Sea Urchin Populations Research Articles

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.

Reproductive periodicity inEvechinus chloroticusin the Hauraki Gulf

Abstract Test diameter and gonad volume were recorded over 1 year for bimonthly samples of Evechinus chloroticus (Valenciennes) from each of 3 subtidal populations 10 km apart in the Hauraki Gulf. Histological analysis was used to relate cellular events during gametogenesis to the annual cycle of change in gonad size. Mean gonad sizes of the different sea urchin populations were compared by analysis of covariance. Gametogenesis began in the spring, and major spawning occurred in mid to late summer. The proportion of the gonads taken up by nutritive phagocyte cells increased from autumn to spring. During proliferation and growth of gametes the nutritive phagocytes declined in abundance and globulation, suggesting that reserves stored in these cells were transferred to developing gametes. Gonad size doubled during gametogenesis, declined after spawning, and remained low in autumn and winter. Differences in gonad size between populations were significant during most of the year, and were most pronounced in m...

Activity and Prey Election in the Sea Otter: Influence of Population Status on Community Structure

The population of sea otters (Enhydra lutris) at Amchitka Island has been at or near equilibrium density for at least 3 decades. At Amchitka, sea otters spent 55% of daylight hours foraging; there were distinct early morning and late afternoon peaks in foraging activity; and fish made up over 60% of their overall diet by volume. In contrast, the more recently established populations at Attu Island and off the coast of Oregon are far below equilibrium density. At both Attu and in Oregon, sea otters spent 17% of daylight hours foraging; there was no peak in foraging activity; and herbivorous macroinvertebrates made up their entire diet. We interpret these observations in the following way. Subsequent to the over-exploitation of sea otters, uncontrolled populations of herbivorous sea urchins greatly reduced or eliminated sublittoral kelp assemblages. In areas only recently reoccupied by otters, sea urchins are abundant and easily accessible as prey. Otters in these areas invest relatively little effort in foraging, and distribute that effort uniformly throughout the day. Where otters are abundant sea urchins are scarce, in turn promoting the growth of kelp beds and an associated fish fauna. Consequently otters at Amchitka invest more time in pursuit of more elusive prey (fish), and distribute that effort disproportionally toward morning and evening to correspond with availability of kelp bed fishes. These observations demonstrate that foraging tactics of a consumer are closely linked to its influence on the structure of the community.

Synchrony of oogenesis in laboratory‐maintained and wild populations of the purple sea urchin (Stronglyocentrotus purpuratus)

AbstractCycles of oogenesis were studied in populations of Strongylocentrotus purpuratus by disaggregation of ovaries and measurement of the number of vitellogenic and mature oocytes. Average egg yields upon spawning were recorded. As reported by many previous authors, fecundity of intertidal S. purpuratus females is seasonal. Two peaks of fertility are observed per year in subtidal animals from a southern California location, and for some months of each year virtually none of these animals contain vitellogenic or mature oocytes. In contrast, a fraction of animals living at 50–70‐foot depths in a kelp bed display vitellogenic and mature oocytes at all seasons of the year. Different individuals in these deep‐water sea urchin populations are in different stages of the process of oogenesis at any one time, and their fecundity is not entrained with the seasons. Annual yield of eggs of average females in deep‐water and intertidal populations is about the same. Groups of mature females derived from both deep‐water and subtidal populations and held in our large scale laboratory S. purpuratus maintenance system (SpMS) resemble the deep‐water populations. Reproductive cycles in these females are in different phases even within the same tank and remain so for at least several years. Initially synchronous populations eventually drift apart in phase when held in the SpMS in the absence of seasonal environmental stimuli. We show that in SpMS females the fertility of each group can be predicted from oocyte count data, and that it can to a large extent be controlled by the frequency of induced spawning.

Stability of sea urchin dominated barren grounds following destructive grazing of kelp in St. Margaret's Bay, Eastern Canada

Kelp regeneration was observed for the first time in St. Margaret's Bay, Nova Scotia, Canada in an area known to have been devoid of macroalgae for several years. The regeneration was destroyed by sea urchins within 10 months. Experimentally induced kelp regeneration met a similar fate under normal grazing pressure. At the lowest sea urchin biomass and density encountered re-establishment of mature kelp stands seems highly unlikely. The sea urchin population required to suppress kelp regeneration is fed by benthic microalgae. Diatoms and other pioneer algal community species were found in the guts of sea urchins. The mean standing crop of benthic microalgae was found to be, 2.2 g C m-2 and production estimated as ca 15 g C m-2yr-1 at 8m depth. Most of the primary production of St. Margaret's Bay has been lost with the disappearing kelp populations.

Sea Urchin Recruitment: Effect of Substrate Selection on Juvenile Distribution

Intertidal field observations have shown that juvenile purple sea urchins Strongylocentwtus purpuratus are found in higher densities near adults . The same is true for subtidal populations of juvenile red sea urchins 5. franciscanus. These distribution patterns could be influenced by any of three elements: substrate selection, early juvenile mortality or juvenile migration. Using laboratory-reared larvae, we conducted experiments designed to gauge the effect of substrate selection on juvenile distributions of these two sea urchin species. In both laboratory and field experiments, larval sea urchins showed no enhanced settlement in the presence of adults or adult-associated substrates. These results imply that early juvenile mortality or migration provide the greater influence on juvenile distributions.

Expansion of a central California kelp forest following the mass mortality of sea urchins

The mass mortality by disease of a localized population of sea urchins, Strongylocencrotus franciscanus, on the seaward side of a kelp forest was followed by the rapid seaward expansion of 4 species of brown algae, Macrocystis pyrifera, Laminaria dentigera, Pterygophora california, and to a lesser extent, Nereocystis leutkeana. One other brown alga, Cystoseira osmundacea, failed to become established in the newly available area. Competition among M. pyrifera, L. dentigera, P. californica, and N. Leutkeana apparently was severe, and within 1 year after the demise of the sea urchins, M. pyrifera formed a dense, nearly monospecific stand. Experimental removal of M. pyrifera demonstrated that the canopy of these plants limited light penetration to levels below that necessary for the growth and survival of other brown and red algae.

Environmental influences on histological changes in the reproductive cycle of four New South Wales sea urchins

The suitability of a sea urchin population for commercial harvesting and culturing depends on the timing and duration of the spawning season. Pilot studies have shown that the New South Wales echinoids produce mature gonads for several months. Investigations were made into the influence of those exogenous factors which could be modified under aquacultural conditions. Temperature and food appeared to affect gonad (roe) development, while depth and density did not. The relationship between macro and micro changes in the gonads was established and methods of estimating stages of gametogensis were compared.

Movement and feeding activity of red sea urchins (Strongylocentrotus franciscanus) adjacent to a kelp forest

Movement and feeding were studied in a population of red sea urchins, Strongylocentrotus franciscanus (Agassiz, 1863), found within and immediately seaward of a kelp forest offshore from Santa Cruz, California, USA. Mean sea urchin movements varied from 7.5 cm/day inside the kelp forest to over 50 cm/day at 15 and 100 m outside the kelp forest. The percentage of sea urchins feeding decreased from 66% inside the kelp forest to 16 and 15% at 15 and 100 m outside the kelp forest. These data indicate that movement by these sea urchins is a response to a low food supply.

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.

Ecological energetics of the seaweed zone in a marine bay on the Atlantic coast of Canada. III. Energy transformations by sea urchins

An energy budget was constructed for a population of the sea urchin Strongylocentrotus droebachiensis in the nearshore area of St. Margaret's Bay, Nova Scotia, Canada. Of the 6 age classes identified, ages 1+and 2+accounted for about 1/2 the population energy flow. Population production efficiencies were: production/assimilation=0.28, production/consumption=0.04 to 0.13, and production/biomass=0.80. Although S. droebachiensis was the dominant herbivore in the seaweed bed, it utilized only 1 to 7% of seaweed production. As with other populations of sea urchins, however, it had a proportionately greater influence on seaweed biomass, and also presumably production, by clearing seaweed from large areas of substrate and maintaining it clear. Loss of dissolved organic matter, the only term in the energy budget not measured, was estimated by substracting the other terms in the energy budget from consumption. In laboratory individuals, this ranged from 40 to 80% of absorption (consumption-faeces). A critical review of energy budgets for 6 other species of marine benthic grazers also revealed large amounts of energy unaccounted for that might be attributed to loss of dissolved organic matter.

The Relation Between Lobster Abundance, Sea Urchins, and Kelp Beds

When subtidal communities are disturbed and sea urchin populations expand, they frequently overgraze their food supply, eliminating large seaweeds from considerable areas. The hypothesis is advanced that the lobster is a key species, controlling sea urchin populations in eastern Canada, and that reduction of lobster populations below a critical density has led to overgrazing of seaweeds in many places.

Studies of Populations of Sea Urchins, "Strongylocentrotus" sp., in Relation to Underground Nuclear Testing at Amchitka Island, Alaska

primarily by fleets from the Soviet Union and Japan, although U.S. fishermen are extending their operations westward to this area. The Soviets harvest Pacific ocean perch and whales; the Japanese, Pacific ocean perch, whales, and salmon. U.S. fishermen catch mostly king crabs, but some Dungeness and snow crabs and halibut are also caught. The salmon resource in the western Aleutians is of the greatest value and concern to commercial fisheries. Japan sends fleets of several hundred salmonfishing vessels each spring and summer to the western Aleutians, and, as long as international agreements are effective in maintaining the productivity of the resource, this high seas fishery will continue at a high level. U.S. and Soviet coastal salmon fisheries also have a large stake in the resource because salmon from Asian and Alaskan streams migrate through the western Aleutians before returning to parent streams as mature adults.

Sea Urchin Population Explosion in Southern California Coastal Waters

The alleged population explosion of coral-eating sea stars (Acanthaster planci) in the west tropical Pacific (1) may be comparable to the apparent population explosion of herbivorous sea urchins (Strongylocentrotus franciscanus and S. purpuratus) along the coast of southern California. Many luxuriant kelp beds, supporting a rich association of other organisms, have all but disappeared during the past few decades, and in the place of the holdluxuriant holdfasts are barren areas supporting extremely dense populations of the urchins (2).

ウニ類の生物学的研究-VI.

The distribution of the sea urchin, Strongylocentrotus intermedius, with successive developmental processes of its life cycle was observed by SCUBA diving at the rocky shore of Ikantai, southern Hokkaido, from August 1966 until November 1968. This shore, supporting the echinoid population, consisted of an extensive flat rock, a boulder field, an eelgrass area, and a sandy area, and was classified into five major habitats according to the nature of the bottom. The boulder field supported more than 90% of the sea urchin population, and this zone was a major habitat for the sea urchin. The flat rock area and the eelgrass one were nearly valueless areas as habitat for the sea urchin. Almost all individuals belonging to larval and young urchins were most dense at the field of small boulders. The 2-year old urchins were also more abundant in this boulder field than in the others, but they dispersed gradually toward the field with large boulders from late spring to late fall. The field with large boulders was appraised as a major habitat for the adult urchins older than 3 years old. Some members of the adult urchins, during the cold months, extended their sphere over the field covered with small boulders. However, during the rest of the year, they were clumped within the large boulder field.

Response of the Testes of Purple Sea Urchins to Variations in Temperature and Light

STUDIES on the reproductive activity of the purple sea urchin, S. purpuratus, made during the past eight years at Pacific Grove, California, indicate the presence of an annual breeding cycle, with maximum gonadal activity occurring during the winter months. In the course of the present investigation, designed to examine the effect of latitude on the breeding activity of the sea urchin, it was observed that the sea urchin populations distributed over a 20° latitudinal range have a uniform spawning period. These observations suggest that the reproductive activity of the purple sea urchin might be independent of temperature over a physiological range represented by this distribution. Therefore a photoperiodic response was considered as a possible explanation for the breeding cycles observed.