Grazing Effect on Sea Grasses by Herbivorous Reef Fishes...

Grazing Effect on Sea Grasses by Herbivorous Reef Fishes in the West IndiesAuthor(s): John E. RandallSource: Ecology, Vol. 46, No. 3 (May, 1965), pp. 255-260Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1936328 .

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Spring 1965 GRAZING OF SEA GRASSES BY NVEST INDIAN FISIIES 255

,and 0. Warbach. 1960. Small-mammal populations of a Maryland woodlot. Ecology 41: 269-286.

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Whitaker, J. O., Jr. 1961. Habitat and food of mouse- trapped young Rana pipients and Rana clamitans. Her- petologica 17: 173-179.

Wright, A. H., and A. A. Wright. 1949. Hanclbook of frogs and toads of the United States and Canada. Ithaca, N. Y.: Comstock. 640 pp.

GRAZING EFFECT ON SEA GRASSES BY HERBIVOROUS REEF FISHES IN THE WEST INDIES'

JOH N E. RANDALL Institute of Marine Biology, University of Puerto Rico, Mayagutez, Puerto Rico

Abstract. A conspicuous band of bare sand averaging about 30 ft in width often separates reefs and beds of sea grasses (Thalassia and Cymodocea) in the Virgin Islands and other islands of the West Indies. This zone of sand appears to be the result of heavy grazinlg by parrotfishes (Scarnts and Sparisomtla) and surgeonfislbes (Acanthuruts) that stay close to reefs for shelter frorm predaceous fishes.

Floating sea grass fragnments are eaten by the halfbeak Hemiramiphus brosiliceisis and occasionally by the Bermuda chub Kyphosus sectatrix and the triggerfish Melichlthys radulla.

Within the beds, the sea grasses are fed upoIn by the simiall residlent parrotfish Sparisonlia radians, the echinoids Lytechinus, Tripuieustes, and D)iadena, the green turtle Cheloniia nziydas, and in part by the queen conch Strom ibuis gigas, and manatee Trichechts tnianiatuts. It is the author's opinion that if the pre-Columbian population of the green turtle could be restored and its fishery properly regulated, the enormous production of the sea grasses in the Caribbean region could be realized more fully for the benefit of man.

The mlarinle spermiiatoplhytes known as sea grasses cover immilense areas of shallow sea bottom in the tropical WVestern Atlanitic. The two most abundant species are turtle grass, Thcalassia testut- dinuml, and manatee grass, Cyniiodocca vianatoruw (- Syrinyodiium filifornue). These plants are imil- portant in resisting erosioni, trapping fine sedi- nienits, and( provi(dinig for attachmnenit, shelter, and foodl for a large number of organismiis (nmaniy as juveniles), amonig them certain conmmlercially im- I)ortant fishes, molluscs, and crustaceans.

In recent years the sea grasses have received conisiderable attentioni fromii biologists. The more significanit papers are those of Tlhorne (1954), Voss and Voss (1955), Huumm (1956), Odum (1957). Margalef and Rivero (1958), Burkholder, Burkholder, and Rivero (1959), Oduim, Burk- holdler, and Rivero (1960), Phillips (1960, 1962), Thomiias, Moore, and Work (1961), Strawn (1961), ancl Moore (1963).

Beginning in 1958 the author participated in a mlarinie b)iological survey of St. John, Virgin Is- lands, with the support of the Nationial Science Founldation (G-5941), Federal Aid in Fish Resto- rationi (Dingell-Johuisoni Project F-2-R of the Vir-

gina Islancds) andl the Nationial Park Service. One of the principal projects of the survey was the clhartinig of the marine enviroinmenits of the island to the ten-fathomii curve (Kunmpf and Randall 1961). -Most of the sea bottom arotunid St. Johni to this depth was found to coInsist of beds of sea grasses.

Aerial photographs from the U. S. Geological Survey were extensively used in the preparation of the clhart of the marine habitats of St. Johln. B3ecause of the clarity of the sea in the Virgin Islands, the shallow water bottom topograplhy is usually discernible on the photographs. One very striking feature of the bottom in the Virgin Is- lands is a band of bare sand that often occturs between the fringing reef and the beds of sea grasses farther offshore (Fig. 1). This feature made the task of mapping the marine environments mucl simpler. It is often not possible to distinguislh between areas of hard bottom and sea grass in black and white aerial photographs. If, however, a pale strip separates the two, their demarkationl is evident. In the series of aerial photographs of St. John and nearby islets the pale band of sand, ahout 30 ft in average width, may be observed intermittently for a total of about one half of the 58 miles of coastline.

' Joint contribution of the Institute of MIarine Science of the University of Miami (No. 585) and the Institute of Mariine Biology, University of Puerto Rico.



256 JOHN E. RANDALL Ecology, Vol. 46, No. 3

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FIG. 1. Aerial photograph of Lameshur Bay, St. John, Virgin Islands showing the pale band of sand separating the reef along the shore from the dark-colored sea grass beds to seaward. "A" designates the location of the small artificial reef of Fig. 2 at Yawzi Point, and "B" the site of a large reef of concrete blocks surrounded by sea grass. The island of Tortola, British Virgin Islands, is in the background. Photograph taken in April 1961.

Speculation naturally arose as to the origin of this feature. Why do the sea grasses fail to grow adjacent to many of the reefs? At first the band was thought to represent a zone of shifting sand between the shore reef and the sea grasses. It was believed to be sloping, with coarser material near the rock and coral of the shore and finer sediment at the outer edge. Presumably the sea grasses could grow only where the sand was not so coarse, the bottom not so sloping, and the effect of surge less. Direct observation of the sand zone at vari- ous localities quickly dispelled this hypothesis. The sand strip is equally well developed in protected bays and off promonitories where wave action is greater. It exists from only a few feet of water to 60 ft or more. (Thalassia was found to a maximum depth of 43 ft in southern St. John; Cymodocea occurred to 72 ft. These depths are greater than those recorded for these plants by most authors, probably because light penetrates the sea deeper in the Virgin Islands than in most Caribbean localities.) Frequently the baud of sand shows no obvious slope from the inner to the outer edge. An analysis of samples of sediment near the fringing reef and from the outer edge of the band, kindly supplied by Francis P. Shepard of the Scripps Institution of Oceanography, showed Ino significant difference in particle size.

Attention was then turned to the possibility that the sand belts are a result of heavy grazing of sea

grasses by herbivorous reef fishes. Tied to the reefs for shelter, these fishes venture only a short distance away, probably to avoid predation by barracuda, jacks, and kingfish. The grazing of the reef fishes would therefore be so concentrated near reefs that the sea grasses could not sustain growth there.

As a first step toward determining if any reef fishes feed upon sea grasses, approximately two ft2 of mixed Thalassia and Cymodocea were dug from a grass bed and placed next to a reef off Yawzi Point, Lameshur Bay, St. John, at a depth of nearly 40 ft. Parrotfishes (Scarus spp. and Sparisow,na spp.) began feeding upon the grasses almost immediately. Six individuals were eating the grass of the first shovel full when the second was brought to the site of transplantation. Twenty- four hours later the two ft2 of grass were found to be almost completely consumed. Only a few basal parts of blades of Thalassia remained.

On April 8, 1960, a one-foot wide strip of sea grass, about half Thalassia and half Cyrnodocea, was planted across the sand band at the same locality. The width of the sand at this place is 20 ft. Two days later the sea grasses were eaten five ft away from the reef. By April 20 the grasses had been removed to a distance of seven ft from the reef, and they were partly grazed all the way to the grass bed. By May 15 they were completely eaten to the edge of the grass bed except for a few basal pieces of Thalassia.

On May 5, two ft2 of Thalassia and Cyntodocea were transplanted next to the reef at Yawzi Point and covered with an enclosure of one-inch mesh chicken wire, one yd2 in area, to exclude the larger herbivores. On June 5, the sea grasses were noted to be entirely intact, although no new growth was observed.

On October 26, 1960, a corridor of concrete blocks (each measuring 16 X8 X 8 in. and con- taining two holes 5 in. square) was laid by Robert E. Schroeder at a depth of 30 ft from the edge of the reef at Yawzi Point (at approximately point A in Fig. 1) across the 20-ft zone of bare sand into the sea grass. At the end of the corridor a circular pile of blocks about 5 ft in diameter was built (Fig. 2). Reef fishes from the Yawzi Point reef could then move to the grass with the shelter afforded by the holes in the individual blocks and the spaces between the blocks.

Gradually a band of bare sand developed around the pile of blocks in the grass zone. On July 2, 1961, a second photograph was taken of the artificial reef (Fig. 3). A broad area of sand is clearly visible around the structure.

The little reef of concrete blocks was revisited



Spring 1965 GRAZING OF SEA GRASSES BY W"EST INDIAN FISHES 257

h~~~~~~V

FIG. 2. Artificial reef of concrete blocks built in sea grass in Lameshur Bay, St. John (at "A" of Fig. 1) on Oct. 27, 1960. A corridor of concrete blocks (right, foreground) crosses band of bare sand to join the fringing reef of Yawzi Point.

November 24, 1963. There was no perceptible change in the nature of the bare sand surrounding it. The dominant fishes living among the blocks as adults were not herbivores; they were the squir- relfish Myripristis jacobus and grunts (Haemulon spp.) which are carnivores. A large green moray (Gymnothorax funebris) occupied the central cav- ern of the terminal pile of blocks (a small green moray, possibly the same eel, was observed at the same place July 2, 1961). These fishes are pri- marily nocturnal and tend to hid during the day. The herbivorous reef fishes of comparable size do not ordinarily restrict themselves to so circum- scribed an area during the day. Their range of movement is much larger; thus adult fishes which would graze on plants on or near the artificial reef also forage for food on the natural reef nearby.

The concrete blocks were notably devoid of benthic organisms which grow well above the substratum. In three years one might have expected well-developed gorgonians and corals, yet no gorgonians were observed and the scattered coral colonies (Agaricia agaricites, Favia fragum, Po- rites porites, and Montastrea annularis) were not more than 30 mm in diameter and 15 mm in height. There were a few patches of Millepora alcicornis on the blocks, some a little larger in area than the corals. The sponges included Microciona junipe- rina, Haliclona variabilis, Callyspongia sp., Hali- sarca sp., and Echinostylinos sp.; the latter, a bright red encrusting species, was the most abundant. The most common tunicates were the soli-

FIG. 3. The artificial reef of Fig. 2 as it appeared July 2, 1961. A broad zone of sand with very sparse growth of Thalassia is now evident around the reef.

tary forms Ascidia nigra and Microcosmus exas- peratus. Arca zebra, Chama macerophylla, Leuco- zonia nassa, and Cerithium litteratum were the most obvious of the molluscs. Perhaps the most conspicuous of the invertebrates were two large individuals of the polychaete Sabellastarte mag- nifica and several subadult Diadema antillarum.2 Algae were at best a low stubble, and coralline reds such as Goniolithon boergesenii seemed to cover more area of the blocks than other plants. The poor development of the algae is probably a result of limited substratum (earlier in the succession of organisms on the blocks, filamentous algae were more prominent, but in time sessile animals covered progressively greater areas) and heavy grazing by herbivores (overgrazing of algae by plant- feeding fishes has been demonstrated by Randall 1961).

A large artificial reef of 800 concrete blocks was built in Lameshur Bay (B of Fig. 1) far from the fringing reefs in 29 ft of water in mixed Thalassia and Cymodocea on April 6, 1960. Few fishes came to this reef as adults (most colonized the reef as juveniles or prejuveniles), and even after two years and four months when all fishes were col- lected from the reef (Randall 1963a), there were relatively few large herbivores. At the time of this collection, no bare sand areas had developed around any part of the reef, although the growth of sea grasses was more sparse within a few feet of the piles of blocks.

2 For assistance in the identification of invertebrate animals on the concrete blocks, the author is indebted to Peter W. Glynn, Ivan M. Goodbody, Willard D. Hart- man, and Germaine L. Warmke.



258 JOHN E. RANDALL Ecology, Vol. 46, No. 3

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4.

i~~~~~~~~~~~~~~~~~~~~~~~~~~~i

5 ^~~~~~~~~k 9. . .

FIG. 4. Aerial photograph of a small patch reef east of New Providence Island, Bahamas. A pale, concentric zone of sand separates the reef from the darker sea grass bottom to the periphery. A small sloop may be seen at the top of the figure. Photograph taken in June, 1962.

The sand band is by no means restricted to the Virgin Islands but has been observed by the author at a number of other West Indian islands. Nor is it a phenomenon restricted to the transition zone between fringing reef and sea grass; it may also be seen as a concentric band around patch reefs such as those which occur commonly in the Ba- hamas (Fig. 4).

The better-developed reefs, hence usually ones with more shelter for fishes, have broader zones of sand separating them from sea grass beds, probably reflecting the greater population of reef fishes. Low-lying reefs with few ledges and holes for shelter for reef fishes have narrow zones of sand or none at all.

Although other factors such as kind of sediment and degree of wave action may exert an effect in certain areas, the principal cause of formation of the band of bare sand or sparse sea-grass growth between well-developed reefs and sea grass beds is the grazing of plant-feeding reef fishes.

Of the herbivorous reef fishes, the most impor- tant group to feed upon the sea grasses, when afforded the opportunity, is the Scaridae (parrotfishes). Nearly all West Indian species of Scarus and Sparisoma (Schultz 1958; Randall 1963b) have been observed grazing on the grasses, and fragments have been found in the digestive tracts of many, at times in large amounts. For example, Cymodocea made up 95% of the volume of the stomach contents of one specimen of Scarus gua- camaia, 516 mm in standard length from Europa Bay near Lameshur, St. John. In the Virgin Is-

lainds area, the parrotfishes appear to be the dominant family of fishes on a weight basis (Randall 1963a).

Adults of all three West Indian species of surgeonfishes (Acanthurus) have been observed feeding upon sea grasses. This was not expected because the jaws and dentition of these fishes are not notably powerful, and the sea grasses are tougher than most species of algae eaten by species of Acanthurus. Fragments of the two common sea grasses have been found among the stomach contents of two large individuals of A. chirurgus (25% by volume of Thalassia) and two of A. bahianus (40% and 85% by volume of Thalassia). The stomach contents of three specimens of A. coeruleus consisted of Cymodocea (about 50% by volume), and a few fragments were found in another specimen; no Thalassia has yet been taken from the stomach of this surgeon- fish, however.

Identification of plant material from parrotfishes and the surgeonfishes except Acanthurus coeruleus may be difficult because these fishes grind up their plant food. The parrotfishes uti- lize their unique pharyngeal mill, often in con- junction with sand or coral rock fragments scraped from the dead coral during feeding (the importance of these fishes in sediment formation has been discussed by Cloud 1959 and Bardach 1961). Acanthurus chirurgus and A. bahianus have thick- walled, gizzardlike stomachs, and they ingest much more sedimentary material with their food than A. coeruleus which lacks a thick-walled stomach.

The only other family of West Indian reef fishes of moderate to large size which is predominately herbivorous is the Kyphosidae (rudderfishes), of which there are two western Atlantic species (Moore 1962), Kyphosus sectatrix and K. incisor. These fishes have not been observed by the author feeding on sea grasses on the bottom, but two of 13 specimens of K. sectatrix (238 and 279 mm standard length) which were examined had eaten pieces of Cymodocea manatorum. In the stomachs of both fish there was also a large amount of Sargassum natans. Since this species of Sargas- sum floats at the surface, it is presumed that the fish had eaten only pieces of manatee grass which had become detached from the bottom and were floating. The two species of Kyphosus feed more on benthic algae, and possibly they occasionally graze on sea grasses on the bottom.

The omnivorous triggerfish Melichthys radula has also been found with large amounts of Sargas- sum natans and Cymodocea in its stomach.

Another fish which has been observed feeding upon Cymodocea fragments at the surface is the halfbeak known as the ballyhoo (Hemiramphus



Spring 1965 GRAZING OF SEA GRASSES BY WEST INDIAN FISHES 259

brasiliensis). The stomach contents of 11 specimens (170 to 217 mm standard length) taken from Lameshur Bay, St. John, on April 28, 1959, consisted of about one-half Cymodocea ground very finely and one-half the remains of small fish (Jen- kinsia lamprotaenia). The manatee grass was not ingested accidentally, for one fish contained only this, and later observations from a dock in St. Thomas demonstrated that the halfbeaks select individual fragments of the grass; the strands pass at a constant rate into the mouth, apparently drawn in by the pharyngeal teeth in the same way a piece of meat is drawn into the mechanism of a meat grinder. Occasionally a second halfbeak would seize a piece of Cymodocea by the free end after the first had begun to consume it. As they ap- proached each other while feeding up the strand, a small struggle ensued and one wrenched the remainder of the grass from the other. The author also observed ballyhoo feeding upon Cymodocea at the surface in a flotsam of Thalassia and Cymo- docea off the dock at the Institute of Marine Sci- ence, University of Miami, on November 3, 1959. Cymodocea was eaten for the most part, but occasional pieces of Thalassia were also ingested. Pieces of the two grasses which were green, straw- colored, or brownish were eaten, but dark brown or black pieces were avoided. Burkholder, Burk- holder, and Rivero (1959) reported Thalassia as the main food of Hemiramphus brasiliensis. They based their conclusion on the study of stomach contents of specimens from Puerto Rico.

The floating pieces of sea grass may ultimately sink to the bottom, probably because of calcareous organisms that begin to grow upon them and make them heavier. While diving, the author has seen parrotfishes rise several feet above a reef to eat slowly sinking pieces of Cymodocea.

Occasional fragments of sea grass are found in the stomachs of predaceous fishes, usually together with the prey. This is especially true of certain grunts (Haemulon spp.) that feed in the grass and sand flats at night and seek the shelter of reefs by day. Since these fishes are normally carnivorous, it may be assumed that the plants are ingested accidentally.

Thus far only the concentrated feeding by fishes on sea grasses near reefs or upon floating pieces at the surface has been discussed. There is one West Indian parrotfish, the relatively small Sparn- soma radians (the largest of many examined by Schultz 1958 was 156 mm standard length) which lives as an adult in the sea grasses and feeds on them. The scalloping of the edges of the blades of Thalassia mentioned by Thomas, Moore, and Work (1961) may be caused principally by this parrotfish. When alarmed, S. radians follows an

escape pattern of darting rapidly away and sud- denly coming to rest motionless in the sea grass, where it assumes a mottled color pattern that makes it difficult to detect. Other species of Sparisoma also practice this mode of escape, but when they attain large size, they are more easily seen.

Contrary to the belief of Robertson (1961), the queen conch (Strombus gigas), a large gastropod which is often found in sea grass beds, feeds in part on Thalassia and Cymodocea; it ingests more algae, however, including those epiphytic on the grasses (Randall 1964). The echinoids Lytechi- nus variegatus and Tripneustes esculentus also feed on Thalassia (Margalef and Rivero 1958; Thomas, Moore, and Work 1961; Moore, Jutare, Bauer, and Jones 1963; Moore, Jutare, Jones, McPherson, and Roper 1963), as does Diadema antillarum (Randall, Schroeder, and Starck, in press).

The green turtle (Chelonia mydas), the most valuable of the sea turtles, is reported (in a review article by Ingle and Smith 1949) to be predomi- nantly herbivorous, feeding mostly on sea grasses. In pre-Columbian days this turtle is said to have been abundant in the Caribbean region. The population has been greatly reduced by the largely unrestricted exploitation of the turtle and its eggs and the encroachment by man of beaches suitable for egg-laying. It is interesting to speculate whether the green turtle ever attained such a population size that the amount of sea grasses became a limiting factor. If so, the population must have been very great.

The manatee (Trichechus manatus) is another animal, highly esteemed as food, which feeds in part on sea grasses in the tropical western Atlantic. The Florida subspecies is protected by law, and in some areas its numbers have apparently in- creased. Little seems to have been done, however, to preserve the West Indian subspecies, and its population has substantially declined (Allen 1942).

At the present time, the utilization of the sea grasses as food by animals, particularly those of economic importance to man, is negligible. The program of Archie Carr and associates to prevent further reduction in the number of green turtles in the Caribbean and to reintroduce young turtles into depleted areas is highly meritorious and wor- thy of the cooperation of all (Carr 1956; Carr and Ingle 1959; Parsons 1962). If the former large population of Chelonia mydas could be fully restored and the resource wisely used, the immense productivity of the sea grass beds of the Caribbean could be more fully appreciated by man.



260 JOIIN E. RANDALL Ecology, Vol. 46, No. 3

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