Temporal variation in community interfaces: kelp-bed boundary dynamics adjacent to persistent urchin barrens

We examined, over 2 years, factors affecting the temporal stability of the lower limit of kelp beds (Alaria esculenta) at five subtidal sites in the Mingan Islands, northern Gulf of St. Lawrence. The position of the lower limit of the beds varied markedly among sites and over time and was largely controlled by the green sea urchin, Strongylocentrotus droebachiensis, which formed dense (up to 500 individuals m^–2) feeding fronts at the lower edge of the beds. These aggregations advanced over the kelp most rapidly during the summer (at rates as high as 2.5 m month^–1), and there appeared to be a threshold urchin biomass of ~5 kg m^–2 below which the fronts could not substantially reduce the limit of the beds. The fronts consisted mainly of large individuals, whereas smaller urchins predominated in the barrens zone below the kelp beds. At one site, we recorded large seasonal shifts in overall urchin densities, with large increases and decreases during the summer and winter, respectively. An urchin exclusion experiment indicated that algal recruitment in the barrens was two orders of magnitude greater in the absence than in the presence of urchins. The kelp Agarum cribrosum greatly restricted urchin movements, and the greater temporal stability of the kelp bed at one site appeared related to the gradual replacement of Alaria esculenta in the lower kelp bed by a large stand of Agarum cribrosum. We propose that perturbations by abiotic factors (e.g., ice scouring and water motion) trigger important but localized changes in urchin densities that, in turn, largely determine the limits of kelp bed distribution in this region of the Atlantic where urchin barrens are a persistent community state.Communicated by R.J. Thompson, St. Johns     

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.     

Nitrogen fixation by blue-green algae associated with the siphonous green seaweed Codium decorticatum: effects on ammonium uptake

Nitrogen fixation (acetylene reduction) at rates of up to 1.2 g N₂ g dry wt^-1 h^-1 was measured for the siphonous green seaweed Codium decorticatum. No nitrogenase activity was detected in C. isthmocladum. The nitrogenase activity was light sensitive and was inhibited by the addition of DCMU and triphenyl tetrazolium chloride. Additions of glucose did not stimulate nitrogen fixation. Blue-green algae (Calothrix sp., Anabaena sp., and Phormidium sp.) were implicated as the organisms responsible for the nitrogenase activity. They occurred in a reduced microzone within the C. decorticatum thallus where nitrogen fixation was optimized. Nitrogen fixation did not affect the kinetic constants for ammonium uptake in C. decorticatum (K_s=12.0 M, V_max=13.4 mol NH₃ g dry wt^-1 h^-1) determined using the perturbation method. Nevertheless, C. decorticatum thalli which fixed nitrogen had internal dissolved nitrogen concentrations which were over 1.4 times higher than in non-fixing thalli. This suggests that if C. decorticatum does derive part of its nitrogen requirement from the blue-green algae which it harbors, the transfer does not involve competition between this process and the uptake of ambient ammonium.     

Experimental evaluation of aggregation behavior in the sea urchin Strongylocentrotus droebachiensis

Defensive aggregation by Strongylocentrotus droebachiensis has been invoked as the fulcrum for the transformation of macroalgal beds into coralline barrens in the northwest Atlantic. We critically examined some of the mechanisms contributing to aggregation behavior by experimentally manipulating sea urchins, purported predators and food during autumn 1983 and spring 1984 both in the laboratory and field. We utilized several approaches to examine a range of sea urchin responses to the presence of food, tethered predators, caged predators, crushed con-specifics in the field and predators in laboratory tanks. Some of the field cages had the property of allowing free passage to sea urchins while retaining lobsters; this allowed distinctions to be made between artifacts caused by cage walls or topographic barriers and unrestricted behavioral responses of sea urchins. The results falsified the hypothesis that aggregations of S. droebachiensis are elicited by predators. Except in the presence of algae, sea urchins always avoided decapod predators (but not sea stars) and fled from the vicinity of injured conspecifics. However, avoidance behavior was subordinate to feeding responses, demonstrated by mass migrations of sea urchins into cages (with lobsters) to feed on algae. We reaffirmed by observation and manipulation previous studies which showed that sea urchins aggregate only in the presence of food. Two types of sea urchin groupings were delineated: (1) surficial two dimensional associations, often caused by topographic or other features which inhibited dispersal and (2) cohesive three-dimensional aggregations induced by food.     

Effects of salinity on respiration and nitrogen excretion in two species of echinoderms

The 30-d survival limit of Eupentacta quinquesemita and Strongylocentrotus droebachiensis is 12–13 S. The activity coefficient (1 000/righting time in seconds) of stepwise acclimated sea urchins declined from 16.3 at 30 S to 3.5 at 15 S. Oxygen consumption rates (QO₂) of both species held at 30 S and 13°C were highest in June and lowest in December. During the summer, when environmental salinity is most variable in southeastern Alaska, the QO₂ of both species held at 30, 20 and 15 S varied directly with salinity. Perivisceral fluid PO₂ varied directly with acclimation salinity in sea urchins, but not in sea cucumbers. Perivisceral fluid oxygen content of acclimated sea urchins was significantly lower at 15 and 20 S than at 30 S due to reduced PO₂ and extracellular fluid volume at the lower salinities. The QO₂ of both species varied directly with ambient salinity during a 30-10-30. semidiurnal pattern of fluctuating salinity. No change occurred in the average QO₂ of either species over a 15-30-15. semidiurnal pattern of fluctuating salinity. Sea urchin perivisceral fluid PO₂ declined as ambient salinity fluctuated away from the acclimation salinity in both cycles and increased as ambient salinity returned to the acclimation salinity. Total nitrogen excretion of stepwise acclimated sea cucumbers declined significantly from 30 to 15 S, but there was no salinity effect on total nitrogen excretion in sea urchins. Ammonia excretion varied directly with salinity in stepwise acclimated sea cucumbers (67–96% of total nitrogen excreted), but there was no salinity effect on ammonia excretion (89–95% of total nitrogen excreted) of sea urchins. Urea excretion did not vary with salinity in sea cucumbers (2–4% of total nitrogen excreted) or sea urchins (2–9% of total nitrogen excreted). Primary amines varied inversely with salinity in sea cucumbers (2–30% of total nitrogen excreted), but did not vary with salinity in sea urchins (2–4% of total nitrogen excreted). The oxygen: nitrogen ratio of both species indicated that carbohydrate and/or lipid form the primary catabolic substrate. The O:N ratio did not vary as a function of salinity. Both species are more tolerant to reduced salinity than previously reported, however, rates of oxygen consumption and/or nitrogen excretion are modified by salinity as well as season.     

Changes in sea urchin populations after the destruction of kelp beds

In St. Margaret's Bay, Nova Scotia, Canada, there are large areas in which sea urchins (Strongylocentrotus droebachiensis) have eliminated beds of kelp (Laminaria spp.). Sites were identified where destruction of kelp beds had taken place 1, 2, 3, 3.5 and 4 years ago. With increase of time since kelp disappearance, the sea urchins showed decreased growth rate, reduced gonad size, but an increase in numbers resulting from high recruitment rates in the first two years after kelp bed destruction. These sea urchin populations, by their browsing, effectively prevent the regeneration of kelp. There is, as yet, no evidence of the sea urchin populations being starved out to allow the kelp to return.     

Diel foraging patterns of the sea urchin Centrostephanus coronatus as a predator avoidance strategy

During the day, the diadematid sea urchin Centrostephanus coronatus occupies holes and crevices in shallow subtidal rocky substrata. Individuals emerge from these after sunset and forage on organisms attached to the surrounding rock surface. Each urchin travels <1 m from its shelter and returns to the same one before sunrise. The sheephead wrasse Pimelometopon pulchrum does not remove urchins from their shelters, but will attack and consume urchins placed in normal feeding locations during the daytime. The active periods of the sheephead and the urchin do not overlap; urchins begin foraging about 20 min after the diurnal sheephead retire in the evening and return to their shelters 1 to 2 h before sheephead resume feeding in the morning. We infer that the urchin's daytime crevice-dwelling and nocturnal foraging habits have evolved as a response to sheephead predation. Moreover, because shelters are limited in supply, shelter fidelity may have evolved to insure refuge from sheephead.     

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.     

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.     

Seasonal variation in movement,aggregation and destructive grazing of the green sea urchin (<Emphasis Type="Italic">Strongylocentrotus droebachiensis</Emphasis>) in relation to wave action and sea temperature

Hydrodynamic forces are an important determinant of subtidal community structure, particularly when they limit the distribution and foraging ability of mobile consumers. We examined the effect of wave action on the rate of movement and destructive grazing of a kelp bed by the green sea urchin (Strongylocentrotus droebachiensis) under field conditions. We measured density and rate of advance at fixed intervals along ∼100 m of a grazing front over 1 year, and quantified individual movement rates in the barrens 5–10 m behind the urchin front using a time-lapse videography. Seasonal variation in the mean rate of advance of the front (range: 0–4 m month⁻¹) was explained by changes in urchin density at the front (120–360 individuals m⁻²), which in turn varied inversely with significant wave height (0.5–2 m). Water temperature (0.8–17.6°C) had no effect on the rate of advance or on urchin density (aggregation) at the front, except when temperature exceeded 17°C. Movement of individual urchins also was affected by wave action: we observed a significant decrease in speed and displacement of urchins with increasing significant wave height. Wave action had no effect on the proportion of urchins moving or the degree of linearity of their movements. We propose that the decrease in urchin density at the front associated with increased wave action, results from de-aggregation, which reduces the risk of dislodgement, combined with a reduction in urchin movement in barrens, which supplies new urchins to the front.     

Changing lobster abundance and the destruction of kelp beds by sea urchins

In a study area in Nova Scotia, Canada, abundance of the lobster Homarus americanus decreased by nearly 50% in 14 years. The lobster is a major predator of sea urchins, and during the past 6 years the sea urchin Strongylocentrotus droebachiensis has destroyed 70% of the beds of Laminaria spp. in the area. Implications for management are discussed.     

Ecological growth strategies in the seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae): Soluble nitrogen and reserve carbohydrates

The seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae) were grown in an outdoor continuous-flow system at both ambient incident light (I₀) and 0.13 I₀. During the winter, both species accumulated substantial soluble nitrogen reserves (up to 1020 g-at N·g dry wt^-1 in G. foliifera and 630 g-at N·g dry wt^-1 in Ulva sp.). The rate at which these N reserves were depleted was proportional to the growth rate. Seaweeds grown at 0.13 I₀ had lower growth rates and higher levels of soluble tissue N than plants grown at I₀. During the spring-summer growing season, peaks in tissue N followed nutrient peaks in the ambient seawater. Ulva sp. had higher nutrient uptake and growth rates than G. foliifera and showed greater fluctuations in soluble tissue N. This may characterize opportunistic seaweed species with high biomass turnover rates. At I₀, the levels of starch (up to 340 mg·g dry wt^-1 in G. foliifera and 170 mg·g dry wt^-1 in Ulva sp.) were highest during the spring and summer. During this period, fluctuations in starch content were inversely related to growth rate and soluble tissue N. Seaweeds grown at 0.13 I₀ did not accumulate starch. Neither species was found to overwinter with starch reserves.     

Anaerobic and aerobic energy metabolism in ovaries of the sea urchin Strongylocentrotus droebachiensis

The gonads of sea urchins undergo large changes in mass during their gametogenic cycle. In addition, they have relatively low aerobic capacities and are poorly perfused by the circulatory system and thus are continually hypoxic or anoxic. The present study of Strongylocentrotus droebachiensis investigates seasonal changes in the relationships among mass of the ovaries, pH and P_O2 of the perivisceral coelomic fluid which bathes the ovaries, and partitioning of ovary energy metabolism into its anaerobic and aerobic components. S. droebachiensis were collected at Blue Hill Falls, Maine, USA, from August 1982 to March 1984. We found that from 76 to 92% of the heat dissipated by isolated ovaries of the sea urchin S. droebachiensis derives from anaerobic energy metabolism at partial pressures of oxygen prevailing in vivo. Ovaries from S. droebachiensis have the capacity to produce large amounts of lactate under imposed anoxia, but lactate accounts for only 37% of the total anoxic heat dissipation, which suggests that other end products of anaerobiosis are present. Seasonal changes in pH and P_O2 of the perivisceral coelomic fluid can be explained by a complex relationship among changes in temperature, reproductive condition, and anaerobic and aerobic metabolism in the ovaries, gut and body wall. Seasonal changes in the buffering capacity of the perivisceral coelomic fluid must be determined before the effects of respiratory and metabolic acid production on the acid-base status of the coelomic fluid can be fully understood.     

Enlarged lantern size in similar-sized,sympatric, sibling species of Strongylocentrotid sea urchins: from phenotypic accommodation to functional adaptation for durophagy

Strongylocentrotus droebachiensis is a well known omnivorous sea urchin with an unrivalled capacity to destroy North Atlantic kelp forests. S. pallidus is a lesser known, morphologically similar, and closely related species with no record of destructive grazing, despite its larger lantern size. I quantify the lantern size of both species using bivariate allometric analysis, and test the hypothesis that enlarged lantern size facilitates durophagy, the consumption of hard prey, by measuring the feeding capacity of urchins with different lantern sizes when offered a hard-shelled prey, the blue mussel, Mytilus edulis. The results suggest that S. droebachiensis has a limited capacity for durophagous feeding irrespective of lantern size, whereas in S. pallidus the ability to exploit hard shelled prey is positively related to lantern size. This is apparently the first evidence of a relationship between trophic morphology and diet in regular sea urchins. The hypothesis of systematic latitudinal variation in the lantern size of S. pallidus is reappraised and rejected. S. droebachiensis had larger gonads than S. pallidus in field samples, confirming that its small lantern is not impeding nutrient acquisition in shallow habitats.     

Seasonal variation in total and soluble tissue nitrogen of Pleurophycus gardneri (Phaeophyceae: Laminariales) in relation to environmental nitrate

Seasonal variations in tissue nitrogen (ethanol soluble nitrate and ninhydrin positive substances, as well as total nitrogen) of different thallus parts of Pleurophycus gardneri Setchell and Saunders were monitored simultaneously with ambient seawater nitrate from 1982 until 1984 in Bamfield, Vancouver Island, British Columbia, Canada. A trend of low, nearly zero levels in ambient nitrate typical for the area in late spring and early summer normally contrasts with average nitrate concentrations of 10 mol NO₃ ^- l^-1 in late fall and winter. Total nitrogen content was greater in the perennial thallus parts, stipe and holdfast than in the annual blade and peaked in fall and early winter. The longitudinal thallus distribution of nitrate revealed a distinct and significant concentration of nitrate in the haptera reaching at maximum 8% nitrate-N of the internal total nitrogen. Internal nitrate concentration ranged from 20 to 5 000 times the ambient nitrate concentration in the midrib, and from 40 to 3 100 times in the wing, while the range was greatest with 400 to 14 000 times in the haptera. P. gardneri contained at most about 7 mol NO₃ ^- g fresh wt^-1 in the blade, which corresponds to about 6% of total tissue nitrogen. Ninhydrin positive substances comprised the major portion of the soluble N pool in P. gardneri and showed a pronounced seasonality. Concentrations of ninhydrin positive substances ranged from 20 to 800 g N g fresh wt^-1 in the midrib and in the wing. In the stipe, ninhydrin positive substances varied from 180 to 2 200 g N g fresh wt^-1, and from 250 to 1 200 g N g fresh wt^-1 in the haptera. Evidence is given that (1) the perennial parts, stipe and haptera of P. gardneri contain the majority of nitrogen products independent of season and ontogenetic stage; (2) ninhydrin positive substances are the most abundant internal nitrogen constituents; (3) the low N values in the blade in summer suggest a nitrogen limited growth; and (4) nitrate may not be the predominant external nitrogen source.     

Effects of food type and ration on reproductive maturation and growth of the sea urchin Strongylocentrotus droebachiensis

We investigated the effects of food quality and quantity on reproductive maturation and growth of juveniles of Strongylocentrotus droebachiensis (Müller) in a 22 month laboratory experiment in which we fed sea urchins four diets: (1) kelp (Laminaria spp.) for 6 d wk⁻¹ and mussel (Mytilus spp.) flesh for 1 d wk⁻¹ (KM); (2) kelp for 7 d wk⁻¹ (high ration, KH); (3) kelp for 1 d wk⁻¹ (low ration, KL) and (4) no food other than encrusting coralline algae (NF). At their first and second opportunity for reproduction, all sea urchins in the KM and KH treatments, and most in the KL treatment were reproductively mature, whereas all sea urchins in the NF treatment remained immature. Gonad index differed significantly among all fed treatments at first and second reproduction, and was highest in the KM and lowest in the KL treatment. Gonad index was similar in both sexes at first reproduction, but it was higher in females than in males at second reproduction. Diet had little or no effect on the relative abundance of spermatocytes, spermatozoa, or nutritive phagocytes in testes at first and second reproduction. In ovaries, nutritive phagocytes were significantly more abundant in females in the KM and KH treatments than in the KL treatment at first reproduction, and significantly more abundant in unfed (NF) than fed (KM, KH, KL) females at second reproduction. Mean oocyte size was similar in all fed females at first reproduction, but significantly larger in fed than unfed females at second reproduction. Mean ovum size was similar in all fed females in both reproductive periods. Increase in test diameter was greatest in the KM treatment and smallest in the KL treatment; sea urchins in the NF treatment decreased slightly in size. Survival was 95 to 100% in all fed sea urchins but significantly lower in unfed ones. The feeding rate on kelp was significantly greater in the KL than the KM and KH treatments. In the KM treatment, the feeding rate on kelp increased significantly over a 6 d period after mussel flesh was provided. Our results demonstrate that a diet of high food quality and quantity accelerates reproductive maturation and growth rate, and enhances gonad production and survival in juvenile and young adult S. droebachiensis. These findings contribute to our understanding of the reproductive ecology of S. droebachiensis in habitats with differing food supplies (e.g., kelp beds and barrens). Our results also can be used to improve aquacultural practices for sea urchins. Received: 3 June 1998 / Accepted: 2 February 1999     

Effects of predators on sea urchin density and habitat use in a southern California kelp forest

Grazing sea urchins can reduce kelp abundance and therefore strongly affect kelp forest community structure. Despite the ecological importance of sea urchins, direct field studies on the role that urchin predators play in shaping urchin populations are rare for southern California. We conducted surveys and manipulative experiments within kelp forests near San Diego, CA, (32–51′28″N, 117–16′00″W) from 2006 to 2009 to determine whether predators such as sheephead (Semicossyphus pulcher) and spiny lobsters (Panulirus interruptus) may be linked to purple urchin (Strongylocentrotus purpuratus) and red urchin (Strongylocentrotus franciscanus) distribution and habitat use, as well as purple urchin density-dependent mortality. Purple urchins were less dense and more cryptic inside a local marine protected area (MPA) that contained high predator abundance than in nearby heavily fished areas, whereas red urchins rarely were found outside the MPA. Urchin proportional mortality was inversely density dependent during the day when sheephead were active, despite fish aggregations in plots of high urchin density, but was density independent during the night when lobsters were active. Urchin mortality was reduced under understory algal cover during the day, but not during the night. Examining whether urchin mortality from predation is density dependent and how habitat complexity influences this relationship is imperative because behavioral changes and increases in urchin populations can have vast ecological and economic consequences in kelp forest communities.     

Incidence,occurrence and distribution of the nematode Echinomermella matsi in its echinoid host,Strongylocentrotus droebachiensis,in northern Norway

The incidence, occurrence and distribution of the nematode Echinomermella matsi (Jones and Hagen) in its host, the echinoid Strongylocentrotus droebachiensis Müller, were examined in Norwegian waters from Nordmre (63° N) to the Russian border (70° N) in a range of habitats (e.g. kelp cover and community type) from 1982 to 1992. The results are used to examine the likely influence of the parasite on fitness (individual growth and gonad production) and its potential in regulating the population size of its echinoid host. The nematode infections are restricted to areas between Vikna (Vega) and middle Troms, with the highest prevalence (40 to 88% infected) in the Bodø area, the focus of the distribution. The prevalence rates are similar on kelp beds and barren grounds. No difference in growth rates between infected and noninfected sea urchins was found. Nematode-infected sea urchins in kelp bed populations had gonads weighing about one-sixth of those of equivalent uninfected individuals. This indicates that nematodes are really parasites causing reduced reproductive capacity. Areas with high prevalence (>40% infected) of nematodes had low sea urchin densities. High nematode intensities were found in the areas with high prevalence. The mean abundance in high prevalence areas was highest (eight nematodes) in the 4-to 7-yr-old age group and decreased to zero among the oldest age groups. The ratio of variance/mean abundance was also highest among 5-to 8-yr-old sea urchins. At highly and slightly infected locations, the relationships between the log of the variance and the log of the mean abundance of parasites were 1.35 and 1.69, respectively. Measurements of the frequency distribution of nematodes in sea urchins and the corresponding estimates of the negative binomial distribution showed significantly higher counts in estimated than observed subpopulations exceeding 20 nematodes per sea urchin. These results may explain nematode-induced sea urchin mortality. The low density of the host in the Bodø area, which had high prevalence, may be a result of increased host mortality caused by nematodes, by lower recruitment because of reduced gonad production, or by other factors. Low prevalence among older sea urchins may be caused by nematodeinduced host mortality or by cycles in infection. However, studies on the life cycle of the nematode and long term investigations of host density and nematode intensity/prevalence will be needed before any true conclusions can can be drawn.     

Bioerosion in coral reef communities in southwest Puerto Rico by the sea urchin<Emphasis Type="Italic"> Echinometra viridis</Emphasis>

Bioerosion is one of the most important structuring forces in coral reef communities. The bioerosion impact of several species of fish, sponges and sea urchins have been estimated in the Caribbean; however, there is no information for one important species, the red sea urchin Echinometra viridis. This species can be found in high densities in many localities. In this study, bioerosion rates for E. viridis were estimated in two patch reefs off La Parguera, southwest Puerto Rico, using the population size-class distribution, average densities, and the CaCO ₃ content in fecal pellets produced over 24 h. Average densities of urchins along four depth intervals were estimated using 40-m transect lines and 1-m ² quadrats. Average size and size-structure distribution were estimated by measuring the diameter of 180–220 urchins haphazardly collected at each of the four depth intervals. The ignition–loss method was used to estimate the daily rate of bioerosion. Fecal pellets produced by the urchins over a 24 h period were collected in buckets, rinsed in fresh water, dried for 24 h at 70°C, and then burned in a furnace at 550°C, first to eliminate organics, and then at 1000°C until constant weight to determine the amount of calcium carbonate (CaCO ₃) in the fecal pellets. HCl (10%) was then added to the remainder of the sample to test for presence of CaCO ₃. Average individual CaCO ₃ bioerosion rates were estimated at 0.181±0.104 g day ^-1. Average densities (0.77–62.0 ind. m ^-2), size (2.01–2.44 cm) and average bioerosion rates (0.114–4.14 kg m ^-2 year ^-1) were significantly higher in shallow areas (1–3 m) in both reefs. Bioerosion rates were low compared to those reported for parrotfish, endolithic sponges and the black sea urchin D. antillarum, but they were higher than those reported for other small-sized sea urchins in the Caribbean and the Indo-Pacific.