Ecological Studies of Black Coral in Hawaii!

Ecological Studies of Black Coral in Hawaii! RICHARD W. GRIGG2 THE BLACK CORALS (Order Anriparharia) are readily be made and the factors limiting distri found in all oceans. However, the great majority bution could be outlined. of the 150 species have been collected with Branches of the living colonies were trans dredges below the limits of human observation planted to various habitats, where regular ob (see Tabl e 1). It is therefore not surprising that servations could be made to determine if the very little ecological work has been done within animal was alive, dying, or dead. These trans this group. The anatomy and taxonomy have plants were put in places where it was hoped the been reviewed by Brook, 1889, Cooper, 1907 effects of environmental extremes could be dis 1909, and Van Pesch, 1914. covered. For example, the headland off Moku In 1958 off Lahaina, Maui, Jack Ackerman Manu Islands, Oahu, was selected because here and Larry Windley, using SCUBA equipment, ac wave action and surge reach a maximum, while cidentally discovered a vast "bed" of black coral turbidity, oxygen concentration, salinity, and in 50 m of water. Previ ous to this find, the temperature are relatively constant. On the other black coral Antipathes grandis Verrill had rarely hand, the muddy bottom of Kaneohe Bay, Oahu, been observed in Hawaii. Only occasionally had was also used, for it is extremely turbid, with no divers found stunted colonies in shallow caves excessive fluctuations in salinity, oxygen con where surge was not excessive. centration, temperature, and surge. Thus, it was At least three species of black coral are rep attempted to measure one factor while keeping resent ed in the Hawaiian Islands, all limited to the other factors relati vely stable, thereby using deeper water generally beyond 30 m. Only the environment as a natural laboratory. Experi one species has been recorded in the literature, ments in the laboratory were impractical be A. grandis, and since it is the most common cause of the problem involved with handling the form, it was selected for this study (Fig. 1). anim al. The purpose of this research was to study and delineate the ecological factors which limit the Collection of Data distribution of this animal to deeper water. LIGHT : Light measurements were carried Out with a flat plate irradiance meter which was METHODS AND MATERIALS calibrated with a foorcandle meter. Two photo Plan of Work electric cells were first connected to a galva A series of stations was selected, some with nometer which was adjusted to a zero reading and some without colonies of actively growing TABLE 1 A . grandis (Fig. 2). Over a period of six months, hydrographic data were collected : light penetra THE BATHYMETRIC DISTRIBUTION OF 66 SPECIES OF ANTIPATHARIA COLLECTED BY THE CHALLENGER tion , current, turbidity, surge, oxygen concentra EXPEDITION DURING THE YEARS 1872-1876 tion , salinity, temperature, and depth. In addi ( BROOK, 1889) tion , the texture and type of substrate were analyzed and population counts were made. By ZONE METERS NUMBER OF SPECIES so doing , a comparison between stations could 1 0-1 8 6 2 18-1 83 25 1 Portion of a Master's Thesis, Universiry of Hawaii, 3 183-914 26 completed in July , 1963. Manuscript received Decem 4 914-1,829 2 ber 8, 1963. 5 1,829-3,658 3 2 Scripps Institute of Oceanography, La Jolla, Cali 6 . 3,658-5,486 4 fornia. 244 Ecology of Black Coral-GRIGG 245 FIG. 1. Large colony of A. grandis, at 58 meters, off Lahai na, Maui (Station 9) . It is app roximately 311:1 meters across and 2. meters high . Photo by Ron Church . while the twOcells were exposed to full sunlight. OXYGEN CONCENTRATION: Dissolved oxygen Then one cell was lowered to the bottom and was measured by the Modified Winkler Method the difference in electrical potential was meas (Hydrographic Office Pub . N o. 607). ured with the galvanometer. This value can be SALINITY: The Knudsen Method was em converted to percent surface light or to gm ployed to measure chlorinity, which is easily cal. cm- 2 (24 hr):", and plotted against depth converted to salinity by the formula : Salinity = (Fig. 6). 0.03 + (1.805 X chloriniry) (Hydrographic Of Th ese measurements were taken at an hour fice Pub. N o. 607) . when the sun was at maximum altitude on days TEMPERATURE : Temperatur e as a functi on of when turb idity was at a minimum. Thus it was depth was recorded by using a bathythermo attempted to measure the maximum penetration graph. Data were also collected with a simple of light for a particular depth. 110° Celsius thermometer carried to the bottom TURBIDITY : Samples of water were collected by a diver. Temperature increase due to hydro from various stations and analyzed with a Tyn dall meter so that turbidity values could be com static pressure was assumed to be negligibl e and pared . Since only relative values were required, thus was not taken into account. it was not necessary to calibrate the Tynd all SUBSTRATE: Portions of the substrate were meter. chiseled with an axe and sledge hamm er and 246 PACIFIC SCIENCE, Vol. XIX, April 1965 carried to the surface by a diver. The substrate and swell. Hence, under very roug h conditions was examined for special features such as tex this instrume nr is not accurate. ture and chemical composition (e.g., CaC0 3 or The third, and simplest, method of measuring basalt). It was possible to draw bottom profiles currenr (and the most accurate) was timing the of the substrate in situ, using a grease pencil horizontal drif t of suspended particl es over a and plastic slate. known distance. Currents less than OJ knots CURRENTS : Currents were measured in three were referred to as slight, from OJ to 1.5 as ways.Data off Sandy Beach, Oahu, and Kaena moderate, and over 1.5 knots as heavy. Point, Oahu, were collected with the aid of the SURGE : Surge is herein defined as a back-and research vessel "N eptune 1." In these cases, sur forth moveme nt of water over short distances face currents were measured by tracking the which is generally caused by long waves (swell). movement of drifting current crosses. It is dependent upon the amplitude and the The Carruthers' Currenr Cone (Carruthers, wavelength of the swell. The period of oscilla 1957) was used when sampling was carried out tion usually varies between 6 and 16 seconds. in an IS-foot skiff. It opera tes on a wa .er Th e moveme nt of water particles in waves with resistance principle, much like a flag fluttering short wavelengths is nearly circular at the sur in the wind. Attached to a stationary line, the face. Th e radii of these circles decrease expo cone is lifted by the currenr at an angle propor nentially with depth and are impercep tible at tional to the inte nsity of flow. At this point a a depth which equals the wavelength (Sverdrup, dissolving cube of sugar triggers the device so Johnson, and Fleming, 1940). In waves wi th that a reading is obtained. Observations of this longer wavelengths, the moveme nr of water par instrument during use showed that the angle of ticles follows a more elliptical orbit. the cone at anyone instant was highly variable, W hen waves begin to "feel" the bottom, the subject to the oscillatory movements of surge movemenr of water particles close to the sub- 0 ..u.. ' HAWAIIAN ISL ANDS "__ :-~. 0'"" s: ,..., . L::5 ~WU; ~~ L.... ~j ~~ ... - _ KAHOOLAWZJ" [ .. HAWAII FIG. 2. The position of selected stations in the H awaiian Islands. Ecology of Black Coral-GRIGG 247 strate will take the form of a flat elipse so that EPIFLORA AND EPIFAUNA: Collection of as the water is practically oscillating back and sociated organisms was made both at the bottom forth in a horizontal plane. This type of surge and on the surface after the colony had been was found to be of particular importance in this brought up (Fig. 5). research. FEEDING: Observations on the feeding habits Currents are often superimposed upon surge. and on the type of food ingested were carried For example, if the current is running in the out. In addition a plankton tow was made at 50 same direction as the swell, it will reinforce meters to determine roughly the type of food surge in one direction and inhibit it in another. present in the natural environment. If the current is strong enough , the latter direc tion can be nullified altogether so that motion is DISCUSSION AND RESULTS uni-directional, speeding up and slowing down Light depending upon whether current and surge are in or out of phase. Since colonies of A. grandis are found only in deeper waters, where the amount of light is con Surge was estimated by watching suspended siderably reduced, it seems probable that strong particles in the water. A back-and-forth move light intensity is important as a limiting factor. ment of water with a horizontal displacement of Indeed, when colonies of black coral are found less than 1 foot was considered slight, of 1-3 in shallow water, their bases are always situated feet moderate, and more than 3 feet heavy. in dimly lit areas such as overhangs and caves. DEPTH : Depth was recorded in two ways. A Peres (1949) has found a similar condi tion in fathometer or continuous depth recorder was the underwater "grotros" off Marseilles, where used to outline bottom profiles, and a depth gage certain deeper sea species were found at much worn by a diver made in sit« measurements higher levels in submarine caves.

Load more