Strongylocentrotus Franciscanus Research Articles

Preparation and characterization of mitotic cytoskeletons from embryos of the sea urchin Strongylocentrotus franciscanus

We present here a preparation protocol and molecular characterization of mitotic cytoskeletons isolated from embryos of the spiny red sea urchin, Strongylocentrotus franciscanus. The mitotic cytoskeletons are produced by detergent lysis of S. franciscanus embryos during cell division and consist of a mitotic apparatus enclosed in a thin shell of cortical filaments. Mitotic cytoskeletons produced in this way are stable for a year at liquid nitrogen temperature. S. franciscanus is of interest because it is commercially available in large quantities and each animal provides prodigious amounts of gametes. Furthermore, embryos from this sea urchin will develop synchronously at high densities and are an excellent source of quantities of cytoskeletal proteins appropriate for biochemical studies.

SEA OTTERS AND BENTHIC PREY COMMUNITIES IN WASHINGTON STATE

AbstractAn August 1987 benthic survey of otter‐free and otter‐occupied areas along the outer coast of Washington State's Olympic Peninsula confirms that this area has been as profoundly influenced by sea otters as other rocky, nearshore communities studied in California, Canada, and Alaska. Prey density, size, and biomass were found to be negatively correlated with sea otter abundance, suggesting that the re‐introduction of sea otters to this area in 1969–1970 has profoundly affected invertebrate prey abundance and distribution, particularly that of the red sea urchin, Strongylocentrotus franciscanus. Red urchin distribution appears to influence algal groups differently and in a manner consistent with current otter/urchin/kelp theory. Foliose red algal abundance was negatively related to urchin numbers and coralline crusts were positively correlated. Aerial photographs of Macrocystis integrifolia cover at Cape Alava suggest an increase since the introduction of sea otters. Given the present distribution of prey along the Olympic Peninsula coast, we conclude that as the sea otter population continues to grow, range expansion is more likely to occur to the north, which may also lead to possible conflicts with an increasing sea urchin fishery and Native American set net activity.

Survival of Juvenile Giant Kelp: The Effects of Demographic Factors, Competitors, and Grazers

Patterns of survival of juvenile giant kelp (Macrocystis pyrifera) were examined in relation to grazing by sea urchins, shading by adult kelp, density of recruits, densities of other algal species, and substrate distribution. Survival was poorer in areas where white sea urchins (Lytechinus anamesus) were abundant or where there was an overlying canopy of adults. At other sites, the density of recruits explained the greatest proportion of variability in survival, and the proportion of juveniles that survived was negatively correlated with the number of recruits. Potential algal competitors (Pterygophora californica and Cystoseira osmundacea), red sea urchins (Strongylocentrotus franciscanus), and substrate distributions had no significant effect on survival. The mechanisms of density—dependent survival were similar to "dominance and suppression" models that explain self—thinning in many terrestrial plant populations. In stands where densities of recruits were high, skewed size distributions developed, probably as the result of competition for light. High—density stands had proportionally more small plants than low—density stands, and, during subsequent storms, only the larger plants survived. This led to a more equitable distribution of survivors than would be expected based on the number of recruits. Intraspecific interactions, both between adults and juveniles and among juveniles of the same cohort, appear to be important structuring forces in most giant kelp populations. The parallels in many terrestrial populations. However, in other algal populations, there is little evidence for density—dependent mortality or for the dominance—suppression hypothesis. These differences may relate to competition for light, which is intense in Macrocystis forests and many terrestrial populations, but less so in assemblages of other benthic algae.

Settlement and recruitment of sea urchins (Strongylocentrotus spp.) in a sea-urchin barren ground and a kelp bed: are populations regulated by settlement or post-settlement processes?

I sampled recruitment of very small sea urchins (Strongylocentrotus spp.) by using the anesthetic magnesium chloride to remove individuals from substrata collected in sea-urchin barren grounds (barrens) and kelp beds at Naples Reef near Santa Barbara, California, USA. Preliminary sampling found low numbers of newly settled individuals(<0.6 mm test diam) from April–July in 1984 and 1985, and in April, 1986. In early May, 1986, I found many newly settled seaurchins (0.3 to 0.6 mm, 5 to 17 d old), and I compared the densities of the cohort on several types of natural substrata in barrens and kelp-bed habitats. Newly settled individuals of both purple sea urchins (S. purpuratus) and red sea urchins (S. franciscanus) were present in similar, high densities (1 000 S. purpuratus m-2) on foliose red algal turf, a dominant substratum ofthe kelp bed, and on crustose coralline algae, the dominant substratum of an adjacent barrens. Larvae of S. purpuratus reared and tested in the laboratory showed high rates of settlement on both red algal turf and on crustose coralline algae, but significantly lower rates on rock. Larvae also settled in response to a partiallypurified extract of coralline algae. The reduced settlement on natural rock surfaces relative to either algal treatment and the significant settlement in response to the extract of coralline algae indicate that larvae discriminate between natural substrata and probably respond to a settlement cue other than, or in addition to, a simple microbial (bacterial) film. The similar densities of young recruits of S. purpuratus on dominant substrata of barrens and kelp bed show that, at least in this case, differential settlement cannot explain the high densities of sea urchins in the barrens habitat. Movement between barrens and kelp bed is unlikely given the small sizes of the newly recruited sea urchins relative to the large distances often involved. Reduced post-settlement mortality of newly settled individuals in the barrens remains the most likely mechanism leading to the higher densities of sea urchins in barrens relative to kelp-bed habitats.

Localization and developmental fate of ovoperoxidase and proteoliaisin, two proteins involved in fertilization envelope assembly

Fertilization of the sea urchin egg leads to the assembly of an extracellular matrix, the fertilization envelope. Ovoperoxidase, the enzyme implicated in hardening the fertilization envelope, is inserted into the assembling structure via a Ca2+-dependent interaction with the protein proteoliasin (P. Weidman and B. M. Shapiro, 1987, J. Cell Biol. 105, 561-567). In the present report, polyclonal antisera were raised to ovoperoxidase and proteoliasin (purified from eggs of Strongylocentrotus purpuratus) and characterized by Western blot analysis and an enzyme-linked immunoabsorbent assay (ELISA). By indirect immunofluorescence microscopy all cortical granules of unfertilized eggs, as well as the fertilization envelope, contained both proteoliasin and ovoperoxidase. At the ultrastructural level both proteins are localized to the electron-dense spiral lamellae of the cortical granules. Western blot analysis revealed that ovoperoxidase and proteoliasin persist in early embryos until hatching, but are absent from later developmental stages. Homogenates of eggs of several other echinoderm species (Strongylocentrotus droebachiensis, Strongylocentrotus franciscanus, Pisaster ochraceus, Dendraster excentricus, and Lytechinus pictus) also contain proteins antigenically similar to ovoperoxidase and proteoliaisin, indicating that many echinoderms utilize a similar strategy for assembly of the fertilization envelope. The results underline the need for postsecretory controls in the extracellular matrix modifications that accompany the cortical reaction.

Recruitment characteristics of the commercially harvested red sea urchin Strongylocentrotus franciscanus in Southern British Columbia, Canada

The test size frequencies of red sea urchin, Strongylocentrotus franciscanus (Agassiz), populations from British Columbia revealed low recruitment overall (≈9.5%) compared with populations from lower latitudes. Juvenile urchins tended to shelter under adult conspecifics. On a fine scale, i.e. over 0.5–8.0 km, high variability in recruitment was evident. Possible effects of commercial harvesting are discussed. Site-specific harvest controls based on local habitat and urchin population (size-frequency) data are desirable as a fishery management tool, but would be too costly to gather over such an extensive coastal area.

Utilization of purple and red sea urchins ( Strongylocentrotus purpuratus Stimpson and S. franciscanus Agassiz) as food by the white sea urchin ( Lytechinus anamesus Clark) in the field and laboratory

White sea urchins ( Lytechinus anamesus Clark) attacked purple ( Strongylocentrotus purpuratus Stimpson) and red ( S. franciscanus Agassiz) sea urchins at Anacapa Island, California. Densities of white urchins were highest in the deep algal crust-dominated community where up to 6% of purple and 25% of red urchins were being attacked by white urchins. Up to 9% of Lytechinus anamesus in an area were actively eating stronglylocentrotids and usually, more than one white urchin was involved in the attack. In areas with low densities of white urchins, no strongylocentrotids were being attacked. After 36 h in the laboratory, there was no difference in the number of white urchins attacking injured or healthy purple urchins in each of the three experimental densities of white urchins. However, both injured and healthy urchins were attacked by more white urchins in high density. When given a choice between injured purple urchins or fresh kelp, white urchins overwhelmingly chose kelp. Data suggest that white urchins utilize other urchin species as an alternative source of food when more preferred food is absent, but will switch to preferred food should it become available.

Experimental evaluations of substrate types as barriers to sea urchin (Strongylocentrotus spp.) movement

Descriptive evidence that sandy surfaces and rock ledges inhibit progress of grazing sea urchins prompted an experimental investigation of physical obstacles to urchin movement in a subtidal area of reef and kelp off southern California in 1980. In laboratory experiments, we found that both red (Strongylocentrotus franciscanus) and purple (S. purpuratus) urchins can negotiate sand using their oral spines, although purple urchins are slower and more hesitant to do so. In field experiments, we observed the fates of starved red urchins transported to replicate plots within stands of sand-or rock-based understory kelp (Pterygophora californica). Urchins in rock plots retreated to nearby crevices from where they ate attached kelp. After finding kelp blades, urchins soon disappeared from sand plots because individuals in small groups may have difficulty holding and eating attached kelps on unconsolidated surfaces. In another experiment, red and purple urchins reached kelp on a rock ledge by mounting an artificial ramp. We conclude that by using their tube feet, individuals of both species move best over flat, hard surfaces, although soft substrate may constitute a major barrier only to purple urchins. In the absence of effective predator control, urchins can surmount most sand or rock barriers when water motion subsides. Hence, their ability to coordinate spine movements to negotiate soft substrates may be an adaptation to invade kelp refuges during quiet periods if preferred drift food is unavailable.

Social behaviour of juvenile red sea urchins, Strongylocentrotus franciscanus (Agassiz)

This study examined social behaviour of juvenile red sea urchins, Strongylocentrotus franciscanus (Agassiz). At four coastal locations in British Columbia, Canada, distributions of juvenile sea urchins were examined in relation to adults. One-third of all juveniles were found underneath an adult “spine canopy”, another third outside the spine canopy but close to an adult, and the remainder were found apart from adults. Juveniles associated with adults were contagiously distributed with each other. In the laboratory, juveniles chose adults over other target locations. They selected adults in particular locations over others, and in one case showed selection among adults; but they did not select between fed and non-fed adults, nor between adults that had been “protective” and “non-protective” of juveniles in the field. In time-lapse film studies, starved juveniles showed little change in social behaviour when food was introduced, but showed a dramatic change in behaviour when predators were active. We suggest that juvenile red sea urchins are found under the spine canopies of adults as a result of juvenile behaviour; and that this interaction functions to protect juveniles from predation. The strong gregarious nature of juveniles among themselves may have evolved in a different selective situation as a defence against predation and to ensure reproductive success.

Evolutionary conservation of DNA sequences expressed in sea urchin eggs and early embryos

DNA sequence divergence measurements indicate that Strongylocentrotus franciscanus is more distinct from S. purpuratus and S. drobachiensis than these two species are from each other, in agreement with paleontological and morphological evidence. The evolutionary divergence of several classes of expressed DNA sequences was compared with that of total single-copy DNA. Between S. franciscanus and S. purpuratus the divergence of cDNA made from gastrula cytoplasmic poly(A)+ RNA is about half that of total single-copy DNA. Similar results were obtained for cDNA made from unfertilized egg poly(A)+ RNA. In contrast, sequences expressed in gastrula nuclear RNA have diverged almost as much as total single-copy DNA.

Evolution of two actin genes in the sea urchin Strongylocentrotus franciscanus.

The complete nucleotide sequences of two chromosomally linked actin genes from the sea urchin Strongylocentrotus franciscanus are presented. The genes are separated by 5.7 kilobases, occur in the same transcriptional orientation, and contain introns in identical positions. The structures and nucleotide sequences of the two genes are extremely similar, suggesting that they arose through a recent duplication. Comparison of the nucleotide sequences of the genes allows inferences to be made about mutational mechanisms active since the duplication event. Whereas point mutations predominate in the coding regions, the introns and flanking DNA are more heavily influenced by a variety of events that cause simultaneous changes in short regions of DNA.

Food Availability, Sea Urchin Grazing, and Kelp Forest Community Structure

Unlike most previously reported kelp forest, which exhibit long—term persistence over large spatial scales, the kelp forest community on the west end of San Nicolas Island, California, occurs as a dynamic patchwork of barren areas characterized by grazing sea urchins and an algal assemblage consisting of upright and encrusting coralline algae, and kelp—dominated areas characterized by high densities of perennial brown algae, including the giant kelp Macrocysits pyrifera. In this study we examine the factor that regulate the grazing of sea urchins (Strongylocentroutus franciscanus), which in turn determines the structure of this community. In the barren area, drift algae sparse, sea urchins were poorly nourished, occupied open, unprotected microhabitats, and actively grazed the substratum. In the kelp—dominated are, drift algae were abundant, sea urchins were well nourished, moved little, occupied cracks and crevices, and probably fed on the drift algae. Early in the study substantial recruitment of brown macroalgae occurred in both sites, and the barren area gradually transformed into a kelp—dominated area. Concomitant with this change, the abundance of drift algae in the barren area increased. Urchins in this area abandoned open microhabitats for protected crevices and pockets, and sea urchin grazing intensity was reduced to levels characteristic of the kelp—dominated area. From these observations we present a qualitative model in which the transformation from one configuration to another is triggered by a behavioral switch in the mode of feeding of red sea urchins. This switch is controlled by the availability of drift algae, which in turn is dependent upon the abundance of attached macroalgae. The adult algal standing stocks appear to be regulated by prevailing hydrographic conditions, which are either favorable or unfavorable for kelp recruitment, survivorship, and growth. In this model grazing intensity is independent of urchin density and therefore provides a new view of how kelp forest communities function.

Severe storm disturbances and reversal of community structure in a southern California kelp forest

Regular observations made over a period of 5 yr in four permanent transects provided data on plant, sea urchin, and fish densities which indicate that two unusually severe winter storms in 1980 (“Storm I”) and 1983 (“Storm II”) had different effects on a southern California kelp-forest community. Storm I removed all canopies of the giant kelp Macrocystis pyrifera, but spared most understory kelps, mainly Pterygophora californica. Hence, the previously large accumulation of detached drift kelp, mostly M. pyrifera, disappeared. Denied their preferred diet of drift kelp, the sea urchins Strongylocentrotus franciscanus and S. purpuratus then emerged from shelters to find alternative food. Without effective predators, they consumed most living plants, including the surviving understory kelps. This weakened the important detritus-based food chain, as indicated indirectly by declining abundances of algal turf and fish (Embiotocidae) that eat small animals living in turf. In 1983, Storm II reversed the process by eliminating exposed urchins, while clearing rock surfaces for widespread kelp settlement and growth. By summer 1984, the kelp grew to maturity to form extensive canopies despite elevated water temperatures during summer and fall of 1983. Thus, severe storms may have vastly different effects on community structure, depending on the state of the community before the disturbance.

Mechanical Properties and Control of Nonmuscular Catch in Spine Ligaments of the Sea Urchin, Strongylocentrotus Franciscanus

ABSTRACT Catch was induced in the spine ligaments of Strongylocentrotus franciscanus (Agassiz) (surrounding muscle and nervous tissue removed) by mechanical agitation. When the catch mechanism is set, the tensile stiffness of urchin ligament can change by nearly an order of magnitude and approaches that of vertebrate tendon. Deflection of a spine in catch through an angle of more than 10° damages the catch mechanism. Setting of catch depends on extracellular calcium. Transglutaminase inhibitors (cadaverine and putrescine) prevent the setting of catch. A model involving Ca-dependent activation of a transglutaminase-like enzyme is discussed as a possible basis for the setting of catch by crosslinking collagen fibrils.

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.

Sea urchin actin gene subtypes: Gene number, linkage and evolution

The actin gene family of the sea urchin Strongylocentrotus purpuratus was analyzed by the genome blot method, using subcloned probes specific to the 3' terminal non-translated actin gene sequence, intervening sequence and coding region probes. We define an actin gene subtype as that gene or set of genes displaying homology with a given 3' terminal sequence probe, when hybridized at 55 degrees C, 0.75 M-Na+. By determining the often polymorphic restriction fragment band pattern displayed in genome blots by each probe, all, or almost all of the actin genes in this species could be classified. Our evidence shows that the S. purpuratus genome probably contains seven to eight actin genes, and these can be assigned to four subtypes. Studies of the expression of the genes (Shott et al., 1983) show that the actin genes of three of these subtypes code for cytoskeletal actins (Cy), while the fourth gives rise to a muscle-specific actin (M). We denote the array of S. purpuratus actin genes indicated by our data as follows. There is a single CyI actin gene, two or possibly three CyII genes (CyIIa, CyIIb, and possibly CyIIc), three CyIII actin genes (CyIIIa, CyIIIb, CyIIIc), and a single M actin gene. Comparative studies were carried out on the actin gene families of five other sea urchin species. At least the CyIIa and CyIIb genes are also linked in the Strongylocentrotus franciscanus genome, and this species also has a CyI gene, an M actin gene and at least two CyIII actin genes. It is not clear whether it also possesses a CyIIc actin gene, or a CyIIIc actin gene. The genome of a more closely related congener, Strongylocentrotus dröbachiensis, includes 3' terminal sequences suggesting the presence of a CyIIc gene. In S. franciscanus and S. dröbachiensis the first intron of the CyI gene has remained homologous with intron sequences of both the CyIIa and CyIIb genes, indicating a common origin of these three linked cytoskeletal actin genes. Of the four S. purpuratus 3' terminal subtype probe sequences only the CyI 3' terminal sequence has been conserved sufficiently during evolution to permit detection outside of the genus Strongylocentrotus. An unexpected observation was that a sequence found only in the 3' untranslated region of the CyII actin gene in the DNA of S. dröbachiensis and S. purpuratus is represented as a large family of interspersed repeat sequences in the genome of S. franciscanus.

Spiny lobsters and sea urchins: Analysis of a predator-prey interaction

Spiny lobsters, Panulirus interruptus (Randall), are important predators of two species of sea urchins, Strongylocentrotus franciscanus (A. Agassiz) and S. purpuratus (Stimpson), which are major herbivores in southern California kelp forest communities. Aquarium experiments indicate that spiny lobsters strongly prefer S. purpuratus over S. franciscanus of comparable test diameter, probably accounting for the higher number of S. purpuratus mortalities observed in the field. Juvenile S. franciscanus (< 60 mm test diameter, TD) are preferred over larger conspecifics, but the smallest S. franciscanus (< 30 mm TD) are protected by association with the adult urchin spine canopy. Mid-sized S. franciscanus too large to fit under the spine canopy are subject to intense predation. All sizes of P. interruptus tested showed a notable reluctance to attack large S. franciscanus (> 90 mm TD) when offered a range of sizes, although the larger lobsters are capable of handling the largest urchins. Relative movement, spine length, test thickness, and gonad weights of sea urchins were examined to evaluate the factors affecting the lobsters' choice of prey, and behavioral responses to lobsters were assessed to determine urchin availability to these predators. The handling problems associated with the long spines of larger S. franciscanus appear to be the major considerations in prey choice despite the much greater potential energy gain from these urchins. Thus predation by spiny lobsters appears to be sufficient to explain the bimodal size-frequency distribution typical of S. franciscanus populations in areas where these predators are common. Heavy fishing pressure on spiny lobsters during this century probably resulted in the release of sea urchin populations and contributed to episodes of destructive urchin grazing observed in the 1950's and subsequent years.

Organization and evolution of the actin gene family in sea urchins.

Genomic libraries of the sea urchins Strongylocentrotus franciscanus and Lytechinus pictus were screened with an actin cDNA clone from Strongylocentrotus purpuratus. Four nonoverlapping clones were isolated and characterized from the S. franciscanus library; three were isolated and characterized from the L. pictus library. Linked genes having the same transcriptional orientation were found on all S. franciscanus clones. Three clones contained two actin genes each; the other clone contained three. In contrast, the L. pictus clones contained only one actin gene. Comparison of actin genomic clones from these three species indicated a difference in the genomic organization of sea urchin actin genes in that the genes appear to be more highly clustered in S. franciscanus than in S. purpuratus and L. pictus. Genomic dot blots and reassociation kinetics demonstrated that the copy number of actin genes in all three species is 15 to 20. Nucleotide sequence homology of actin genes within and among the species was measured by thermal elution. These experiments indicated that there is a high degree of interspecies actin gene sequence homology but that, within each species, actin gene sequences may differ by as much as 30%. Sequencing of two S. franciscanus actin genes revealed introns at the same amino acid positions, 121 and 204, reported for S. purpuratus actin genes. These data demonstrated that the genomic copy number, the transcriptional orientation of linked genes, and, to the extent studied, the intron position of actin genes have evolved similarly in these three species. In contrast, significant change has occurred in the chromosomal arrangement of sea urchin actin genes.

Organization and evolution of the actin gene family in sea urchins.

Genomic libraries of the sea urchins Strongylocentrotus franciscanus and Lytechinus pictus were screened with an actin cDNA clone from Strongylocentrotus purpuratus. Four nonoverlapping clones were isolated and characterized from the S. franciscanus library; three were isolated and characterized from the L. pictus library. Linked genes having the same transcriptional orientation were found on all S. franciscanus clones. Three clones contained two actin genes each; the other clone contained three. In contrast, the L. pictus clones contained only one actin gene. Comparison of actin genomic clones from these three species indicated a difference in the genomic organization of sea urchin actin genes in that the genes appear to be more highly clustered in S. franciscanus than in S. purpuratus and L. pictus. Genomic dot blots and reassociation kinetics demonstrated that the copy number of actin genes in all three species is 15 to 20. Nucleotide sequence homology of actin genes within and among the species was measured by thermal elution. These experiments indicated that there is a high degree of interspecies actin gene sequence homology but that, within each species, actin gene sequences may differ by as much as 30%. Sequencing of two S. franciscanus actin genes revealed introns at the same amino acid positions, 121 and 204, reported for S. purpuratus actin genes. These data demonstrated that the genomic copy number, the transcriptional orientation of linked genes, and, to the extent studied, the intron position of actin genes have evolved similarly in these three species. In contrast, significant change has occurred in the chromosomal arrangement of sea urchin actin genes.

The effects of sheephead (Semicossyphus pulcher) predation on red sea urchin (Strongylocentrotus franciscanus) populations: an experimental analysis.

An experimental evaluation of the effect of sheephead (Semicossyphus pulcher) predation on red sea urchins (Strongylocentrotus franciscanus) was initiated at San Nicolas Island, California in Sept. 1980. All sheephead (N=220) were removed from an area of approximately 12,700 m2 and subsequent changes in sea urchin numbers and microhabitat utilization were monitored along permanent transects for 24 months. The sea urchins within the transects were also measured in situ to determine changes in the size frequency distribution of the population. Although sea urchins rank only seventh in relative importance in the sheephead's diet at San Nicolas Island, there has been a significant increase in sea urchin numbers in the experimental area (26% increase/year) but there has been no change in the control site. Further, there has been a slight increase in the proportion of sea urchins occupying exposed versus sheltered microhabitats in the removal site. Changes in the size frequency distribution of the urchins were minimal (probably due to an observation period of only one year); the size frequency distribution was unimodal and skewed to the left, indicating weak recruitment of urchins. Comparisons of sheephead densities and the percent of sea urchins in exposed microhabitats (i.e. available to predation by sheephead) were also made at four areas around San Nicolas Island and three areas in Baja California. In areas with low sheephead densities (0-35/hectare) sea urchins were highly exposed, and in areas where sheephead densities were high (200-500/hectare), no urchins were exposed.This study demonstrates that sheephead are capable of regulating the density and microhabitat distribution of sea urchin populations. These results are discussed in relation to other factors which may affect sea urchin populations.