to observe various species (Chupp, personal communication, August 9, 2009). We saw three marine iguanas basking on the lava rocks. We then moved to the other side of the island which was around 60-70 feet in elevation, where I was surprised to spot marine iguanas (approximately six) on the top of the cliff. This area, known as the arid zone, features the marine iguanas as they had climbed the cliff from the coastal zone in order to sun-bathe (Saa, personal communications, August 17, 2009). This highlighted flora of this island was the red carpetweed that covers the island from zone to zone. This Sesuvium edmonstonei carpetweed hosts the land iguana and can sometimes be the second food of choice for iguanas (Saa, personal communications, August 18, 2009). Many cacti were seen also on this small island. We were also able to observe the iguanas sneeze out excess salts as they were sunning. In the late afternoon to early evening (2:30-5:00pm) we also took a walk on Santa Fe at Barrington Bay. I did not observe any marine iguanas while on Santa Fe Island because the beach we visited was mainly inhabited by sea lions.

August 18, 2009: For our planned three hour walk on Espanola at Punta Suarez we began at 8:15am (Figure 1.3). The weather was slightly cooler (67°F) and more overcast (typical of Galapagos in its early winter months) than the previous day at South Plaza and Santa Fe. The island was inhabited by large numbers of marine iguanas to the point where taking a population count became difficult because of the sheer numbers grouped together on the lava rocks. This clumped dispersion is advantageous for the marine iguanas for warmth and protection. We observed the marine iguanas up close on the beach as well as from a distance when we walked up to the top of the cliff, known as Albatross airport, to see the blowhole. To the best of our ability, we averaged a total count of 729 individuals with an approximate ratio of adults to juveniles of 3:1. In the afternoon we went snorkeling around Gardner Bay but I did not see any iguanas either on the beach or in the water.

August 19, 2009: We started our walk on Floreana at Punta Cormorant at 7:45am and the weather was similar to the previous day on Espanola with a temperature of 68°F and overcast skies. The island was different from South Plaza because it has some larger hills of about 1500-2000 feet elevation, called tuff cones. The first beach that we visited was a green-sand beach (olivine in the sand) and I did not observe any marine iguanas (Figure 1.4). Next, we walked inland to view flamingos in a lagoon. The second beach we explored, opposite of olivine beach, consisted of very fine white sand, which is sometimes called the flour sand beach (Fitter, Fitter, Hosking, 2000). The surf line was overrun by stingrays making beach access difficult for many species including marine iguanas and sea lions, thus I did not observe any marine iguanas. In the afternoon we also visited Post Office Bay to play soccer, drop off letters and climb down a lava tube; however, I did not find any iguanas on that beach either.

August 20, 2009: We had a dry landing in Puerto Ayora on the island of Santa Cruz where we then traveled by bus into the highlands of the islands. This excursion took us up into the scalesia, brown, miconia and pampa zones (Figure 1.5). We arrived in the highlands at 9am, where it was 66°F and more humid than at the coastal zone. We were in the highlands to observe the giant tortoises and I did not see any marine iguanas because they inhabit the coastal/littoral zone and not the greener vegetative areas.

August 21, 2009: In the morning we rode in pangas around Rabida before taking our walk on the island (Figure 1.6). While in the pangas we spotted approximately 68 marine iguanas on the rocky shoreline surrounding the island. After viewing the island from the pangas, we had a wet landing on the red-sand beach (red sand from high iron content) at 9:15am. The day was warmer at 72°F and partly cloudy. In the afternoon, after a navigation, we snorkeled around Sombrero Chino or ‘Chinese Hat’ and I sighted approximately ten marine iguanas on the rocky outcroppings as well as one marine iguana swimming in the water. This was the first time I had observed one of the reptiles in the water and was surprised to watch its ease of movement as it whipped its tail side to side.

August 22, 2009: In the morning we climbed the stairs on Bartolome to the lookout point of Pinnacle Rock (Figure 1.7). We did not spot many organisms, in general, because the walk was mainly to learn about the geology and unique volcanic properties on the islands; however, we did find six marine iguanas on the lava rocks surrounding the landing site for the pangas. We also used this day to get additional information about our organism of focus from the naturalist, Danny Saa, which helped fill in any questions we still had about our research

August 23, 2009: For our final walk in the Galapagos, we visited North Seymour at 7:00am. It was a partly cloudy morning, at high tide, with a cool temperature of 67°F. I observed marine iguanas on the lava rocks adjacent to the beach and can approximate the number counted to be 41. While on the beach, I also found the carcass of a dead iguana lying on the sand. I could not determine the cause of death, but it was interesting because it was the first deceased specimen I had come across.

Results

As a species, marine iguanas are well suited to their surroundings, thanks to adaptations as the reptiles evolved from their South American ancestors. “Geography is life’s limiting factor…Speciation and life itself-is ultimately a matter of warm and cool currents, rich and bare soils, deserts and forests, fresh and salt waters, deltas and jungles and plains” (Dillard, 1988). With the environment that the archipelago provides, these iguanas have filled a niche uniquely their own.

Physical Characteristics: The marine iguana has unique physical characteristics that make it well-adapted to its niche and aquatic lifestyle. The reptiles have flattened and rounded faces which allow them to reach algae on submerged lava rocks (Figure 1.8). If marine iguanas had long snouts like their mainland ancestors or even the land iguanas on the islands, they would be unable to feed on the marine algae because it grows low on the rocks. The iguanas also have longer claws that improve their ability to grip slippery algae-covered rocks (Saa, personal communications, August 18, 2009). These longer claws aid the iguanas when feeding because they can remain on the rocks even when there are strong tidal currents and surge. Another physical characteristic of marine iguanas is their flattened tails which provide greater propulsion and ease of movement in the water.

Swimming: Not only is the marine iguana the only lizard species to adapt to an oceanic environment, it has also gained a remarkable ability to swim. “When in the water this lizard swims with perfect ease and quickness, by a serpentine movement of its body and flattened tail-the legs being motionless and closely collapsed on its sides” (Darwin, 2001). The iguanas are capable of breathing under water and are able to hold their breath for up to sixty minutes while submerged (Saa, personal communications, August 22, 2009). Generally it is the larger males that dive at high tide (usually ten minutes) because reptiles lose body heat quickly and must then return to the surface to sun-bathe (Saa, personal communications, August 18, 2009). The smaller juveniles and females tend to feed at low tide on the exposed tidal rocks.

Food source: Marine iguanas are herbivores that feed almost solely on the Ulva red and green algae (Constant, 2007). However, they have also been known to feed on their own feces and that of sea lions and crabs (Fitter, Fitter, Hosking, 2006). The unique convergence of currents surrounding the Galapagos makes marine algae abundant because upwelling churns nutrients to the ocean’s surface (Chupp, personal communications, August 10, 2009). While adapting to its nice, the marine iguana took advantage of a plentiful resource where little competition existed. In years where El Niño has caused a scarcity of algae, the iguanas have been sighted eating Sesuvium (Saa, personal communications, August 23, 2009). Scientists have also discovered that during times of famine lizards lose weight as well as length. They are capable of shrinking their vertebrae, disguising portions of their bones, and then regrowing when food sources return (What You Will See, 2008). This is a unique and incredible adaptation that benefits the iguanas during adverse conditions.

Salt expulsion-Marine iguanas ingest an abundance of salt when they eat marine algae so they must get rid of excess salts. The reptiles have an uropigyal gland for filtering salts from the bloodstream which are then sneezed out in a brine solution ((Saa, personal communications, August 17, 2009). The iguanas can regularly be seen sneezing, which is the only noise that the lizards make. This behavior also contributes to the iguana’s appearance because it leaves salty crust on its face. The salt spray is ejected through the nostrils about two feet and it is thought the behavior could even be an adaptation to warn away or scare predators (Saa, personal communications, August 18, 2009).

Habitat: The marine iguana has limited competition for resources within its niche because its habitat is the coastal/littoral zone (Chupp, personal communications, August 9, 2009). The iguanas live on land and feed inshore and in the intertidal zone as well as often being found sunning on the sand or nearby lava rocks. The number of individuals on each beach varies island to island (Table 1.1).

Intraspecific competition: These reptiles exhibit gregarious or thigmotatic behavior, meaning that they benefit from touching other individuals. The marine iguanas often herd together in groups and even lie on top of each other to stay warm (Saa, personal communications, August 18, 2009). For this reason, they are in clumped dispersion around the islands and generally the clumps are high density around lava rocks where Ulva algae are more abundant (Figure 1.10).

Coloration: Juvenile marine iguanas begin life almost completely black in color. This allows the young reptiles to be camouflaged against the lava rock and avoid predation by the Galapagos hawk. As the iguanas mature they gain reddish patches (males are more colorful than females) along the length of their body but retain much of their black color. Being dark in color is beneficial to marine iguanas because it allows them to store heat more efficiently-which is critical for this cold-blooded reptile which forages for food in the cold ocean waters and must rely on the sun to maintain body temperature.

Homeostasis: The marine iguana is poikilothermic, meaning that it can adapt to wide temperature variations, so even though it’s optimal body temperature is 95-98°F, it can withstand extremes such as 104°F and go as low as 75.2°F (Constant, 2007). With their dark body color, the iguanas can store heat as well as being able to warm themselves quickly by sunning. To avoid over-heating, the lizards are often seen switching their direction toward the sun and lifting themselves above the ground to allow air to circulate (Saa, personal communications, August 18, 2009).

Development/Maturation: Juveniles are smaller versions of adult iguanas and can be recognized by their darker colorations (almost completely black) as well as their smaller size of about ten centimeters (Figure 1.11). “The males are larger and more brightly and distinctly colored than the females” (Fitter, Fitter and Hosking, 2000). Full-grown adults are usually three feet in length but can be as long as four feet because the size of adult of iguanas varies island-to-island (Darwin, 2001). After becoming full-grown, the lizards reach sexual maturity at 5-6 years of age. The lifespan of a marine iguana is 25-30 years (Saa, personal communications, August 18, 2009).

Mating: These generally nonaggressive lizards become territorial during the mating season (February) because males want to protect their group of females (Saa, personal communications, August 18, 2009). Males create a territory (approximately 100 square feet) and will drag females back if they try to leave. During this season, makes become redder in color and often a green color is also present on the back, forelegs and hind legs (Saa, personal communications, August 18, 2009). This type of sexual selection allows the female marine iguana to choose her mate best on the best male colors. Typically, the more red on the male, the more likely a female thinks that male is worth reproducing with. The male iguana will also lift its head up and down, like shaking his head in a “yes” motion during mating season to attract a female for mating. After mating, females need sandy beaches for nesting and compete for space so they can lay their eggs. “The females excavates a nest in the sand, a burrow up to one meter long, then lays a clutch of up to four leathery elongated eggs, which take between three and four months to incubate” (Fitter, Fitter Hosking, 2009).