Sea Urchin Recruitment Research Articles

Changes in fish abundance do not cascade to sea urchins and erect algae in one of the most oligotrophic parts of the Mediterranean

The abundance of fish, sea urchins, and erect algae over carbonated and non-carbonated rocks was measured in two areas differing in the level of fishing intensity off northern Minorca (Balearic Islands, western Mediterranean) to test the hypothesis that changes in the abundance of invertebrate-feeding fish cascade to erect algae through changes in the density of sea urchins in oligotrophic areas, as has been reported previously for eutrophic areas in the northwestern Mediterranean. Although the biomass of invertebrate-feeding fish and the cover of erect algae increased where fishing intensity decreased, independently of rock type, the biomass and the horizontal test size of sea urchins, represented almost exclusively by Paracentrotus lividus, also increased. Furthermore, the total cover of erect algae and that of the two dominant algae species were not correlated with sea urchin density or sea urchin biomass. Rock type did not affect the total cover of erect algae, but Cystoseira brachycarpa J. Agardh preferred non-carbonated rocks and Dictyota dichotoma v. intricata (C. Agardh) Greville favoured carbonated ones. The only herbivorous fish ( Sarpa salpa) and the omnivorous breams of the family Sparidae were more abundant around carbonated rocks, although only inside the marine protected areas. The wrasses of the family Labridae were insensitive to rock type and the level of fishing intensity. The overall evidence indicates that trophic cascades do not operate in the area studied, as changes in the abundance of invertebrate-feeding fish did not cascade to sea urchins and changes in the abundance of sea urchins did not affect erect algae. The limited recruitment of fish and sea urchins due to extreme oligotrophy is invoked as an explanation, in agreement with the ecosystem exploitation paradigm.

Overgrazing of Kelp Beds Along the Coast of Norway

The aim of this study was to better understand the down-grazing of kelp beds by sea urchins (Strongylocentrotus droebachiensis) along the coast of Norway. Barren grounds were first observed in sheltered areas along the coast of the counties of Trondelag, Nordland and Troms in 1974. In the 1980s, the barren grounds spread to areas more heavily exposed to waves. In the 1990s, the kelp beds were re-established in some localities in southern Trondelag, initially in wave-exposed areas. In the northernmost parts of Norway, i.e. the counties of Troms and Finnmark, the barren ground areas may still be increasing. Crabs (Cancer pagurus) and common eiders are the most common predators on urchins. Predation on sea urchins in kelp beds is probably not among the factors that limit the sea urchin populations. Along the coast of Nordland and further north, sea urchins are infected by nematodes, resulting in a low, but significant increase in their mortality. No re-growth of kelp beds has been found in the most infected areas. In the late 1960s and the early 1970s, a high occurrence of echinoderm larvae was observed in deeper waters. This was a period with cold water, which may have caused high recruitment of sea urchins. The bet-hedging life strategy of sea urchins may account for the sudden increase in the size of the populations. In the present paper I propose the hypothesis that higher individual growth rates and higher mortality rates in the south than in the north may explain the decrease in the populations, which may in turn account for the re-growth of kelp in the southern areas.

Physical influences on recruitment to California Current invertebrate populations on multiple scales

Studies of recruitment to Dungeness crab (Cancer magister) and red sea urchin (Strongylocentrotus franciscanus) populations have focused on recruitment variability on interannual, 1000-km scales. These studies have identified correlations between recruitment and variables indicating ENSO conditions and wind-forced larval transport. On longer scales, our understanding of biologically important physical variability in the California Current has recently been enhanced by an appreciation of semi-basin, decadal changes. Intensification of the Aleutian low-pressure zone in the mid-1970s had a clear positive effect of biological productivity in the Gulf of Alaska, and less obvious negative effects on productivity in the California Current. On shorter scales, an increasing appreciation of the effect of dispersal patterns on population dynamics and the potential of spatially explicit management schemes provide the impetus for studying processes such as alongshore spatial variability in recruitment and the underlying daily variability in circulation over 100-km distances. In the 1980s, physical studies of coastal circulation in response to daily fluctuations in upwelling winds identified brief periods of northward, onshore flow interrupting the offshore, southward flows associated with active upwelling. In the 1990s, concurrent monitoring of biological and physical conditions revealed that these flows transport late-stage invertebrate larvae from a retention zone in the lee of a local promontory to settlement locations, producing specific spatial patterns of recruitment. Variability in recruitment of both crabs and sea urchins is thus driven by daily temporal variability in upwelling winds and 100-km spatial variability in coastal topography. Interannual recruitment variability appears to depend on ENSO-related biological productivity and larval transport in ways that vary among species, but are not completely understood. Observations are consistent with coastwide, interannual variability being driven by biological productivity (i.e. ENSO/non-ENSO conditions), while the upwelling relaxation mechanism sets the spatial scale, and decadal, semi-basin variability modulates the frequency of ENSO conditions.

Spatial variability in red sea urchin (Strongylocentrotus franciscanus) recruitment in northern California

To better understand the spatial distribution of recruitment in the northern California red sea urchin (Strongylocentrotus franciscanus) population, we sampled size distributions at each of 12 locations in 1995 and 1996, two of those locations in 1994, and 5 of those locations in 1997. An index of recent recruitment in these size distributions and an estimate of density of recent recruitment reflect a similar spatial pattern of recruitment. This pattern appears to be determined by the effect of coastal circulation on larval delivery during relaxation of upwelling, and not the result of the positive effect of the adult spine canopy on juvenile survival. Recent recruitment of red sea urchins in northern California is higher in areas more frequently subjected to onshore and poleward flow during relaxation of upwelling. These results are consistent with a mechanism by which alongshore spatial variability in southward, offshore flow during upwelling winds allows larvae to maintain latitudinal position, whereas flows during event‐scale relaxations in upwelling winds serve to distribute settling larvae alongshore, favouring areas north of promontories. The consequent spatial pattern of red sea urchin settlement varies from year to year, and there are not yet sufficient data to demonstrate the degree to which this spatial pattern in recruitment determines a spatial pattern in fishable adult abundance.

Changes in sea urchins and kelp following a reduction in sea otter density as a result of the Exxon Valdez oil spill

Interactions between sea otters Enhydra lutris, sea urchins Strongylocentrotus droebachiensis, and kelp were investigated following the reduction in sea otter density in Prince William Sound, Alaska, after the Exxon Valdez oil spill in 1989. At northern Knight Island, a heavily oiled portion of the sound, sea otter abundance was reduced by a minimum of 50 % by the oil spill, and from 1995 through 1998 remained at an estimated 66 % lower than in 1973. Where sea otter densities were reduced, there were proportionally more large sea urchins. However, except in some widely scattered aggregations, both density and biomass of sea urchins were similar in an area of reduced sea otter density compared with an area where sea otters remained about 10 times more abundant. Furthermore, there was no change in kelp abundance in the area of reduced sea otter density. This is in contrast to greatly increased biomass of sea urchins and greatly reduced kelp density observed following an approximate 90 % decline in sea otter abundance in the western Aleutian Islands. The variation in community response to a reduction in sea otters may be related to the magnitude of the reduction and the non-linear response by sea urchins to changes in predator abundance. The number of surviving sea otters may have been high enough to suppress sea urchin populations in Prince William Sound, but not in the Aleutians. Alternatively, differences in response may have been due to differences in the frequency or magnitude of sea urchin recruitment. Densities of small sea urchins were much higher in the Aleutian system even prior to the reduction in sea otters, suggesting a higher rate of recruitment.

Relationship between Year-to-year Fluctuations in Recruitment of Juvenile Sea Urchins <i>Strongylocentrotus nudus</i> and Seawater Temperature in Southwestern Hokkaido

Relationship between Year-to-year Fluctuations in Recruitment of Juvenile Sea Urchins <i>Strongylocentrotus nudus</i> and Seawater Temperature in Southwestern Hokkaido

Isolating settlement from recruitment of sea urchins

Recruitment of benthic marine invertebrates with planktonic larvae is composed of three components: larval supply; settlement and metamorphosis of larvae; and survival and growth in the benthic environment. We offer a technique that isolates settlement from the other processes for the sea urchin Strongylocentrotus. Settlement samplers constructed of common and inexpensive materials were deployed in a central California kelp forest for 1 yr and sampled monthly. The samplers effectively induced larval sea urchins Strongylocentrotus purpuratus (Stimpson) to settle both in the laboratory and in the field. Other invertebrates occurred in the samplers as both adults and juveniles, and predation on settled sea urchins in the samplers probably influenced the results. We suggest that long-term use of settlement samplers will contribute to a better understanding of the population dynamics of sea urchins and other benthic invertebrates with planktonic larvae.

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.

Heat waves, baby booms, and the destruction of kelp beds by sea urchins

Large populations of sea urchins, Strongylocentrotus droebachiensis (Muller), destroyed kelp beds along the Atlantic coast of Nova Scotia in the 1960's and 1970's. The origin of these large sea urchin populations is not understood. We have investigated the potential influence of variable growth and development of the planktonic larvae of sea urchins (in response to temperature and food abundance) on recruitment of benthic juveniles. The adult sea urchins were collected at Sandy Cove, Digby County, Nova Scotia, Canada, in December 1986. Temperature strongly affected larval size and the growth of the echinus rudiment within the range 3° to 9°C, and larvae grew most rapidly at 14°C. Food abundance had a smaller effect on larval growth, and these effects were apparent only at high temperature. Larvae fed the same concentration of two different algal food species grew and developed similarly. Correspondence between spring temperature variation and qualitative variation in sea urchin recruitment, as well as strong temperature effects on larval growth in culture, and the occurrence of a large, positive temperature anomaly in June 1960, all suggest that temperature effects on larval growth and development may have led to intense sea urchin recruitment in 1960 and the appearance of large adult populations 4 to 6 yr later. This result invites further research.

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.

Sea urchin recruitment patterns and implications of commercial fishing.

Coexisting sea urchins Strongylocentrotus purpuratus and S. franciscanus exhibit different recruitment patterns. Juveniles of the former species are found in a variety of habitats, whereas juveniles of the latter occur almost exclusively under the spine canopy of conspecific adults. The commercial harvest of S. franciscanus thus seems to affect nursery grounds as well as the reproductive potential of exploited populations.