Food preferences and related behavior of the browsing sea urchin <Emphasis Type="Italic">Tripneustes gratilla</Emphasis> (Linnaeus) and its potential for use as a biological control agent

The short-spined toxopneustid sea urchin Tripneustes gratilla feeds on a wide variety of algal species and on sea grasses. However, the urchin does show preferences when offered a selection of macroalgal species, which it encounters in nature. Preferences among macroalgae were evident in field-collected urchins exposed to pair-wise tests where the variable was either the consumption rate of the algae or observation of which algal species the urchins chose to touch with their lantern teeth. Exposure of lab-housed urchins to one of five species of macroalgae for 5 months did not seem to alter preferences of urchins in three of the exposure groups, but those exposed to Padina sanctae-crucis seemed to show an enhanced preference for this species when offered a choice of the five species of macroalgae at the end of the exposure period, and those exposed to Gracilaria salicornia seemed to avoid the species when offered the choice of the five species. Perhaps more ecologically important than their preferences were two other observations on these urchins: first, when offered only a single species of algae, the urchins on four of five diets ate the same quantity per day. Second, when simultaneously offered the choice among the five macroalgal species, the urchins consumed more macroalgae per day than when offered only one species. These urchins move about a meter a day. They probably encounter food resources in a relatively coarse-grained fashion and have evolved to eat what is available. Because of their limited movements, their habitat overlap with grazing fishes, their acceptance of a wide variety of macroalgae and their preference for macroalgae, these native urchins are thought to have the potential to serve as biological control agents of alien and invasive macroalgae, which have come to dominate some reef zones normally occupied by corals in Hawaii.     

Mass mortality ofDiadema antillarum

It has been hypothesized that herbivorous fishes and the regular echinoidDiadema antillarum Philippi compete for benthic algae as their major food resource. Mass mortality ofD. antillarum in February 1984 provided the opportunity to test the hypothesis that herbivorous fishes and sea urchins were competing previously. Visual censuses of herbivorous fishes conducted over 4 yr in four reef zones on Tague Bay Reef, St. Croix, U.S. Virgin Islands, before and after the mass mortality indicated that population densities increased approximately three-fold in backreef and shallow (2m) forereef zones and two-fold, and four-fold in mid (5m) and deep (10m) forereef zones, respectively. Juvenile parrotfishes constituted the major component of these increases, except in the shallow forereef where acanthurids became most abundant. Grazing intensity by herbivorous fishes increased in three of the four reef zones immediately following the mass mortality. These data support the hypothesis that exploitative competition for algal resources was occurring prior to the sea urchin mass-mortality, although alternative hypotheses cannot be discounted completely. Despite the increases in the abundances of, and grazing by, herbivorous fishes, the algal community continued to increase in percent cover and biomass, indicating that increased grazing by fishes does not compensate for the loss of grazing byD. antillarum in controlling algal abundance and community structure.     

Effects of grazing by two species of sea urchins (Strongylocentrotus franciscanus and Lytechinus anamesus) on recruitment and survival of two species of kelp (Macrocystis pyrifera and Pterygophora californica)

We studied the effects of grazing by two species of sea urchins on two species of kelp (Macrocystis pyrifera and Pterygophora californica) in the San Onofre kelp bed in southern California from 1978 through 1981. Both red sea urchins, Strongylocentrotus franciscanus, and white sea urchins, Lytechinus anamesus, were abundant and lived in aggregations. The purple sea urchin (S. purpuratus) was rare at the study site and was not studied. The aggregations of red urchins were either relatively small and stationary (for over 3 yr) or relatively large and motile (advancing at about 2 m mo^–1). Both stationary and moving aggregations were observed at the same time, and within 100 m of one another. Stationary aggregations of red urchins probably subsisted mainly on drift kelp and had no effect on kelp recruitment or on adult kelp abundance. In contrast, red sea urchins in large, motile aggregations or fronts ate almost all the macroalgae in their path. The condition of their gonalds indicated that red urchins in fronts were starved relative to red urchins in the small, stationary aggregations. Large, motile aggregations developed after 2 yr of declining kelp abundance (probably due largely to storms). We propose that a scarcity of drift algae for food results in a change in the behavior pattern of the red urchins and thus leads to the formation of large, motile aggregations. The aggregations of white urchins, which occurred along the offshore margin of the kelp bed, were large, but relatively stationary. The white urchins rarely ate adult kelps, but grazed extensively on early developmental stages of kelps and evidently prevented seaward expansion of the bed. The spatial distribution of both types of red urchin aggregations appeared to be unrelated to predation pressure from fishes or lobsters.Please address all requests for reprints to the senior author at his present address.     

Trophic cascades result in large-scale coralline algae loss through differential grazer effects

Removal of predators can have strong indirect effects on primary producers through trophic cascades. Crustose coralline algae (CCA) are major primary producers worldwide that may be influenced by predator removal through changes in grazer composition and biomass. CCA have been most widely studied in Caribbean and temperate reefs, where cover increases with increasing grazer biomass due to removal of competitive fleshy algae. However, each of these systems has one dominant grazer type, herbivorous fishes or sea urchins, which may not be functionally equivalent. Where fishes and sea urchins co-occur, fishing can result in a phase shift in the grazing community with subsequent effects on CCA and other substrata. Kenyan reefs have herbivorous fishes and sea urchins, providing an opportunity to determine the relative impacts of each grazer type and evaluate potential human-induced trophic cascades. We hypothesized that fish benefit CCA, abundant sea urchins erode CCA, and that fishing indirectly reduces CCA cover by removing sea urchin predators. We used closures and fished reefs as a large-scale, long-term natural experiment to assess how fishing and resultant changes in communities affect CCA abundance. We used a short-term caging experiment to directly test the effects of grazing on CCA. CCA cover declined with increasing fish and sea urchin abundance, but the negative impact of sea urchin grazing was much stronger than that of fishes. Abundant sea urchins reduced the CCA growth rate to almost zero and prevented CCA accumulation. A warming event (El Ni?o Southern Oscillation, ENSO) occurred during the 18-year study and had a strong but short-term positive effect on CCA cover. However, the effect of the ENSO on CCA was lower in magnitude than the effect of sea urchin grazing. We compare our results with worldwide literature on bioerosion by fishes and sea urchins. Grazer influence depends on whether benefits of fleshy algae removal outweigh costs of grazer-induced bioerosion. However, the cost-benefit ratio for CCA appears to change with grazer type, grazer abundance, and environment. In Kenya, predator removal leads to a trophic cascade that is expected to reduce net calcification of reefs and therefore reduce reef stability, growth, and resilience.     

Feeding aggregations of sea stars (<Emphasis Type="Italic">Asterias</Emphasis> spp. and <Emphasis Type="Italic">Henricia sanguinolenta</Emphasis>) associated with sea urchin (<Emphasis Type="Italic">Strongylocentrotus droebachiensis</Emphasis>) grazing fronts in Nova Scotia

Migrating feeding aggregations (or fronts) of sea urchins can dramatically alter subtidal seascapes by destructively grazing macrophytes. While direct effects of urchin fronts on macrophytes (particularly kelps) are well documented, indirect effects on associated fauna are largely unknown. Secondary aggregations of predators and scavengers form around fronts of Strongylocentrotus droebachiensis in Nova Scotia. We recorded mean densities of the sea stars Asterias spp. (mainly A. rubens) and Henricia sanguinolenta of up to 11.6 and 1.7 individuals 0.25 m⁻² along an urchin front over 1 year. For Asterias, mean density at the front was 7 and 15 times greater than in the kelp bed and adjacent barrens, respectively. There was strong concordance between locations of peak density of urchins and sea stars (Asterias r = 0.98; H. sanguinolenta r = 0.97) along transects across the kelp–barrens interface, indicating that sea star aggregations migrated along with the urchin front at rates of up to 2.5 m per month. Size–frequency distributions suggest that Asterias at the front were drawn from both the barrens (smaller individuals) and the kelp bed (larger individuals). These sea stars fed intensively on mussels on kelp holdfasts and in adjacent patches. Urchin grazing may precipitate aggregations of sea stars and other predators or scavengers by incidentally consuming or damaging mussels and other small invertebrates, and thereby releasing a strong odor cue. Consumption of protective holdfasts and turf algae by urchins could facilitate feeding by these consumers, which may obtain a substantial energy subsidy during destructive grazing events.     

Influence of some coral reef communities on the calcium carbonate budget of Tiahura reef (Moorea,French Polynesia)

The calcium carbonate budget of coral reefs is the result of the interaction of the processes of calcification and biological degradation, and is reflected in the chemical properties of the seawater overlying the reefs. A series of experiments at Moorea Island (French Polynesia) in 1988 monitored the diurnal and nocturnal variations in the chemical properties of seawater under field and laboratory conditions. Our results revealed that in the study area (Tiahura barrier reef flat), the calcium carbonate budget varied over space and time as a function of location in the water current. Two in-situ sites were investigated; one was situated 100 m from the algal crest of the barrier reef, the other 300 m further downstream. As a result of cumulative upstream events, the daily net calcification was ten times higher at the downstream (5.22 gm^-2 d^-1) than at the upstream (0.45 gm^-2 d^-1) site. The carbonate uptake by in situ Porites lobata in enclosures (8 kgm^-2 yr^-1) was ten times higher than the uptake by the whole community in the surrounding water (0.8 kgm^-2 yr^-1) and five times higher than that recorded for P. lobata in laboratory experiments (1.4 kgm^-2 yr^-1), where illumination levels were 10% of in situ levels. In laboratory experiments, the planktonic fraction of the seawater had no perceptible influence on the calcium carbonate budget. In the absence of bioeroders, living coral totally depleted the carbonate content of the seawater (3.7 gm^-2d^-1). Bioerosive organisms played an important role in restoring this calcium carbonate; e.g. sea urchins grazing on algal turf covering dead coral ingested CaCO₃ and released this as a carbonate powder (1.26 gm^-2d^-1); a form of carbonate which is extremely accessible to chemical dissolution.     

Fine-scale distribution of larval fishes: patterns and processes adjacent to coral reefs in Kaneohe Bay,Hawaii

Plankton samples were taken from January to June 1987 in Kaneohe Bay, Oahu, Hawaiian Islands, with a free-fall plankton net, to investigate the fine-scale distribution of larval fishes around coral reefs. Daytime samples indicated that the postflexion larvae of two gobiids (Psilogobius mainlandi and an unidentified species) were significantly more abundant at stations immediately adjacent to reefs (near-reef) than at stations in open water off the reef (off-reef). These postflexion gobiid larvae appeared to be capable of resisting advection and dispersal while remaining in the water column near suitable adult habitats. The larvae of Foa brachygramma (Apogonidae) and Encrasicholina purpurea (Engraulidae) were significantly more abundant at off-reef stations than at near-reef stations. Nighttime samples indicated that the gobiid larvae depend on visual cues to remain near the reef. The horizontal distributions of F. brachygramma and E. purpurea larvae appeared to be related to their vertical positioning. These data suggest that typical ichthyoplankton surveys which do not sample close to adult fish habitats would greatly underestimate the abundances of larvae such as the gobiids.     

Annual reproductive cycle of the green sea urchin, Strongylocentrotus droebachiensis, in differing habitats in Nova Scotia, Canada

We monitored the reproductive cycle of Strongylocentrotus droebachiensis (OF Müller) between April 1993 and August 1995 in kelp beds, barren grounds and grazing fronts at both a wave-exposed and a sheltered site along the Atlantic coast of Nova Scotia. Gonad index and histological analyses showed that S. droebachiensis has an annual reproductive cycle that is synchronous across sites and habitats, and between females and males. Spawning occurs in March/April of each year but a small proportion of sea urchins in the study populations also spawned in fall 1995. During most of the year, sea urchins in kelp beds and grazing fronts have a higher gonad index than those in barren grounds. Gonad indices also tended to be higher at the wave-exposed than the sheltered site. Interannual variability in peak gonad index was significant in the barren grounds at the wave-exposed site and in the grazing front at the sheltered site. The gametogenic cycle is characterized by six stages based on the abundance of nutritive and germinal/gametic cells. Nutritive phagocytes are abundant after spawning and replaced by increasing numbers of germinal and gametic cells as the gametogenic cycle progresses. The temporal patterns of abundance of each cell type were similar among habitats indicating that the gonads were qualitatively similar despite large differences in gonadal mass. The quantity of gut contents (ratio of food volume to body volume) was similar among habitats, but the quality (percentage of organic material) tended to be higher in kelp beds and grazing fronts than in barren grounds suggesting that differences in gonad index of S. droebachiensis in different habitats are related to differences in diet. The high density of sea urchins in grazing fronts combined with their high fecundity suggests that they make the greatest contribution, per unit area, to the overall larval pool. Received: 20 May 1997 / Accepted: 21 January 1998     

Copépodes planctoniques du lagon de Nouvelle-Calédonie: facteurs écologiques et associations d'espèces

In the southwestern part of the lagoon of New Caledonia (South Pacific Ocean), a plankton sampling programme was conducted from February 1978 to April 1979. During 11 cruises, 5 stations in the open sea and various bays were sampled at approximately monthly intervals. A transect of 3 stations (mid lagoon, near-reef and barrier-reef channel) visited each fortnight, completed the sampling programme. The copepods collected were identified to the specific level, counted and the 52 most abundant species analysed for seasonal and regional variations. Cruise and transect data, dealt with separately, were analysed as follows: (a) Correspondence analyses (reciprocal averaging) of qualitative and quantitative copepod counts were made; these revealed the relationships between species and between samples by means of factorial design. After computation of factorial axes, the barycentres of different sets of samples were projected as reference points (zero weight) in the factorial space; these reference points, characterizing different stations and sampling times, facilitate ecological interpretation. (b) Species partition was achieved by two successive methods: non-hierarchical, followed by hierarchical classification. Between-species distance was computed from their co-ordinates on the factorial axis. (c) The clusters of species obtained were plotted in the factorial planes to assess ecological preference. Then, clusters from cruise and transect data sets were compared to improve the copepod classification. The main ecological factors appear to be (i) spatial patterns, (ii) seasonal temperature cycle, (iii) changes in wind force and direction. Different populations inhabit the open sea, near-reef, mid-lagoon, shallow and deep-bay waters. Acartia australis outnumbers all other species in the reef vicinity. A. amboinensis is the most abundant in the deep, fjord-like bay. Canthocalanus pauper, Paracalanus parvus, Bestiola sp., Centropages orsinii, A. bispinosa are characteristic of shallow bays. In the factorial structure produced by correspondence analysis, certain seasonal barycentres appear close to some station barycentres: summer close to mid-lagoon, winter close to open sea, and spring close to near-reef barycentres, respectively. This may be explained by the seasonal dynamics of the lagoon water. After the heavy summer rainfalls, freshwater runoff carries midlagoon plankton towards the open sea; conversely, during the winter westerly gales, oceanic species enter the lagoon through the barrier-reef channel or above the reef with the swell breakers. In October, the surface-layer current induced by the strong trade wind carries large swarms of A. australis out to the open sea. Finally, the variations of plankton populations and biomass in the lagoon seem to be governed by the direction of water flow across the reef channels. Enrichment factors are terrestrial sediment wash-out after rainfall and, probably, trade wind-induced upwelling. Therefore, the more or less steady state of the lagoon's plankton biomass may result from the fact that enrichment factors are also exportation factors. Reciprocally, the entry of plankton-poor oceanic water cannot increase the plankton biomass in the lagoon.     

Effect of sunlight intensity and albinism on the covering response of the Caribbean sea urchin<Emphasis Type="Italic"> Tripneustes ventricosus</Emphasis>

At Discovery Bay, Jamaica, Tripneustes ventricosus lives in beds of the turtle grass Thalassia testudinum. Especially during daylight hours, it covers its aboral surface with fragments of this plant and other objects. Normally pigmented, wild-type sea urchins covered themselves significantly less with Thalassia when sunlight was experimentally decreased to 66% or 32% ambient intensity. Consistent with this result, naturally occurring sea urchins exhibited significantly less covering at a deep (3.5 m) site than at a shallow (1 m) site, where light intensities at the bottom were 619 and 946 mol s^–1 m^–2, respectively. The graded covering response to light intensity suggests that covering is a defense against damaging solar radiation. Albino sea urchins covered themselves significantly more with Thalassia than wild-type sea urchins in both full and 66% sunlight. In addition, at the shallow site where they accounted for about 4% of the population, they showed significantly greater covering than wild-type urchins. The greater covering response of albino sea urchins suggests a greater susceptibility to solar radiation.Communicated by P. W. Sammarco, Chauvin     

Foods and predators of the green sea urchin Strongylocentrotus droebachiensis in Newfoundland waters

Gut analyses of the green sea urchin Strongylocentrotus droebachiensis (O. F. Müller) demonstrated that perennial phaeophytes, mostly fucoids and Alaria esculenta, were predominant in the diet. Ephemeral species, coralline algae and animals, were consumed in smaller amounts when available. Grazing by the urchins is evidently responsible for the dearth of non-coralline sublittoral algae in Newfoundland waters. Lobsters, rock crabs, purple sea stars, other urchins, and a variety of fishes and birds feed on S. droebachiensis, but predation is apparently not effective in limiting the abundance of the urchin.Studies in Biology from the Memorial University of Newfoundland No. 234.Contribution from the Marine Sciences Research Laboratory No. 66.     

Comparative feeding preference of Strongylocentrotus droebachiensis (Echinoidea) for the invasive seaweed Codium fragile ssp. tomentosoides (Chlorophyceae) and four other seaweeds

Laboratory experiments conducted during 1987 on Appledore Island, Maine, USA, tested whether feeding preference or the absence of an attractant was the cause for the occurrence of beds of Codium fragile ssp. tomentosoides (herein referred to as Codium fragile) within rocky barrens grazed clear of kelp by the sea urchin Strongylocentrotus droebachiensis. Consumption of C. fragile in single-diet experiments (1 seaweed/sea urchin) was highly variable and was not significantly different from that for several other seaweeds (Agarum cribrosum, Ascophyllum nodosum, Chondrus crispus, and Laminana saccharina) important in the field diet of the green sea urchin. In multiple-diet experiments (5 seaweeds/sea urchin) significantly less Codium fragile was eaten than Chondrus crispus, but significantly more Codium fragile was eaten than A. cribrosum. Chemosensory experiments suggest that C. fragile does not attract the sea urchin. Sea urchins are unable to detect C. fragile but will eat it when they come in contact with it.     

Tolerance to high temperatures and potential impact of sea warming on reef fishes of Gorgona Island (tropical eastern Pacific)

Knowledge of upper thermal-tolerance limits of marine organisms in the tropical eastern Pacific (TEP) is important because of the influence of phenomena such as El Niño and global warming, which increase sea temperature. Laboratory experiments were conducted to determine the critical thermal maximum (CTM) of reef fishes from the TEP. In 15 reef fishes of Gorgona Island (TEP) the CTM was between 34.7°C and 40.8°C. None of these CTMs was exceeded by sea temperature in the TEP during any of the strongest El Niño events in this century (32°C during El Niño 1982-1983 and 1997-1998), which indicates that all species studied here may tolerate El Niño maximum temperatures. In addition, the CTM of the least-tolerant species was 8°C above the current mean sea temperature in a wide range of latitudes in the TEP. This suggests that fishes live far from their upper thermal tolerance limits and that the current global-warming trend is still unlikely to be dangerous for these species. If sea temperature continues to increase at the current rate, in about a century sea temperature could exceed the thermal tolerance of some reef fishes and threaten them with extinction. Such risk, however, might occur sooner if the sea temperature during El Niño also increased in step with the global warming, but also because other processes involved in maintaining population, such as reproduction, can be affected at lower temperatures. The possible ability of reef fishes to adapt to increases in sea temperature is discussed.     

Feeding macroecology of territorial damselfishes (Perciformes: Pomacentridae)

The present study provides the first analysis of the feeding macroecology of territorial damselfishes (Perciformes: Pomacentridae), a circumtropical family whose feeding and behavioral activities are important in structuring tropical and subtropical reef benthic communities. The analyses were conducted from data collected by the authors and from the literature. A strong positive correlation was observed between bite rates and sea surface temperature (SST) for the genus Stegastes. A negative correlation was found between bite rates and mean body size for the genera Stegastes and Pomacentrus, but this relationship was not significant when all territorial pomacentrids were analyzed together. A negative correlation between body size and SST was observed for the whole group and for the genera Stegastes, and Pomacentrus. No relationship was found between territory size and feeding rates. Principal Components Analysis showed that differences in feeding rates accounted for most of the variability in the data. It also suggested that body size may be important in characterizing the different genera. In general, tropical species are smaller and have higher bite rates than subtropical ones. This study extended the validity of Bergmann’s rule, which states that larger species or larger individuals within species occur towards higher latitudes and/or lower temperatures, for an important group of reef fishes. The identification of large-scale, robust ecological patterns in the feeding ecology of pomacentrid fishes may establish a foundation for predicting large-scale changes in reef fish assemblages with expected future changes in global SST.     

Utilisation of mangroves and seagrasses by fishes in the Negombo Estuary,Sri Lanka

In two years of sampling, 10 287 fishes belonging to 62 species (39 families) were collected from mangroves and seagrasses of the Negombo Estuary. Of the species collected, 70% were marine, 27% estuarine and 3% freshwater. Aplocheilus melastigma, Ambassis dayi, Etroplus suratensis, E. maculatus, Bathygobius fuscus and Arothron immaculatus constituted 90% of the catch. Sixteen species were caught only from mangroves, 6 only from seagrasses, and 40 from both. The number of species, individuals, biomass and diversity were higher in seagrasses than in mangroves. Gobies and some siganids moved to the mangrove habitat as they grew, but cichlids, triacanthids, some eleotrids and tetraodontids preferred seagrasses. The number of species and individuals caught increased at the beginning of the north-east monsoon when the rainfall was low. The number of species and individual numbers were positively correlated with salinity and dissolved oxygen and negatively correlated with water temperature. Of the 33 species examined, seagrasses formed >25% of the gut content in juvenile Lates calcarifer, Apogon thermalis, E. suratensis, E. maculatus, Siganus javus, Butis butis and Bathygobius fuscus. Mangrove litter was the main gut content of the family Syngnathidae. E. suratensis, E. maculatus and S. javus from seagrasses contained more filamentous algae than seagrasses in their stomachs. S. javus collected from mangroves also contained more seagrasses in their stomachs than mangrove litter, indicating that the association of juvenile fishes with mangroves or seagrasses may not be obligate in estuaries subject to low tidal fluctuations.     

Influences of habitat and natural disturbances on contributions of massive Porites corals to reef communities

We compared densities, distributions and size frequencies of massive corals in the genus Porites on five relatively exposed, mid-shelf reefs (50 km offshore) in the central Great Barrier Reef with those on a sheltered inshore reef (10 km offshore). Data included various transect and mapping studies between 1984 and 1990, estimates of size-dependent damage from the crown-of-thorns starfish Acanthaster planci, estimated densities of herbivorous sea urchins (potential predators of juveniles), and observations of size-specific effects of tropical cyclones. Assemblages of Porites spp. on mid-shelf reefs were dominated by small colonies (2 to 10 cm diam) established either from planula larvae or from small tissue remnants that had survived A. planci predation in the early to mid-1980s. Large colonies (up to 10 m diam) were scarce, except for localized aggregations on terraces at the base of reef slopes (6 to 12 m deep). Extensive space suitable for settlement by coral larvae can be attributed to recurrent cyclones and A. planci outbreaks. Despite low sea urchin predation, the slowly growing Porites juveniles are likely to die from overgrowth by numerous, much faster growing corals. On the sheltered inshore reef, the coral community was dominated by very large (>5 m diam) Porites colonies, several centuries old; recruitment was mainly by fragmentation of large colonies; there was little space available for settlement, and probabilities of juvenile mortality from grazing urchins were high. Differences in settlement and early survival of Porites spp. are exacerbated by different regimes of storm damage. A model is proposed that links wave climate with the size and age reached by corals before dislodgement by storm waves, and which is consistent with observed densities and size-frequency distributions of Porites in sheltered and exposed areas.     

Chemical defense in the eggs and embryos of antarctic sea stars (Echinodermata)

Fifteen species of reproductively mature echinoderms (11 sea stars, 3 sea urchins, 1 sea cucumber) were collected from McMurdo Sound, Antarctica, during austral spring and summer of 1985 and 1986; eggs and embryos were obtained, and were tested for ichthyonoxicity using the common marine killifishFundulus grandis as a model predator. Chemical deterrents occurred in the large, yolky eggs of the pelagic lecithotrophic sea starPerknaster fuscus and the planktotrophic sea starPorania antarctica. Brooded embryos of the sea starsDiplasterias brucei andNotasterias armata were also noxious. Significant ichthyonoxicity was not detected in the remaining 7 species of sea stars, 3 sea urchins, and 1 sea cucumber. Chemical deterrents were generally effective at concentrations below a single egg or embryo per agar test-pellet. Although chemicals found in these eggs and embryos are noxious to an allopatric fish, they may not be noxious to sympatric fish.     

Effects of seasonality and population density on the reproduction of the Indo-Pacific echinoid<Emphasis Type="Italic"> Echinometra mathaei</Emphasis> in Kenyan coral reef lagoons

Reproduction in the widely distributed tropical sea urchin Echinometra mathaei (de Blainville) was examined in three Kenyan reef lagoons that differed in substrate cover and E. mathaei population density. Histological examination of the gonads and gonad index measurements for 3 years showed a seasonal reproductive cycle with gametogenesis commencing in July, when temperature and light are at their lowest, and spawning commencing in December and peaking in February to May, when temperature and light reach their annual maxima. Monthly gonad indices correlated significantly with seawater temperature and light intensity. Male urchins had mature gametes for a longer period (8 months) than females (6 months), possibly an adaptive strategy that increases the probability of fertilization. Lunar periodicity was not observed, as male and female gonads were full of gametes on all days of the lunar cycle during the period of spawning. The peak in spawning activity coincides with the peak in phytoplankton abundance, which could ensure high food availability for the planktonic larvae. Gonad weights were significantly higher relative to urchin weight at the reef having the largest mean sizes and lowest population of urchins, indicating availability of food resources for growth and reproduction. E. mathaei at the reef with the highest density had the smallest urchins and high relative gonad sizes, indicative of a tradeoff between growth and reproduction when food is limited.Communicated by O. Kinne, Oldendorf/Luhe     

Diet,behaviour and reproduction of the whelk Buccinum undatum in the northern Gulf of St. Lawrence,eastern Canada

A laboratory experiment was used to examine whether a decrease in the width of sutures is the basis for shrinking of the test in echinoids under conditions of low food availability and whether the body condition of sea urchins is likely to be retained when shrinking occurs. This experiment was performed with H. erythrogramma specimens collected in January 1987 from Pt. Lillias in Corio Bay, Australia. Suture widths in the test and gonad volume were significantly less (relative to body volume) in Heliocidaris erythrogramma (Val.) in the low food treatment than the high food treatment after 4 mo. These results imply that sea urchins which shrink may have a poorer body condition and be less able to reproduce than sea urchins that do not shrink. A comparison of the expected to the observed changes in suture width showed that a decrease in suture width is involved in the shrinking of the test in H. erythrogramma and may be of sufficient magnitude to fully explain the observed reduction in test size. This suggests that the amount a sea urchin may shrink under low food conditions is constrained by the degree to which the sutures can be reduced in width.