Importance of the fatty acids 20:5ω 3 and 22:6ω 3 in the diet of plaice (Pleuronectes platessa) larvae

The requirement of plaice (Pleuronectes platessa L.) larvae for the fatty acids 20:5 3 and 22:6 3 was studied. Larvae were reared from first feeding to beyond metamorphosis on Artemia sp. nauplii (EG brand), whose nutritional content had been manipulated by enrichment. Some larvae were fed Artemia rich in 20:5 3 and 22:6 3, and others were fed nauplii with no 22:6 3 and low levels of 20:5 3 (1.9% of the total fatty acids). The differences in diet had no significant difference on growth or survival of the plaice larvae. The results indicate that the plaice larvae in this study did not require 22:6 3 in their diet, even when the levels of 20:5 3 were low.     

Studies on the nutritional requirements of the larval stages of Penaeus japonicus using microencapsulated diets

Fatty acid biosynthesis in the larval stages of Penaeus japonicus Bate was examined by feeding microencapsulated diets containing (1-¹⁴C) palmitic acid, and fat-free diets supplemented with defined fatty acids. Highest larval growth rates were achieved on diets containing Tapes philippinarum lipid and, when defined fatty acids were substituted, on diets containing 20:63 fatty acid. The radioactive tracer experiments indicate that 16:17, 18:0 and 18:19 fatty acids may be synthesised from palmitic acid and that P. japonicus larvae may possess the ability to elongate 18:33 to 20:53 and 22:63, and 18:26 to 20:46. However, the rates of these reactions appear to be too slow to meet the larval requirements for essential fatty acids and the 3 series of polyunsaturated fatty acids must be provided in the diet.     

Lipid biochemistry and reproductive biology in two species of Gammaridae (Crustacea: Amphipoda)

Gammarus oceanicus Segerstråle, 1947 and Echinogammarus marinus (Leach, 1815) were sampled during the breeding season from Oslofjord in 1984, and their lipid composition examined in relation to reproductive condition. In G. oceanicus, female lipid content increased as the ovary matured. Both the amount of lipid stored and the rate of accumulation were greater in spring than in winter. Spring eggs contained 12.4 g lipid, of which 63% was triacylglycerol and 27% phospholipid. Both fractions decreased steadily during embryonic development. Winter eggs contained 19.2g lipid, of which 52% was triacylglycerol and 43% phospholipid. During the early stages of embryonic development the amount of phospholipid decreased sharply, whereas that of triacylglycerol increased, suggesting that some of the fatty acid released from phospholipid was sequestered temporarily as triacylglycerol. When newly spawned, both winter and spring eggs were richer in monoenoic fatty acids than adult amphipods and these acids were the major fuel consumed during development. 6 fatty acids were utilised more slowly than 3 acids, and egg carotenoid pigment content remained constant. Female E. marinus increased in lipid content as the ovary matured. Spring eggs contained 14.7 g lipid when newly spawned and this increased to 16.6 g during the early stages of development. This increase was entirely triacylglycerol, which declined in later stages; the source of the extra lipid was unclear. Eggs contained very little phospholipid or sterol, and both of these components remained at a steady low level during development. E. marinus eggs were not significantly rich in thonoenoic acids compared with adults, and saturated, monoenoic and polyenoic acids were utilised about equally during development. Both adults and eggs were rich in 20.46, which was utilised at a slower rate than the 3 polyunsaturated acids during embryonic development; again, egg carotenoid pigment content remained constant. In both species there was a decrease in the size of the egg (and as a result, of the newly hatched juvenile), but an increase in total reproductive output (i.e., the total weight of the egg clutch) per female as the breeding season proceeded. The reproductive output of an individual female is probably related to food availability during the period of ovarian maturation, whereas the size of an individual egg is dictated largely by feeding conditions for the juveniles once they are independent of the female. The different patterns of lipid utilisation during development found in this study emphasize the flexibility of response in the reproductive biology of gammarid amphipods. It is not yet possible, however, to relate the differing patterns in a simple way either to egg size or total female reproductive output. Two outstanding problems are the source of extra triacylglycerol during the early stages of development of E. marinus and the metabolic cost of brooding eggs.     

Biochemical composition of Nyctiphanes australis and its possible use as an aquaculture feed source: Lipids,pigments and fluoride content

Nyctiphanes australis contained, on a dry weight basis, an average of 52% crude protein and 5.0 to 9.5% lipid. The fatty acid profile of N. australis was markedly unsaturated, with a mean total 3 fatty acid content of 48.6±2.4% of total fatty acids. N. australis contained high levels of the essential long-chain polyunsaturated fatty acids eicosapentaenoic (EPA, 20:53) and docosahexaencic (DHA, 22:63), ranging from 16.6 to 36.5% and 11.1 to 24.8%, respectively. The concentration of total carotenoids ranged from 137 to 302 g g^–1 dry wt, with no significant differences in concentrations found with season or life stage. The carotenoids were comprised of 79.5% astaxanthin and 20.5% canthaxanthin. The lipid and pigment compositions of N. australis suggest that the species could serve as a suitable feed source for cultured salmonids. Like other euphausiids, N. australis contained high levels of fluoride, with a seasonal range between 277 and 3507 g g^–1 dry wt. The high fluoride levels found in N. australis would not detract from its potential as a feed source for salmonids because ingested fluoride is largely absorbed by the skeleton.     

Modeling Free-Surface Fluctuation Effects for Calculation of Turbulent Open-Channel Flows

A new method of introducing the free-surface effects in the calculation of turbulent open-channel flows using the amplitude of the free-surface fluctuation is proposed along with a modeling method of the equation for the free-surface fluctuation. It can be incorporated in two-equation models like k-or k-type models by introducing the damping factor to represent the interaction of the eddies with the fluctuating free-surface. Test calculations for fully developed flows and those over backward-facing step indicate good agreement with direct numerical simulation results as well as experimental results.     

Incorporation and utilization of bacterial lipids in theSolemya velum symbiosis

We undertook a detailed analysis of the lipid composition ofSolemya velum (Say), a bivalve containing endosymbiotic chemoautotrophic bacteria, in order to determine the presence of lipid biomarkers of endosymbiont activity. The symbiont-free clamMya arenaria (L.) and the sulfur-oxidizing bacteriumThiomicrospira crunogena (Jannasch et al.) were analyzed for comparative purposes. The ¹³C ratios of the fatty acids and sterols were also measured to elucidate potential carbon sources for the lipids of each bivalve species. Both fatty acid and sterol composition differed markedly between the two bivalves. The lipids ofS. velum were characterized by large amounts of 18: 17 (cis-vaccenic acid), 16:0, and 16 : 17 fatty acids, and low concentrations of the highly unsaturated plant-derived fatty acids characteristic of most marine bivalves. Cholest-5-en-3-ol (cholesterol) accounted for greater than 95% of the sterols inS. velum. In contrast,M. arenaria had fatty acid and sterol compositions similar to typical marine bivalves and was characterized by large amounts of the highly unsaturated fatty acids 20 : 53 and 22 : 63 and a variety of plant-derived sterols. The fatty acids ofT. crunogena were similar to those ofS. velum and were dominated by 18:17, 16:0 and 16:17 fatty acids. Thecis-vaccenic acid found inS. velum is almost certainly symbiontderived and serves as a potential biomarker for symbiontlipid incorporation by the host. The high concentrations ofcis-vaccenic acid (up to 35% of the total fatty acid content) in both symbiont-containing and symbiont-free tissues ofS. velum demonstrate the importance of the endosymbionts in the lipid metabolism of this bivalve. The presence ofcis-vaccenic acid in all the major lipid classes ofS. velum demonstrates both incorporation and utilization of this compound. The ¹³C ratios of the fatty acids and sterols ofS. velum were significantly lighter (–38.4 to –45.3) than those ofM. arenaria (–23.8 to – 24.2) and were similar to the values found for the fatty acids ofT. crunogena (–45); this suggests that the lipids ofS. velum are either derived directly from the endosymbionts or are synthesized using endosymbiontderived carbon.Woods Hole Oceanographic Institution Contribution No. 7356Please address all correspondence and reprint requests to Dr Conway at her present address: Department of Biological Sciences, University of Pittsburgh, Pennsylvania 15260, USA     

Effects of nitrogen and phosphorus enrichement on in vivo symbiotic zooxanthellae of Pocillopora damicornis

The reef coral Pocillopora damicornis (Linnaeus) was grown for 8 wk in four nutrient treatments: control, consisting of ambient, unfiltered Kaneohe Bay seawater [dissolved inorganic nitrogen (DIN, 1.0 M) and dissolved inorganic phosphate (DIP, 0.3 M)]; nitrogen enrichment (15 M DIN as ammonium); phosphorus enrichment (1.2 M DIP as inorganic phosphate); and 15 M DIN+1.2 M DIP. Analyses of zooxanthellae for C, N, P and chlorophyll a after the 8 wk experiment indicated that DIN enrichment increased the cellular chlorophyll a and excess nitrogen fraction of the algae, but did not affect C cell^-1. DIP enrichment decreased both C and P cell^-1, but the decrease was proportionally less for C cell^-1. the response of cellular P to both DIN and DIP enrichment appeared to be in the same direction and could not be explained as a primary effect of external nutrient enrichment. The observed response of cellular P might be a consequence of in situ CO₂ limitation. DIN enrichment could increase the CO₂ (aq) demand by increasing the net production per unit area. DIP enrichment could slow down calcification, thus decreasing the availability of CO₂ (aq) in the coral tissue.Hawaii Institute of Marine Biology Contribution No. 920     

Aggregation and foraging behavior of whirligig beetles (Gyrinidae)

Summary Whirligig beetles aggregate in the daytime into dense single-and multispecies groups (rafts) of hundreds or thousands of individuals. On the 22km shoreline of Lake Itasca in northern Minnesota, these aggregations were on the average 0.8 km apart, and they were usually found day after day in the same ocations.Most beetles apparently do not home to the aggregation of their origin after dispersing at night because (a) the species composition of some aggregations changed greatly, and (b) paint-marked beetles (Dineutus horni) moved overnight from one aggregation as far as 4km, joining 11 of the 14 large (>300 beetles) D. horni groups on the lake.Throughout the night, the largest concentrations of beetles remained within 100m of the diurnal aggregation sites. Beetles reconvened into the compact rafts before daybreak, in part by following each other in sometimes long single files or trains. Their forward motion stopped after they joined large number of other beetles. We infer that following behavior enables those individuals that have dispersed from their original aggregations (during their nocturnal foraging) to find and join other aggregations before daylight.Naive fish ate the beetles despite their noxious secretions. However, fish living near rafting sites and feeding on insects on the water surface in daylight should soon learn to avoid the beetles. The rafting sites would then become safe places. We observed fish attacking only those beetles that had been either dispersed from their rafts or released into open water away from raft sites in the daytime. We speculate that the evolutionary significance of the aggregation behavior is related to predator (fish) avoidance.     

Influence of irradiance on the nutritional value of two phytoplankton species fed to larval Japanese scallops (Patinopecten yessoensis)

Japanese scallop (Patinopectin yessoensis Jay) larvae grew faster and were larger after 18 d when fed a diet of high-light(HL)-grown Chaetoceros simplex or HL Pavlova lutheri relative to diets of the same phytoplankton species grown at low light (LL). When provided as saturating rations to larval scallop, these diets could be ranked: HL C. simplex>LL C. simplex>HL P. lutheri>LL P. lutheri. In both phytoplankton species, HL-grown cells contained more of the short-chain saturated fatty acid (FA), 16:0 than LL-grown cells. Scallop growth rates were a significant function of the amounts (mg g^-1 dry wt) and the proportions (as percentage of total FAs) of the FAs 14:0 and 14:0+16:0 (total saturated FAs) in their diet. The proximate biochemical composition of HL- versus LL-grown phytoplankton showed no significant differences in protein, total lipid, carbon, carbohydrate or nitrogen per cell which were consistently associated with the greater nutritional value of HL cells. In spite of this high variability in proximate composition, the larval growth rate was a significant function of the average carbon content, nitrogen content and cell volume of the phytoplankton cells. Increased amounts of the essential polyunsaturated FAs 20:5 3 and 22:6 3 in the phytoplankton were negatively correlated with larval scallop growth rates. Thus HL-grown phytoplankton cells were nutritionally superior to LL-grown cells. This nutritional superiority seems to be determined by the fatty acid composition of the cells which, in turn, is controlled by variation in irradiance. The general tendency of predator FA profiles to resemble that of their prey was not observed in larvae fed P. lutheri. The much greater amounts of 18:4 3, 20:5 3, and 22:6 3 FA in P. lutheri relative to C. simplex were not evident in the scallop larvae fed these cells.     

Impacts of copepods on marine seston,and resulting effects on<Emphasis Type="Italic"> Calanus finmarchicus</Emphasis> RNA:DNA ratios in mesocosm experiments

We investigated the impact of copepods on the seston community in a mesocosm set-up, and assessed how the changes in food quantity, quality and size affected the condition of the grazers, by measuring the RNA:DNA ratios in different developmental stages of Calanus finmarchicus. Manipulated copepod densities did not affect the particulate carbon concentration in the mesocosms. On the other hand, chlorophyll a content increased with higher copepod densities, and increasing densities had a positive effect on seston food quality in the mesocosms, measured as C:N ratios and 3:6 fatty acid ratios. These food quality indicators were significantly correlated to the nutritional status of C. finmarchicus. In contrast to our expectations, these results suggest a lower copepod growth potential on higher quality food. However, in concordance with earlier studies, we found that when copepods were in high densities the large particles (>1000 µm³) decreased and that the smaller particles (<1000 µm³) increased in number. These patterns were closely linked to the condition of C. finmarchicus, which were of better condition (RNA:DNA ratios) with increasing biovolumes of large particles, and, conversely, lower RNA:DNA ratios with increasing biovolumes of smaller particles. Consequentially, the selective grazing by copepods stimulated increased biovolumes of smaller plankton, and this increase was responsible for the increased food quality, in terms of C:N and 3:6 ratios. Thus, we conclude that the decreasing growth potentials of C. finmarchicus were a result of a decrease of favourably sized food particles, induced by copepod grazing.Communicated by O. Kinne, Oldendorf/Luhe     

Stable isotopic investigation of physiological and environmental changes recorded in shell carbonate from the giant clam Tridacna maxima

The aragonitic shell of the photosymbiont-bearing bivalve Tridacna maxima contains a record of the physiological and environmental changes the organism has experienced during its lifetime. This record is preserved as chemical and microstructural variations throughout the shell. Stable isotopic analyses of oxygen (¹⁸O/¹⁶O) and carbon (¹³C/¹²C) in shell carbonate were combined with growth increment studies to interpret the shell record of specimens collected from the Rose Atoll (Lat. 14°31S; Long. 168°10W) in April 1982. The seasonal water temperature cycle is recorded in the oxygen isotopic signature of the clams, permitting the recognition of annual cycles in the ¹⁸O profile. The total number of these cycles corresponds to the age of a specimen, while the cycle length is a measure of the yearly growth rate. Large-amplitude cycles, reflecting year-round calcification, characterize the early portion of the growth record. With the onset of sexual maturity and slower growth at an age of approximately ten years, the cycles decrease in amplitude and become more erratic. During this later growth phase calcification is limited to the cooler months of the year, perhaps in response to a re-ordering of energy priorities between growth and gametogenesis. A growth curve developed from the ¹⁸O profile indicates rapid juvenile shell growth followed by slower growth thereafter producing a lifespan of several decades. Carbon isotopic analyses of T. maxima were compared to analyses of the symbiont-barren gastropod Terebra areolata collected from the same locality in April 1984. A 2 depletion in the ¹³C composition of T. maxima shell carbonate is attributed to a symbiontenhanced metabolic rate and an increased flow of isotopically light, respired CO₂ into the carbon pool used in calcification. Such a depletion may prove useful in identifying the presence of photosymbionts in extinct species of fossil mollusks.     

Expansive covers of turf-forming algae on human-dominated coast: the relative effects of increasing nutrient and sediment loads

Turf-forming algae form more extensive habitat on subtidal rock adjacent to urban than non-urban coast of South Australia. This pattern is frequently observed on the worlds temperate coasts and is variously considered to be a result of enhanced concentration of nutrients or rates of sediment deposition on urban coasts. We experimentally tested which of three components of environmental change (increased nutrients in water, increased nutrients in sediments and increased sediment deposition) best explain the expansive covers of turf-forming algae on urban coasts. All three treatments had independent and positive effects on the percentage cover of turf-forming algae. The addition of nutrients from the water column had the largest influence (²=0.55), which was more than six times greater than the effect of nutrients added to sediments (²=0.08). An increase in rate of deposition of sediments had substantial effects (²=0.35), which were about one third less than those of water-borne nutrients. Importantly, the combined effect of all three treatments caused a 77% increase in percentage cover of turf-forming algae, which is comparable to the observed difference in covers between urban and non-urban coast in South Australia (93%). These results suggest that human activities that reduce water quality in both nutrient and sediment loads account for major change observed on human-dominated coasts. Despite this knowledge, we still lack complete information on the mechanisms that switch the primary subtidal habitat from canopy-forming algae to turf-forming algae on human-dominated coasts.Communicated by M.S. Johnson, Crawley     

The evolution of cheating and selfish behavior

Summary We examine the necessary conditions for the spread of genes that determine selfish and cheating behaviors and the rate of spread of these genes through structured populations, in order to address the question of the invadability of altruistic systems by anti-social mutations. We find that, although cheaters always have a higher relative fitness than altruists within groups, population structures which permit the evolution of altruism also preclude invasion by anti-social mutations. These results are related to a discussion by Hamilton (1971) concerning the limits to the evolution of altruistic and selfish behaviors.     

Calcification in the maerl coralline alga Phymatolithon calcareum: Effects of salinity and temperature

The coralline alga Phymatolithon calcareum was dredged from 13 m in the Kattegatt, Baltic Sea, in December, 1980, and its rate of calcification was measured by ⁴⁵Ca⁺⁺-uptake methods. Light-saturated calcification rates at 5°C ranged from 15.8 g CaCO₃ g^-1 dry wt h^-1 for the basal parts of the plants to 38.7 g CaCO₃ g^-1 dry wt h^-1 for the tips. These age gradients were not apparent when calcification rates were expressed on the basis of surface area. Experiments with salinity (10, 20, 30) and temperature (0°, 5°, 10°, 20°C) indicated that optimum conditions for calcification were at 30 S and at temperatures above 10°C. Salinity had a greater influence on calcification rate than did temperature, and there was a positive relationship between salinity and calcification rate at all temperatures. In 6 mo old cultures, salinity was again the important factor, with all plants remaining healthy at 30 except those at the highest temperature (20°C). These trends, and the low calcification rates at 10S (4.6 g CaCO₃ g^-1 dry wt h^-1 at 5°C to 8.6 g CaCO₃g^-1 dry wt h^-1 at 20°C) suggest that low salinity may be the explanation for the general absence of P. calcareum from the brackish waters of the Baltic Sea. Short-term experiments in which salinity was kept constant while Ca⁺⁺ concentration was altered, and experiments in which salinity was varied and Ca⁺⁺ concentration kept constant, suggest that it is the calcium ion concentration and not salinity per se which affects calcification rates.     

Circadian rhythmicity of locomotor behavior and oviposition in female Teleogryllus commodus

Summary Locomotor activity of the Teleogryllus commodus is under circadian control, with _LL=25.3 h and _DD=23.4 h. In LD 12:12 h, running occurs either exclusively during darkness (57%), mainly in the dark phase (35%), or with substantial activity peaks in both phases (8%). For oviposition, _LL>24 h and _DD<24 h; in LD 12:12 h, 80% of the studied deposited their eggs mainly during the light phase and 20% primarily during darkness. In the discussion, a temporal correlation between stridulation, spermatophore formation, and locomotion, and oviposition is established, which serves to make an encounter and mating between the sexes as likely as possible.Dedicated to Prof. Dr. M. Gersch on the occasion of his 70th birthday     

The influence of irradiance on the biochemical composition of three phytoplankton species and their nutritional value for larvae of the Pacific Oyster (Crassostrea gigas)

Three species of phytoplankton grown at high (HL) or low light (LL) were fed as saturating rations to laboratory-reared larval Crassostrea gigas. Larval C. gigas fed diets of HL grown Chaetoceros gracilis and HL grown Isochrysis aff. galbana grew faster than those fed LL grown cells of the same phytoplankton species. Faster growth of C. gigas larvae was consistently associated with increases in the percent composition of short chain saturated fatty acids (FA) 14:0+16:0 in the HL grown cells. There were no consistent and significant differences between HL and LL grown phytoplankton cells in their content of carbon, nitrogen, protein, lipid or carbohydrate. Intraspecific increases in percent composition of essential fatty acids (EFAs), 20:53 and 22:63, in the phytoplankton were not associated with improvements in the growth or survival of the oyster larvae. Oyster larvae fed diets of Phaeodactylum tricornutum with a relatively high proportion of EFAs grew more slowly than those fed C. gracilis. In this experiment the proportion of dietary EFA 20:53 was negatively correlated with oyster growth rates. The faster growing oyster larvae contained relatively more of the FAs 14:0+16:0 which may be useful as measures of larval oyster condition. After a diet of one phytoplankton species for ca. 10 d, oyster larvae acquired distinctive FA profiles resembling that of their phytoplankton prey.     

Effects of monospecific algal diets of varying biochemical composition on the growth and survival of Pacific oyster (Crassostrea gigas) larvae

Using monospecific diets of Thalassiosira pseudonana cells grown under different steady-state conditions, it was determined that higher growth rates of larval Crassostrea gigas Thunberg were obtained when fed T. pseudonana cells grown under high light. High light grown T. pseudonana cells consistently contained relatively more of the saturated fatty acids 14:0 and 16:0. Considered over three independent experiments, high light grown T. pseudonana cells were lower in protein and higher in carbohydrate than low light grown cells. Higher growth rates of larval C. gigas were obtained on diets with more of the essential fatty acid (EFA) 22:63, and less of the other EFA, 20:53. The relative requirements of C. gigas larvae for the essential fatty acids 20:53 and 22:63 are discussed. Faster growing larvae contained higher percentages of the fatty acids 14:0 and 16:0, and lower percentages of 22:2j. Oyster growth rates were correlated with their content of the fatty acids: 14:0, 16:0 and 22:2j in two experiments utilizing separately spawned batches of larvae. Fatty acid profiles are proposed as a technique for assessing larval condition. C. gigas larvae contained ten times the percent composition of the FAs 16:43, 18:17, 20:17 and 22:2j compared with their diet. Correlation analysis suggests that the dietary source of 18:17, 20:17 and 22:2j was 16:17. It is concluded that T. pseudonana cells grown under high light are a superior diet for C. gigas larvae in comparison with low light grown cells of the same species.     

Variation in lipid classes during the molting cycle of the prawn Penaeus japonicus

The variation in the concentration and fatty acid composition of lipid classes during the molting cycle of the prawn Penaeus japonicus was investigated. The lipid concentration of the whole body reached a maximum at mid-premolt (Stage D₂) and then decreased to low level at late premolt (Stage D_3–4). The accumulation of lipids during the premolt period seemed to be attributable to the increase of both polar and neutral lipids. The increase of neutral lipids at Stage D₂ was derived from not only triglycerides but also free sterols and free fatty acids. Regarding the fatty acid composition of every lipid class, a marked variation occurred mainly at the intermolt (Stage C). In this stage, the polar lipids were rich in monoenoic acids such as 18:1 and poor in polyenoic acids such as 20:53 and 22:63. The triglycerides were rich in polyenoic acids at Stage C, but poor in monoenoic acids such as 16:1 and 18:1. The steryl esters contained large amounts of saturated acids such as 16:0 and 18:0 throughout the molting cycle, however the level of polyenoic acids increased at Stage C.     

13C/12C ratios and the trophic importance of algae in Florida Syringodium filiforme seagrass meadows

Over 380 stable carbon isotope (¹³C) analyses made during 1981–82 showed that Syringodium filiforme Kutz seagrass meadows in the Indian River lagoon of eastern Florida have food webs based on algal rather than seagrass carbon. Seagrasses averaging approximately-8 were isotopically distinct from algae epiphytic on seagrass blades (X=-19.3) and particulate organic matter in the water column X=-21.6. ¹³C values of most fauna ranged between-16 and-22, as would be expected if food web carbon were derived solely from algal sources. These results counter the idea that seagrass detritus is the dominant carbon source in seagrass ecosystems. Two factors that may contribute to the low apparent importance of seagrass in the study area are high algal productivities that equal or exceed S. filiforme productivity and the high rates of seagrass leaf export from meadows.     

Spatial mesoscale patterns of West Pacific picophytoplankton as analyzed by flow cytometry: their contribution to subsurface chlorophyll maxima

Vertical distributions of picophytoplankton (ppp) (<2 m) were studied by ship-board flow cytometry during two cruises in Western Pacific waters to Palau and to Australia in 1990. Weak red-fluorescing small ppp, supposed to be free-living prochlorophytes (Chisholm et al. 1988), were abundant in the area surveyed. These ppp, designated the prochlorophytes, were abundant in the surface waters (>10⁴ cells ml^-1) at the northern region (27°03N; 7°11N) in November, whereas in December at the southern tropical stations (0°23.54S; 9°20.30S; 13°50.6S), they formed subsurface maximum layers (>10⁵ cells ml^-1) on a nitracline at a depth of 3.5 to 5.4% surface irradiation. Their fluorescence intensity increased with depth below 10% surface irradiation. The prochlorophytes at a depth of 1% surface irradiation had ten times higher fluorescence than those at the surface layer. The total fluorescence intensity of the prochlorophytes accounted for 32 to 63% of the sum of the total fluorescence intensity of all fluorescing phytoplankton detected at subsurface chlorophyll maxima in the tropical area. These results suggest that distribution of the prochlorophytes is greatly affected by nitracline and by light intensity and that their chlorophyll is a major contributor to the subsurface chlorophyll maximum in the pelagic West Pacific Ocean.