Bioenergetics of the Benthic Herbivorous Populations in a Rocky Intertidal Habitat

Bulletin of the Japanese Society of Scientific Fisheries 49(1), 33-40 (1983) Bioenergetics of the Benthic Herbivorous Populations in a Rocky Intertidal Habitat Mitsuhiro NAGATA* (Received May 31, 1982) Energy transformations through the population of rocky intertidal herbivores, Strongyl ocentrotus intermedius, Chlorostoma argyrostoma turbinatum and Omphalius rusticus, were ana lyzed from Oct. 1978 to Oct. 1979, using estimates of average weight and numbers of individuals in each size-group and observations on respiratory rates. The energy entering the population, which is the sum of the energy assimilated as food and the rate of immigrants, was 118.4 Kcal m-2 yr-1 for the Strongylocentrotus population, 12.6 Kcal m-2 yr-1 for the Chlorostoma popula tion, and 6.6 Kcal m-2 yr-1 for the Omphalius population. The amount of energy lost from the population, expressed as respiration, gametes ejected, predation, mortality and emigration, was estimated at 119.6, 13.4 and 6.5 Kcal m-2 yr-1 for the Strongylocentrotus, Chlorostoma and Omphalius populations, respectively. Since the rocky intertidal regions are strictly Hokkaido. The low-tide-platform, which meas exposed to changes in many environmental ured about 58.6•~104m2 in area, is about 3000 m factors, the rocky intertidal fauna is characterized along the shore-line and about 200m off-shore. largely by fluctuation in aspects of community The tidal excursion at this site is about 0.6 m in structure, including space utilization patterns, vertical distance, and 80% of the platform is composition, trophic structure, and body size exposed at low water. Mean sea temperature structure. For example, studiesl-2) in rocky in ranges from a maximum of 26•Ž (August) to a tertidal fauna suggest that patterns of space utili minimum of 4•Ž (March). zation and size structure of predominant species Invertebrate organisms in this community reflect the operation of important organizing include gastropods, bivalves, barnacles, arthro agents within the community. Therefore, we feel pods and echinoids as major components, and that close attention needs to be paid to the varia are similar to those in other rocky intertidal tions in certain aspects of community structure of habitats in southern Hokkaido. Of these species, rocky intertidal fauna, and that further information Strongylocentrotus intermedius, Chlorostoma concerning the population dynamics and produc argyrostoma turbinatum and Omphalius rusticus tivity of the predominant members is necessary are more abundant in biomass and density, over for a better understanding of the rocky intertidal the year, than others. benthic fauna. However, few reports3-7) have been published concerning the bioenergetic studies Methods on the grazer and deposit-feeder populations as major components of the community. In the The present study is based on the bimonthly present study, the specific objective were to clarify samples of the above three species taken from the population structure and to assess the annual the low-tide-platform from October, 1978 to population production of the predominant mem October, 1979. The abundance of these species bers of each species within a rocky intertidal was measured by counting the number of speci community located to the low-tide-platform in mens found in a known area of the seafloor at a Moheji. right angle with the shore-line; a quadrat of 1m2 (1•~1m) area was put on the sea-floor of 20m intervals along the tapeline. Thus, the popula Study Area tions were examined all along the regular eighteen The study site (Fig. 1) was located on the west sections running out from shore-marks a-r, as side of Hakodate Bay, along the coast of southern shown in Fig. 1. * Faculty of Fisheries, Hokkaido University, 1-1 Minato-3, Hakodate 041,Japan(永田光博:北海道大学水産学部). 34 NAGATA Fig. 1. Diagramatic representation of topograph (A), sampling station (B), and mean surface sea temperature (C). In the top figure, each Arabic number shows the depth in meter. The test diameters of Strongylocentrotus ob of body size distribution. The measurementsof tained by this procedure were measured, and this body size were divided into groups each represent was followed by the removal of gonad tissue. ing a range of 2mm in test diameter for Stron After measuring the shell diameter of Chlorostoma gylocentrotus and 1mm in shell diameter for and Omphalius, shell and body tissue, including Chlorostoma and Omphalius. Then the frequency gonads were removed separately. These tissues of each , group was represented by percentage. were placed in a drying oven at 60•Ž for three The bimodal frequency distribution obtained was days, until constant weight was achieved. After graphically analyzed by making use of a pro drying, the weight was measured. The dry sub bability graph paper method derived from stances were minced, and they were stored in a HARDING.9)The size-frequency distribution of desiccator to analyze caloric content by means of each group is represented in Fig. 2, together with YM-Nenken digital type adiabatic bomb calo several fitted normal curves which were calculated rimeter. by this procedure . By this process, it is possible The rates of oxygen consumption of the animals to determine the growth rate of the mean body were determined bimonthly over a wide range of size of each size-group and to estimate the size animal sizes under the "closed system". An group structure in percentage occupancy. The oxycalorific equivalent') of 4.83 cal m1-1 O2 was results are illustrated in Table 1, and Fig. 3, re used to convert oxygen consumption to energy spectively. In the Strongylocentrotus population, units. II-size-group represents the distinguished per centage occupancy (75-90%) over the year. Results Therefore, even if the quantitative fluctuation of the population was replaced by that of II-size Population Structure group, it would be unlikely for the estimates to The size structure was estimated on the basis be biased. It is possible to calculated the popula- Bioenergetics of the Benthic Herbivorous Populations 35 Fig. 2. Histogram showing the size frequency and several fitted normal curves calculated by the probability graph paper method. A: Strongylocentrous population, B: Chlorostoma popula tion, C: Omphalius population. Table 1. Seasonal changes in the average test diameter (mm) of Strongylocentrotus intermedius and in the average thell diameter (mm) of Chlorostoma a. turbinatum and Omphalius rusticus 36 NAGATA weight of individual body components belonging to various size-groups as a function of mean shell or test diameter. Here the weight of each size-group is estimated as the product of the number of individuals and the mean weight. The sum of the weight of the separate size-groups gives the biomass (Table 4). Respiratory Rate In general, it is well known that the logarithm of oxygen consumption (R) is linearly related to the logarithm of an individual dry weight (W). The exponential equation is R=aWo, where a is a constant, and b is a regression coefficient. Fig. 4 shows the oxygen consumption per unit time per animal plotted against dry body weight on loga rithmic scale. Although the regression coefficient differs with season (Ft0.05)=2.53>F0=1.52 for Strongylocentrotus, F(0.05)=3.58>F0=3.32 for Chlorostoma, F(0.05)=2.50>Fo=2.37 for Ompha Fig. 3. Size-group composition of Strongylocen lius), there was clearly a seasonal change in the trotus population (a), Chlorostoma population respiratory rates over the whole range of animal (b), and Omphalius population (c). ?? : 0 size-group, ?? : I size-group, ?? : II size sizes. When the constant a in the oxygen con sumptiondry body weight regression equation group, ?? : III size-group, ?? : IV size-group, ?? : V size-group. was plotted against temperatures of 6.0•Ž to 22.2•Ž, came close to forming a straight line. tion size for the entire area of the platform on Consequently, it is possible to express oxygen the basis of the average density and size-group consumption (R) as a function of temperature (t) structure in percentage occupancy. This estimate and body weight (W), as shown below: is shown in Table 2. Strongylocentrotus log R=log (0.0070t+0.052) It is generally agreed that the logarithm of an +0.9351 log W individual dry body weight or of dry weight of Chlorostoma log R=log (0.1021t-0.5896) gonad tissue is linearly related to the logarithm +0.7887 log W of body size. These equations lead to dry weight Omphalius log R=log (0.0913t-0.4612) for an individual body or gonad from mean body +0.7951 log W size (shell or test diameter) in each size-group These equations are used to estimate the respira belonging to the total population. The regression tory metabolism of each of the above three animal statistics are illustrated in Table 3. The above populations as a function of sea temperature, dry equations were used to estimate the mean dry weight, and population size. Then the energy Table 2. Population sizes in number per square meter. Each Roman number indicates size group Bioenergetics of the Benthic Herbivorous Populations 37 Table 3. Dry weight of body components (body and gonad: g, shell and flesh: mg) regression statistics at several body size (test and shell diameter: mm) Strongylacentrotus intermedius Table 4. Changes in the biomass (g/m2) of the three benthic populations with the progressive months 38 NAGATA Fig. 4. Logarithmic plot of respiratory rate against body weight. •¤: Oct. (15.5•Ž), •£: Dec. (10.2•Ž), •¢: Mar. (6.0•Ž), • : Apr. (12.0•Ž), •œ: June (18.0•Ž), •¡: Aug. (22.2•Ž), •›: Oct. (16.0•Ž). (A): Strongylocentrotus population, (B): Chlorostoma population, (C): Omphalius population.

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