Sea Urchin Strongylocentrotus Franciscanus Research Articles

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.

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.

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.

Scanning electron microscopy of echinoid podia.

Tube feet of the sea urchin Strongylocentrotus franciscanus were studied with the scanning electron microscope (SEM). By use of fractured preparations it was possible to obtain views of all components of the layered tube-foot wall. The outer epithelium was found to bear tufts of cilia possibly belonging to sensory cells. The nerve plexus was clearly revealed as being composed of bundles of varicose axons. The basal lamina, which covers the outer and inner surfaces of the connective tissue layer, was found to be a mechanically resistant and elastic membrane. The connective tissue appears as dense bundles of (collagen) fibers. The luminal epithelium (coelothelium) is a single layer of flagellated collar cells. There is no indication that the muscle fibers, which insert on the inner basal lamina of the connective tissue layer are innervated by axons from the basi-epithelial nerve plexus. The results agree with previous conclusions concerning tube-foot structure based on transmission electron microscopy, and provide additional information, particularly with regard to the outer and inner epithelia.

Changes in Subtidal Community Structure Associated with British Columbia Sea Otter Transplants

Sea otters Enhydra Iutris were re-introduced into coastal waters of British Columbia, Canada, in 1969-1972, after being hunted to extinction In the previous 2 centuries. In 1979, we visited an area where 55 individuals had been seen in 1978, and we made subtidal observations of the abundance and distribution of red sea urchins, other grazers, and kelps. Earlier observations, made in the area before sea otters were re-introduced, confirmed that changes have taken place in subtidal communities since the re-introduction. Where sea otters had been observed feeding, sea urchins Strongylocentrotus franciscanus were scarce and restricted to crevices or beneath boulders, other grazers were scarce, and kelps colonized the bottom to 10-m depths. The algal communities in these areas appeared to be simple downward extensions of sublittoral fringe communities. Where sea otters had not fed, kelps were limited to shallow water by abundant sea urchins. The observed differences between the 2 types of areas are concluded to have been caused by early ellmlnation of sea urchins by sea otters, as reported in Alaska and California. From the pattern of sea urchin abundance in the general area of the transplant, we were able to delineate the feeding range of this small population of sea otters.

Population Structure, Recruitment and Mortality of Two Sea Urchins (Strongylocentrotus franciscanus and S. purpuratus) in a Kelp Forest

Population Structure, Recruitment and Mortality of Two Sea Urchins (Strongylocentrotus franciscanus and S. purpuratus) in a Kelp Forest

Dispersion and Food Differences Between Two Populations of the Sea Urchin Strongylocentrotus franciscanus

The spatial pattern differs between two allopatric populations of the adult sea urchin Strongylocentrotus franciscanus. The amount of macroscopic algal food is correlated with this difference. In the population with abundant food, the urchins are highly aggregated; in the other food is limited and the spatial pattern of the urchins is random. Urchins aggregate in laboratory aquaria when well supplied with food, while those without food tend to be positioned more at random. This tendency toward random dispersal in food-limited areas is due to the increased and apparently random movement of the urchins in their search for food.

Evolutionary change in the repetition frequency of sea urchin DNA sequences

The frequency of occurrence of particular repetitive sequence families has been estimated in the DNA of the three sea urchin species Strongylocentrotus purpuratus, Strongylocentrotus franciscanus and Lytechinus pictus using individual cloned S. purpuratus repetitive sequence elements. Cloned repetitive sequence elements as described by Scheller et al. (1977a) were prepared by reassociation of S. purpuratus DNA fragments to repetitive Cot, digestion with single-strand-specific nuclease S1 and ligation of synthetic restriction sites to their ends. The sequences were cloned by insertion at the Eco RI site of plasmid RSF2124, labeled, strand-separated and reassociated with 800–900 nucleotide long unlabeled DNA. Both kinetic (genomic DNA excess) and saturation (cloned DNA excess) estimates of frequencies were made. For nine cloned fragments, the ratio of the repetition frequency in S. purpuratus DNA to that in S. franciscanus DNA ranges from about 20 to about 1. In the four cases examined, only a few copies were detected in the DNA of L. pictus. Estimates have also been made of frequency changes in many repetitive families by measuring the reassociation of labeled repetitive DNA fractions of each species with total DNA from other species. In each reciprocal comparison, the labeled repetitive sequences reassociate more slowly with DNA of other species than with DNA of the species from which they were prepared. Thus it appears that the dominant repetitive sequence families in the DNA of each species are present at lower frequencies in the DNA of closely related species. Measurements of thermal stability have been made of S. purpuratus cloned repetitive sequences reassociated with S. franciscanus DNA or S. purpuratus DNA. Most families have changed both in frequency and sequence, although some have changed little in sequence but show great changes in frequency.

Ultrastructure of sea urchin tube feet. Evidence for connective tissue involvement in motor control.

An analysis of the ultrastructure of the tube feet of three species of sea urchins (Strongylocentrotus franciscanus, Arbacia lixula and Echinus esculentus) revealed that the smooth muscle, although known to be cholinoceptive, receives no motor innervation. The muscle fibers are attached to a double layer of circular and longitudinal connective tissue which surrounds the muscle layer and contains numerous bundles of collagen fibers. On its outside, the connective tissue cylinder is invested by a basal lamina of the outer epithelium to which numerous nerve terminals are attached. These are part of a nerve plexus which surrounds the connective tissue cylinder. The plexus itself is an extension of a longitudinal nerve that extends the whole length of the tube foot. It is composed of axons, but nerve cell bodies and synapses are conspicuously lacking, suggesting that the axons and terminals derive from cells of the radial nerve. Processes of the epithelial cells penetrate the nerve plexus and attach to the basal lamina. There is no evidence that the epithelial cells function as sensory cells. On the basis of supporting evidence it is suggested that the transmitter released by the nerve terminals diffuses to the muscle cells over a distance of several microns and in doing so affects the mechanical properties of the connective tissue.

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.

Evidence for Myosin-like Intermediates in the Hydrolysis of Adenosine Triphosphate by Sperm-Tail Flagella

Abstract The ATPase of glycerinated sperm flagella from the sea urchin (Strongylocentrotus franciscanus) catalyzes the preferential exchange of water oxygens into the phosphate originating from ATP. This type of exchange, so far known only for myosin-related ATPases, occurred under conditions (60 mm Mg2+, 1 to 10 mm Ca2+) in which no inorganic phosphate-water exchange or exchange into unhydrolyzed ATP was observed. These results suggest that similar intermediates are formed by both skeletal myosin and flagellar ATPase during the cleavage of ATP, in spite of the known large differences in physical and enzymatic properties of the two contractile systems.

Purification and Properties of Deoxyribonucleic Acid Polymerase from Nuclei of Sea Urchin Embryos

Abstract Deoxyribonucleic acid polymerase has been purified from nuclei of rapidly dividing sea urchin embryos, Strongylocentrotus franciscanus and Strongylocentrotus purpuratus. No gross differences between the enzymes from the two species were observed. The DNA polymerases are heat-labile and are stabilized in the presence of polyglycols. This enhanced stabilization was exploited throughout the purification procedure. The enzymes, purified more than 300-fold, have a molecular weight of approximately 150,000. However, under certain conditions larger aggregates have also been noted. No DNase activity was detected in the most purified fraction. The DNA polymerase activity is totally dependent on the presence of added DNA. For maximum activity the presence of all four deoxynucleoside triphosphates and Mg++ are required. Native double stranded DNA is a more effective primer than single stranded DNA. Native DNA which has been degraded to a limited extent with pancreatic deoxyribonuclease is the most efficient primer.

Sulphation as a metabolic pathway for oestradiol in the sea urchin Strongylocentrotus franciscanus

1. Aerobic incubation of [(14)C]oestradiol, in the presence of surviving gut tissue of the sea urchin Strongylocentrotus franciscanus, or a soluble enzyme system prepared therefrom, resulted in rapid formation of a water-soluble metabolite, identified as oestradiol 3-sulphate. 2. No evidence was obtained for the formation of other metabolic derivatives by the urchin-gut enzymes, or for the presence of the sulphating capacity in any other tissues of the organism under the conditions used. 3. The data are consistent with the possibility that oestradiol, previously detected in the gonads, is synthesized therein and excreted in a conjugated, highly water-soluble form via the gut.

Preliminary communication on the cytolytic action of ox-blood serum upon sea-urchin eggs, and its inhibition by proteins

1. It has been shown by Loeb1 that the eggs of sea urchins (Strongylocentrotus purpuratus and Strongylocentrotus franciscanus) may be fertilized by the blood-sera of mammalia, provided the eggs be previously sensitized by a brief immersion in a solution of SrCl2 which is approximately isotonic with sea water.2. I find that if ox-serum be rendered sufficiently potent by dilution (cf. below) the formation of a fertilization-membrane by the action of the serum is succeeded by cytolysis or may even be accompanied by marked agglutination of the eggs, thus confirming Loeb's view that the formation of a fertilization membrane is essentially a phenomenon of incipient cytolysis.3. The cytolytic (and fertilizing) action of ox serum (rendered isotonic to sea water) upon sensitized sea-urchin eggs is enhanced by dilution with sea water, a maximum potency being attained at a dilution of about 1/16.4. I find that the increase in the cytolytic activity of serum which accompanies dilution is attributable to the fact that...