Feeding Ecology of Four Species of Sea Urchins (Genus <I>Echinometra</I

BULLETIN OF MARINE SCIENCE, 81(1): 85–100, 2007 FeeDinG EcoloGY of Four SPecies of Sea URCHins (Genus ECHINOMETRA) in OKinaWA Yuji Hiratsuka and Tsuyoshi Uehara Abstract We studied the feeding ecology of four Echinometra species at five different sites in Okinawa in terms of distribution, food availability, and gut contents. Echinome- tra sp. A was widely distributed from the lower intertidal to submerged areas, while Echinometra mathaei (Blainville, 1825) was common in the upper subtidal and lower intertidal areas. Echinometra sp. C and Echinometra oblonga (Blainville, 1825) were restricted to the upper intertidal zone. Although the diets of Echinometra spp. showed remarkable differences among sites and between seasons, 39.2%–80.7% of their gut contents usually consisted of plant material. Echinometra sp. A ingested a greater variety of plants than the other three species. The availability of major plants in the field was generally proportional to their abundance in the gut, except when Echinometra relied on imported drift plants or when feeding was restricted because of severe hydrodynamics. Benthic grazing was the primary feeding mode of Echinometra living on rock platforms, while drift feeding was pronounced when macrophytes were abundant in the habitat or surroundings. Our results suggest that despite differences in physical and nutritional environments among their micro- habitats, the four urchin species have essentially similar feeding types, modes, and preferences. The sea urchinEchinometra mathaei (Blainville, 1825) is reported to be widely distributed throughout the tropical to warm Indo-Pacific regions and shows extensive morphological variation in test shape and spine color (Mortensen, 1943). Tsuchiya and Nishihira (1984, 1985) divided E. mathaei at Okinawa into two types—type A and type B, based on the differences in distribution pattern, habitat, and agonistic behavior. Uehara and Shingaki (1984) demonstrated that these two types would not cross-fertilize and differed in larval morphology and karyotype. Uehara and Shin- gaki (1985) and Uehara et al. (1986, 1990) discovered two additional types—type C and type D—from studies of chromosomes, spicule characteristics, and gamete incompatibility. Biochemical studies of enzyme electrophoresis (Matsuoka and Hat- anaka, 1991), mitochondrial DNA (Palumbi and Metz, 1991), and the binding of gamete recognition proteins (Metz and Palumbi, 1996) suggest that these four types are distinct but very closely related species. Currently, type B is regarded as Echinometra mathaei (Arakaki et al., 1998), while type D is recognized as Echinometra oblonga (Blainville, 1825), which may be a cryptic species composed of at least three species (Arakaki and Uehara, 1999). It is likely that both type A and type C are new species (Arakaki et al., 1998), but because they have not been described and named, types A and C are herein referred as Echinometra sp. A and Echinometra sp. C, respectively. Recent studies of the taxonomy, ecology, and molecular biogeography of sea urchins have referred to the occurrence of these four species of Echinometra from various lo- cations in Indo-West Pacific P( alumbi, 1996; Uehara et al., 1996; Arakaki and Kusen, 2000; Peyrot-Clausade et al., 2000; Paulay, 2003; Appana et al., 2004). Feeding ecology of E. mathaei in various regions of the Indo-West Pacific has been examined through gut content analysis or direct observation (Mortensen, 1943; Khamala, 1971; Herring, 1972; Black et al., 1984; McClanahan, 1988; Coppard and Bulletin of Marine Science 85 © 2007 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 86 BULLETIN OF MARINE SCIENCE, VOL. 81, NO. 1, 2007 Campbell, 2005). These studies have concluded that E. mathaei is a generalist her- bivore, utilizing a variety of vegetal diets by grazing on the substrate or by captur- ing drift materials. However, it should be recognized that some of these studies of E. mathaei may include Echinometra sp. A and/or Echinometra sp. C. Hawaiian E. mathaei and E. oblonga, probably a different species from theE. oblonga of Okinawa (Arakaki and Uehara, 1999), have been the subject of numerous ecological studies that have suggested that Hawaiian populations of the two species mainly subsist on drift materials washed into their burrows (Kelso, 1970; Russo, 1977; Ogden et al., 1989; Hart and Chia, 1990). Unfortunately, very few studies have documented the feeding habits of Echinometra sp. A and sp. C. Much more is known about the feeding strategy of Echinometra lucunter (Linnaeus, 1758), a congeneric species distributed from Florida and Bermuda through the Caribbean and the Gulf of Mexico to Dester- ro, Brazil (Mortensen, 1943; Stevenson and Ufret, 1966; McLean, 1967; McPherson, 1969; Abbott et al., 1974). For example, Abbott et al. (1974) thoroughly documented its feeding behavior, and provided evidence that both drift plants and attached algae were its major foods. Elsewhere, brief reference has been made to the diets of Echi- nometra vanbrunti A. Agassiz, 1863, occurring mainly from central California to Zorritos on the coast of Peru (Mortensen, 1943), and Echinometra viridis A. Agassiz, 1863 confined to south Florida and the West Indies (Mortensen, 1943; McPherson, 1969; McClanahan, 1999). To understand the feeding ecology of the species of Echinometra in Indo-Pacific region, our study focused on the distribution, food availability, and gut contents of the four species at five different sites on Okinawan coral reefs. Materials and Methods Study Sites.Field studies were conducted from August 2003 to March 2004 at five different sites (Usahama, Bise, Onna, Yonashiro, and Ikei) located on the coral reefs at Okinawa Island (Fig. 1): Usahama is located at the northernmost part of Okinawa Island, and consists of a narrow fringing reef approximately 1 km long and 150–200 m wide. The moat is a submerged habitat 0.3–1.5 m in depth. The reef margin is affected by both strong wave action at high tide and exposure at low tide. Two study areas were established (1) in the moat to study Echinometra sp. A and E. mathaei, and (2) at the reef margin to study the other two species (Fig. 2A). Bise is on the northwest coast of the Motobu Peninsula in the northern part of Okinawa Island and consists of a well-developed fringing reef. A seagrass bed of about 1 km long and 100–200 m wide occurs on the landward side of the reef flat. The study area lies at the northern part of the seagrass bed, where the depth ranges from 0.5 m at low tide to 2 m at high tide. The bottom of the study area is composed primarily of coral rubble, which is covered by dense stands of seagrass, Thalassia hemprichii(Ehrenberg, 1832), branching corals, and a variety of macroalgae throughout the year (Fig. 2B). Onna is situated on a narrow fringing reef, 1 km in length and 100–300 m in width, on the west coast of the central part of Okinawa Island. A study area was established on the intertidal wave-cut platform near the overhanging cliff (Fig. 2C). The platform, having an area of approximately 250 m2, is subjected to complete exposure during spring tides and intense wave action during high tides. Yonashiro, on the northwest coast of the Katsuren Peninsula of Okinawa Island, has a shallow reef slope extending for several kilometers. The shallow reef bottom consists of a complex of seagrasses and macroalgae interspersed with small round sandy flats (10–20 m in diameter) HIRATSUKA AND UEHARA: FEEDING ECOLOGY OF ECHINOMETRA IN OKINAWA 87 Figure 1. Location of the five study sites at Okinawa Island. lacking vegetation (Fig. 2D). A study area was established within one of the sandy flats (about 300 m2 in area). Ikei, one of the neighboring islands located off the east coast of the Katsuren Peninsula, is surrounded by a 400–600 m wide fringing reef. A study area was set up in the subtidal region near the shore on the south coast of Ikei (Fig. 2E). Distribution of Sea Urchins.The distribution of Echinometra spp. was investigated by measuring the density of sea urchins at each site (except Ikei) during August and Septem- ber 2003. At Ikei, measurements were made in February 2004 because of difficulty in finding urchins in summer 2003 when the bottom was almost entirely covered by dense Sargassum spp. The density of urchins was measured at each site by counting all specimens > 1 cm in test diameter that occurred along ten 10 × 1 m belt transects laid parallel to the shore at intervals of 1.0–1.5 m. Food Availability.Food availability (benthic cover) for Echinometra spp. in the field was investigated at each site in August and September 2003 (summer) and in February and March 2004 (winter). The percent cover of various components was visually estimated from 50 × 50 cm quadrats placed at four evenly spaced points along each of ten 10 × 1 m belt transects deployed for the measurement of urchin density (i.e., 40 quadrats per site). The points where the quadrats were laid in the summer investigation were permanently marked with nails driven into the bedrock, and were surveyed again in the winter. Benthic composition was categorized into the following groups: seagrasses, macroalgae (> 2 cm in height), turf algae (< 2 cm in height; the algal community covering hard substrates, comprising various species of blue-green algae and microscopic algae such as Cladophora, Ec- tocarpus, Sphacelaria, Ceramium, Centroceras, Taenioma, Herposiphonia, and Polysiphonia), encrusting coralline algae, live coral, dead bleached coral, Porifera, Foraminifera, Ascidia, 88 BULLETIN OF MARINE SCIENCE, VOL. 81, NO. 1, 2007 Figure 2. Profiles of the study sites showing substrates, vegetation, and distribution patterns of Echinometra (°: Echinometra sp. A; l: E. mathaei; r: Echinometra sp. C; p: E. oblonga). Rectangles represent the study areas.

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