SUBTIDAL ENVIRONMENTS AND ECOLOGY 



125 



effects of storms on the corals and butterflyfishes (chaeto- 

 lontidae) of "Tunnel Pinnacle" and other pinnacles at 

 Enewetak. 



The lack of significant cementation on upper surfaces 

 of patch reefs and pinnacles by coralline algae may reduce 

 greatly the amount of wave energy required to crumble 

 the structure. Protection of the lagoon from storm swells 

 by a complete or nearly complete atoll rim with no deep 

 passages may also be important. At Ujilang Atoll, which 

 has no major passes allowing ocean swell to enter the 

 southern lagoon, lagoon margin patch reefs examined were 

 near planar on top at about the level of spring low tides 

 with near vertical edges dropping to a few meters in 

 depth. The patch reefs were well cemented by coralline 

 algae on their upper surfaces. Ujilang is exposed to essen- 

 tially the same oceanic conditions (wind, waves, and 

 storms) as Enewetak, yet patch reefs of such morphology 

 are not found at Enewetak. 



Herbivory in Subtidal Communities 



Much work on herbivory and its impact on the ecosys- 

 tems at Enewetak has been undertaken on intertidal areas 

 because of the large, accessible area of such environments 

 at the atoll and the abundance of herbivores there. Her- 

 bivory is a major factor influencing subtidal communities in 

 the lagoon and on the seaward margin. Evidence of intense 

 grazing pressure can be found in many subtidal areas, both 

 on hard and soft substrata. 



Unlike the intertidal areas, subtidal areas are accessible 

 to herbivores at all times. On exposed rock substrates, 

 both seaward and lagoonward of the reef flat at Enewetak, 

 tooth marks from the action of grazing fishes arc nearly 

 ubiquitous in areas to at least 15 to 20 m depth. Many 

 show considerable erosion from grazing (Fig. 23) with 

 angular facets on the rock, and deep tooth marks, particu- 

 larly from large parrot fishes, are often densely grouped. 



The principal grazers of hard substrates at Enewetak 

 are fishes, particularly parrot fishes (Scaridae) and sur- 

 geonfishes (Acanthuridae). In general, algae on subtidal 

 rock surfaces are close-cropped except in the case where 

 the species may be heavily calcified {Halimeda spp.) or 

 potentially toxic or distasteful {Li;ngbia sp). In this respect 

 subtidal rock surfaces are not qualitatively different from 

 the intertidal reef flat. Macroalgae and algal films are found 

 at all depths in the lagoon. 



Surgeonfishes show a well-defined zonation among this 

 largely herbivorous family. Acanthurus triostequs. A. achil- 

 les. A guttatus. and A. lineatus are principally found on 

 windward and leeward reef flats, back reef areas, and the 

 spur and groove zone — all shallow-water environments. In 

 somewhat deeper water on the seaward reefs, lagoon 

 patch reefs, and pinnacles are found species of 

 Ctenochaetus, Zebrasoma, Acanthurus nigrofuscus. and 

 A. olivaceus. Two species of Acanthurus at Enewetak, 

 A. thompsoni and A. bleekeri, feed on zooplankton as do 

 most species of Naso. The species of surgeonfishes with a 

 well-developed gizzard-like stomach commonly feed on 



sediment bottoms and ingest, along with the algae, consid- 

 erable sand (Randall, 1956; Hiatt and Strasburg, 1960) 

 (Fig. 23). 



Herbivory occurs on sediment bottoms where macroal 

 gae and microalgac occur. Macroalgae can occur as dense 

 stands, as exemplified by the species of Halimeda and 

 Caulerpa, and many are probably unpalatable to her- 

 bivores. Microalgae can occur as nearly invisible films on 

 sediment grains on the surface of the sediment bottom, but 

 easily apparent films (algal mats) covering many square 

 meters are often found from a few meters depth to the 

 deepest portion of the lagoon. Epiphytic algae also grow 

 upon larger algae and are often more desirable to her- 

 bivores than the plants on which they grow. 



Invertebrate grazers of rock surfaces are not as impor- 

 tant at Enewetak as in the western Atlantic. In many 

 Caribbean locations sea urchins, particularly Diadema antil- 

 larum, are extremely abundant and as herbivores have an 

 impact equaling, or exceeding, that of fishes (Ogden, 

 1976; Ogden and Lobcl, 1978). Sea urchins, particularly 

 diademnids, are not nearly as abundant, except in localized 

 areas, on reefs at Enewetak. 



Fishes are important herbivores of sediment bottoms at 

 Enewetak. The principal herbivore families of hard 

 substrata, parrot fishes and surgeonfishes, range onto sedi- 

 ment bottoms also (Fig. 23). Different species from those 

 that remain on the reef are often involved. As distance 

 increases from the shelter of the reef, the grazing pressure 

 of reef-based herbivores decreases. They are exposed to 

 increasing risk of predation with increasing distance from 

 shelter. Therefore, soft bottoms near reef shelter are more 

 heavily grazed, often to the extent that no visible bcnthic 

 plant growth except the less desirable species mentioned 

 previously occur near the reef. This results in a 

 phenomenon most easily visible from the air in which reefs 

 arc surrounded by a light colored band, compared to sedi- 

 ment substrata farther away, representing the denuded 

 substatum close to the reef. This area has been termed a 

 "halo" (Randall, 1965) or the "Randall zone" (Ogden, 

 1976) and is a feature found near both Indo-West Pacific 

 and Atlantic reefs. 



Other herbivores, particularly invertebrates, exist far 

 from the reef, either remaining on or above the sediment 

 surface or burying and burrowing in the bottom. Dense 

 stands of macroalgae provide excellent shelter for small 

 herbivores, both fishes and invertebrates which can hide 

 among the thalli. Although these macroalgae are not pri- 

 mary foods for these herbivores, the environment created 

 provides abundant spaces for epiphytic algae (and cpizoic 

 organisms also) w)iich are suitable for the small herbivores. 



In areas without dense algal cover, burrowing organ- 

 isms can function as herbivores without the need of 

 shelter. Irregular sea urchins (Spatangoidea) occur abun- 

 dantly in Enewetak Lagoon sediment bottoms and 

 apparently process sediment grains for the algal matter on 

 their surfaces. These and other "sediment processors" 

 must pass relatively large amounts of sediment to obtain 

 sufficient organic matter. 



