Learn About Sea Urchins: Population & community ecology

Interactions with kelps, other macroalgae, & predators

Topics relating to population & community ecology include INTERACTIONS WITH KELPS, OTHER MACROALGAE, & PREDATORS, considered here, and REMOVAL-TYPE STUDIES, MASS MORTALITIES, and GENE FLOW, considered in other sections.

photograph of a green urchin Strongylocentrotus droebachiensis and a red urchin S. franciscanus

Sea urchins, by their large size, voracious appetites, and often dense numbers potentially have profound effects on intertidal and subtidal community structure. World-wide, they have been popular candidates for removal-type experiments and, on the west coast, there has been keen interest in the interaction of sea urchins, kelps, and sea otters.

Research study 1

artist's drawing of a sea otter eating red urchins courtesy Sue ColemanDwindling giant-kelp beds in the Point Loma area near San Diego, California during the mid-1900's prompted members of the Kelp Habitat Improvement Project to eliminate sea urchins, both by application of quicklime (CaO) and crushing with hammers. The chief aim of the project was to "enhance" the underwater environment by encouraging kelps to grow and thereby improve habitat for fishes. What caused the decline of kelps in California? Several authors attribute this to almost complete removal of sea otters Enhydra lutris from Californian waters during the 1800's. This allowed red sea-urchins Strongylocentrous spp. to flourish, which caused kelp beds to diminish. When sea otters re-appear in an area, change can come quickly. For example, prior to 1963, southern Monterey Bay had dense populations of sea urchins, but little kelp. Within a year of incursion of sea otters, the area became mostly free of sea urchins and beds of giant kelp Macrocystis and other seaweeds were abundant. Cause and effect, no, but suggestive...yes. McLean 1962 Biol Bull 122: 95; Leighton et al. 1966 p. 141, In Proc Int Seaweed Symp, 5th, Halifax (Young, ed.) Oxford: Pergamon Press; Kelp Habitat Improvement Project 1967, 1972, 1973 WM Keck Laboratory of Environmental Health Engineering, Cal Institut Tech; Lowry & Pearse 1973 Mar Biol 23: 213. Drawing of sea otter E. lutris eating red urchins S. franciscanus courtesy Sue Coleman.

Research study 1.1

graph showing relationship of kelps Laminaria spp. and sea urchins Strongylocentrotus polyacanthus in Amchitka Island, AlaskaAlthough primarily dealing with competitive interactions between kelps, a study by a researcher from Scripps Institution of Oceanography, California at Amchitka Island, Alaska includes some points of interest relating to sea urchins Strongylocentrotus polyacanthus. First, sea urchins are all but absent in depths shallower than about 20m and their presence in large numbers at that depth sets the lower limit of distribution of the deep-dwelling kelp species, mainly Laminaria spp. (see graph) Second, kelp holdfasts in shallow water may provide shelter for small sea urchins against sea-otter Enhydra lutris predation. This is suggested by disappearance of the urchins when the researcher removes the overlying kelp canopy as part of another experiment, presumably leaving the urchins potentially more open to discovery by otters. Dayton 1975 Fishery Bull 73 (2): 230.

NOTE the author notes that at the time of writing the actual species name was uncertain, either polyacanthus or droebachiensis

NOTE sea otters are present in the area during the study period and are inferred by the author to be feeding intensively on sea urchins, but no otter-free control areas are surveyed

Research study 2

histograms showing effect of sea otters on kelp diversity in areas of AlaskaWhat effect do sea otters Enhydra lutris, active consumers of sea urchins, have on seaweed biodiversity? This is assessed in Alaska by comparing 3 different bays: 1) Torch Bay, where sea otters are absent, 2) Deer Harbor, where sea otters have been present for less than 2yr (Time 0yr = 1976), and 3) Surge Bay, an area in which sea otters have been present for about 10yr.Note that with increasing presence of sea otters, numbers of sea urchins correspondingly decrease, and densities of seaweeds increase. For example, where sea otters are present and sea-urchin densities low (Deer Harbor), annual kelps predominate and the perennial kelp Laminaria groenlandica is in low density. Where sea otters have had a long presence and sea urchins are absent (Surge Bay), the perennial kelp L. groenlandica dominates. The study demonstrates the competitive superiority of L. groenlandica, the preference of Strongylocentrotus species for this seaweed as food, and the keystone-predator role played by Enhydra lutris in the dynamics of the nearshore marine community. Duggins 1980 Ecology 61: 447.

Research study 2.1

Seven years after sea otters Enhydra lutris are re-introduced into the Bunsby Islands, British Columbia researchers from the Pacific biological Station, Nanaimo compare “before and after” areas for differences in community composition. At 7 locations where sea otters are thought to have been feeding, sea urchins Strongylocentrotus franciscanus are scarce and restricted to crevice and under-rock habitats. Kelps and other algae colonise the bottom to 10m depths, extending downwards from sublittoral fringe communities (such areas are apparently identifiable from the air). Other sea-otter prey items, including abalone, top shells, rock scallops, and green and purple sea-urchin species are also scarce. In contrast, at 5 sites where sea otters are thought not to have been feeding, sea urchins are abundant and large in size, and kelps are limited to shallow water. Extensive grazing by sea-urchins at these sites creates “barrens” areas that begin in shallow waters. Although direct evidence photograph of red sea-urchins Strongylocentrotus franciscanus in an algal "barrens" areais lacking, the researchers infer that sea-otter depredation on sea urchins is the cause of these community differences. Breen et al. 1982 Mar Ecol Progr Ser 7: 13.

NOTE from the time of Cook’s explorations in 1778 to the early 1900s, sea otters were hunted to virtual extinction. Introductions were made to the Bunsby Islans area of Vancouver Island in 1969-72

NOTE this inferred hiding behaviour is a common finding in such studies, but no researcher has yet questioned how sea urchins are aware that predators are in the vicinity. Is a “fright” pheromone released from damaged conspecifics that induces a hiding behaviour? or is it just that individuals with a propensity to hide are more likely to survive?

Large numbers of red sea-urchins Strongylocentrotus franciscanusin an algal "barrens" area about 1m below the seaward edge of a kelp bed Nereocystis luetkeana in Barkley Sound, British Columbia. The only plants visible in the area are coralline algae Lithothamnion spp.

Research study 3

photograph of several red sea-urchins Strongylocentrotus franciscanus eating kelpA study conducted in southern California over a 3yr period provides insight into the way in which food, or the lack of it, changes the behaviour of large aggregations of red sea-urchins Strongylocentrotus franciscanus. The urchins live either in small stationary aggregations or in large motile ones, located within 100m of one another. The stationary groups subsist mainly on drift kelp and play little or no role in population dynamics of the kelp. The motile ones, however, advance in “fronts” at about 2m . mo-1 and consume almost all macroalgae in their paths. Examination of relative gonad size in individuals of these moving groups shows that they are starved. It appears that a scarcity of drift algae changes the behaviour of the stationary urchins, leading to the moving groups. These motile urchins run into other stationary aggregations, quickly outstrip the food supply, and move on in a “snowballing” manner to form the large, motile aggregations, or fronts. White urchins Lytechnus anamesus, also present in aggregations but at the offshore periphery of the kelp bed, remain relatively stationary over the study period. This species rarely eats adult kelps, but feeds instead on the developmental stages and thus prevents seaward expansion of the bed. In different ways, then, the 2 species of urchins profoundly affect the distribution and survival of large kelps. Dean et al. 1984 Mar Biol 78: 301.

NOTE the San Onofre kelp bed is located offshore from Oceanside, California and, at the time of the study, occupied about 10,000 hectares. Principal kelps at a depth of 12-15m are giant Macrocystis pyrifera and smaller understory kelps Pterygophora californica

NOTE while red urchins average about 11cm test diameter, white urchins are quite a bit smaller, averaging a little over 1cm test diameter

Research study 3.1

A 5yr study on the effects of 2 severe winter storms on kelp-forest community structure at Naples Reef, California by researchers at the University of California, Santa Barbara provides an interesting timeline of how such effects can differ depending upon past history. The first storm broke off large pieces of shale rock and removed all giant kelp Macrocystis pyrifera including large accumulations of drift kelp, thus depriving sea-urchin populations Strongylocentrotus franciscanus and S. purpuratus of their preferred foods. The urchins were forced out of shelter and ate most of the remaining algae, including most of the understory kelps Pterygophora californica and Laminaria farlowii. Disruption of the detritus-based food chain had important secondary negative effects on abundance of turf algae and, down the line, on surfperch fishes that were accustomed to eating turf-inhabiting invertebrates. A second severe storm 3yr later killed off most of the sea urchins, thus allowing extensive kelp canopies to become re-established by the following summer (see drawings below). The study illustrates the different effects that storms may have on community structure depending upon the state of the community prior to the disturbance. Ebeling et al. 1985 Mar Biol 84: 287.

diagram of Naples Reef before the first severe winter storm in 1979In summer 1979 before the first winter storm sea urchins shelter beneath the widespread kelp canopy and feed mainly on drift algae
diagram of Naples Reef one year after the first severe winter storm in 1979By autumn 1980 the urchins have left the kelp-forest area and feed near the last remaining Pterygophora beds. Most of the reef is now bare rock (“barrens”)
diagram of Naples Reef 2 years after the first severe winter storm in 1979By spring 1981 most of the reef is bare and urchins are constrained to a small area. The second storm hits in winter 1983and wipes out most of the remaining sea urchins
diagram of Naples Reef after the second severe winter storm in 1983By summer 1984 the reef is essentially restored. “Barrens” areas are minimal and sea urchins are returning. Lines on each figure indicate transecti lines surveyed by SCUBA

Research study 3.2

red urchin Strongylocentrotus franciscanus with corallimorpharian colony Corynactis californica

Could the formation of such “fronts” by sea urchins be related to an inability or perhaps reluctance to cross physical barriers such as sand patchs, vertical rock walls, and the like, or perhaps discomfort to cross biological barriers such as large colonies of stinging corallimorpharians Corynactis californica? These ideas are tested by researchers at the University of California, Santa Barbara for short-spined purple urchins Strongylocentrotus purpuratus and long-spined red urchins S. franciscanus in both laboratory and field situations. Results show that both species can negotiate sand using their oral spines, but with purple urchins being much less adept than red urchins. Sloping ramps installed in the field are readily climbed by both species to reach kelp growing on higher ledges. Colonies of Corynactis californica represent a barrier to the urchins, mostly owing it seems to lack of suitable surface for attachment of tube feet. Laur et al. 1986 Mar Biol 93: 209.

NOTE the authors refer to this species as a "coral-like anemone", but this is not correct. Sea anemones are in Order Actiniaria, while Corynactis is in O. Corallimorpharia

NOTE spines are used for locomotion only on soft substrata; on hard surfaces both species locomote using their tube feet

A red urchin Strongylocentrotus franciscanus (trapped?) in a field of corallimorpharians Corynactis californica. Too soft? Too stinging? Too little food? 0.5X

Research study 4

graph showing decline of kelp cover Macrocystis pyrifera in relation to densities of red and purple sea-urchins photograph of sea urchins Strongylocentrotus purpuratus and S. franciscanus in a kelp bed

In spring 1984 a large settlement of red sea-urchins Strongylocentrotus franciscanus and purple urchins S. purpuratus occurred within forests of kelp Macrocystis pyrifera in Carmel Bay, California. By end of summer 1986 these cohorts of urchins had removed most macro-algae from one large reef area (Outer Pinnacles), and a good portion of the sessile invertebrates as well (most notably compound ascidians and bryozoans). By 1989 most purple urchins are gone, but the population of red urchins remains intact. Study by researchers at the Monterey Bay Aquarium reveals that settlement of algal spores remains high during the grazing episode, but algal recruitment does not occur until the sea-urchin numbers have declined. The authors discuss reasons for the deline, such as predation and disease, but discount the possibility that predation by sea otters could have had such a major impact on one, but not the other, sea-urchin species. Watanabe & Harrold 1991 Mar Ecol Progr Ser 71: 125.

Mixed populations of purple and red sea-urchins in a kelp bed, S. purpuratus and S. franciscanus

Research study 5

Sea urchins attack kelps in several ways, including at the juncture of stipes with holdfast, often leading to entire loss of plant biomass, and also by crawling up and weighing down individual stipes, leading to consumption of entire fronds by individuals on the rocks below.graph showing effects over time of sea urchins on kelp holdfasts in the Point Loma, California area during the early 1990s The holdfasts are perennial, may last for 4-7yr in kelps such as Macrocystis pyrifera, and can be massive in size. Sea urchins around Point Loma, California, most notably Strongylocentrotus franciscanus and S. purpuratus, may burrow deeply into these holdfasts and the “cavitation” produced can lead to wholesale structural failure of the plant in storm photograph of several kelp plants along with anchoring rocks cast up on a beach after a stormsurge. Studies of M. pyrifera holdfasts at 18m depth off Point Loma reveal high incidence of lethal cavitation, correlative with high densities of both species of sea urchins. Tegner et al. 1995 J Exp Mar Biol Ecol 191: 83.

Holdfasts and stipes of bull kelps Nereocystis luetkeana, an annual species and much smaller than Macrocystis spp.

Research study 5.1

In addition to their own study on the interrelationships between sea urchins, kelps, and sea otters in the Aleutian Islands and Southeast Alaska, researchers from University of Santa Cruz and University of Washington provide a summary of 17 studies to date on the subject done in different areas of the west coast. Of these, 6 show that sea otters reduce sea-urchin population numbers but with no reference to secondary effects on kelps, 7 show that sea-otter predation on urchins increases algal biomass, and 4 show that sea urchins affect algal biomass but do not involve sea otters. In their own study at several sites in the Aleutian Islands the authors report that where sea otters have beeneither continuously present or continuously absent, kelp and sea-urchin abundances remain generally unchanged over periods ranging from 3-15yr. However, at sites where sea otters are newly colonising, sea-urchin numbers decline significantly (by 50-100%), with concomitant increases in photograph of sea urchins Strongylocentrotus polyacanthus eating kelp in Aleutian Islands, Alaskakelp biomass. The value of the study is in the selection of 2 widely separated sites and in the long period of study at one of the sites (15yr). The researchers are satisfied that they have answered most of the criticisms put forward by other authors that past studies have focused too much on the top-down role of sea otters, with not enough attention being paid to other factors, such as physical disturbances. The authors provide an excellent review of the subject of sea urchins, sea otters, and kelp on the west coast. Estes & Duggins 1995 Ecol Monogr 65 (1): 75. Photograph courtesy Museum of Aleutians, Unalaska, Alaska MOA.

NOTE species comprise Strongylocentrotus polyacanthus in the Aleutian sites, and S. franciscanus, S. droebachiensis, and S. purpuratus in Southeast Alaska

NOTE the authors actually list 23 papers, but 6 have little or no relevance to this section of the ODYSSEY. Synopses of all of the remaining 17 papers can be found in this or other sections of the ODYSSEY. However, given that there have been at least 6 west-coast publications concluding that sea otters eat sea urchins with resultant increase in algal biomass, one wonders how long interest in, and originality of, such studies will be maintained

A large stand of kelps in Aleutian Islands, Alaska showing grazing effects of many sea urchins Strongylocentrotus polyacanthus

Research study 6

maps of before and after sea-otter presence on the Olympic Peninsula coastExpansion of the sea-otter Enhydra lutris population along the Olympic coast of Washington after their introduction in 1969 allows an assessment of their impact on sea-urchin stocks. An initial survey in 1987 sets the baseline for distribution and numbers of otters and sea urchins, with a follow-up survey in 1995 assessing changes due to the sea otter population moving northwards into previously unoccupied areas. The map shows the sea-otter range in 1987 and 1995 with the benthic sites sampled on both dates indicated by black dots. Note the northwards expansion in range of Enhydra during 1987-1995. The data presented in the paper are complex but can be summarised as an order of magnitude decline photograph of sea otter Enhydra lutrisin numbers and biomass of urchins Strongylocentrotus franciscanus in the areas newly colonised by sea otters, as well as further decline in areas previously occupied. The only exception to this is the wave-exposed and current-swept areas around Cape Flattery, including Tatoosh Island. Here, urchin numbers and biomass are higher than anywhere else sampled in the 2 studies. The otters appear to have skipped over this area, perhaps because of the typically heavy seas and strong tidal currents found there. At sites monitored for seaweeds, cover of red foliose species increases greatly in correspondence with the decline in sea-urchin numbers. As a result of being covered by new growths of foliose species, coralline-crust algae drop from virtually 100% cover to 42% cover. The authors’ results support previous conclusions that sea otters eliminate sea-urchin grazing as a dominant community-structuring force in near-shore benthic regions. Kvitek et al. 1998 Mar Mammal Sci 14: 895; see also Kvitek et al. 1989 Mar Mammal Sci 5 (3): 266.

NOTE the founding members of the population were introduced from Alaska in 1969-70, with numbers being augmented in 1987 and again in 1995.

Research study 7

The Exxon-Valdez oil spill in 1989 caused an immediate 50% reduction in abundance of sea otters Enhydra lutris in northern Knight Island, Alaska. Nine years after the spill numbers of sea otters were still about 2/3rds fewer than they had been in 1973. A decade later, at the time of the present study, athough some areas with reduced sea-otter densities have proportionately more larger green sea urchins Strongylocentrotus droebachiensis, in other areas there appears to have been little or no effect of sea-otter absence on either sea-urchin abundance or kelp abundance. This contrasts with data from the western Aleutian Islands that show greatly increased sea-urchin biomass and greatly reduced kelp density after a 90% reduction in sea-otter abundance. The authors discuss possible reasons for the differences. Dean et al. 2000 Mar Ecol Progr Ser 199: 281.

NOTE overall, the spill killed an estimated 1000-2800 sea otters, most in Prince William Sound

Research study 8

One doesn’t think that kelps could defend themselves from sea-urchin grazing, but in Shemya Island in the Alaskan archipelago sea urchins Strongylocentrotus polyacanthus having found themselves deprived of food in barren areas, may actually be prevented from crossing into the food-rich kelp stands by the sweeping motion of the kelp fronds over the sea floor. When researchers from the University of California, Santa Cruz experimentally translocated sea urchins into the kelp forest, the urchins prospered there until winter storms swept them away or caused them to seek shelter in crevices. Gonad sizes of these kelp-inhabiting individuals are significantly larger than those in the barren areas, indicating that a kelp diet will improve fitness. The finding that kelp plants are able to defend themselves from sea-urchin grazing in this way may explain why kelp-forest ecostystems often exist as stable patchwork mosaics of kelps and barren areas. In Alaska, only in areas where sea otters are present is the stability of this system upset, for otters preferentially consume the sea urchins which prevents the formation of barren areas. Konar & Estes 2003 Ecology 84 (1): 174.

Research study 8.1

histograms comparing grazing responses of green urchins Strongylocentrotus droebachiensis and red urchins Mesocentrotus franciscanus to the scent of upstream predatory sunflower stars Pycnopodia helianthoidesTo what extent does the scent of an upstream predator disrupt feeding activities of a prey herbivore? This time-honoured question is investigated at Friday Harbor Laboratories, Washington using sunflower stars Pycnopodia helianthoides and two species of urchins, Strongylocentrotus droebachiensis and Mesocentrotus franciscanus, both types being prey of the sea star. The study is done in the laboratory, first testing predator preference for small and large red urchins, both when tethered and free-crawling. The second part of the study assesses disruptive effects of scent of an upstream predator on grazing rates of each species, with brown algae Laminaria saccharina being the food. Results show that when red urchins are able to move freely, small and large individuals are caught and consumed more or less evenly. When tethered, though, the small-sized ones are clearly favoured. These small ones are generally swallowed whole by the predator, which is a much quicker process that with large individuals, which require extra-oral digestion. Green urchins are small in size and, with relatively shorter spines than red urchins, are preferred as prey by sunflower stars. As for non-lethal predator effects, adult S. droebachiensis and young M. franciscanus of comparable size (about 7cm test dia) significantly reduce their grazing activities when bathed in runoff water from sunflower stars caged upstream, but large M. franciscanus do not (see graph; 6d experiments). Note in the figure that feeding of adult green urchins and equal-sized red urchins is suppressed by an average of about 95% in the presence of predator scent-cues. The explanation for the differential size response of red urchins to predator cues is unclear. Sunflower stars may avoid large red urchins owing to their spine protection, but how could the adult urchins "know" to ignore the scent and continue to feed normally, when as youngsters they would presumably have curtailed their feeding (a"ghost" of past interactions?)? One wonders if it would be possible to find M. franciscanus in areas totally devoid of sunflower stars to test and compare these apparent age-related responses. Are these escape/feeding responses genetically endowed from fertilisation or are they "learned"? Also, are the urchins' responses specific to sunflower stars, or also to other sea stars? Freeman 2006 Behav Ecol 17: 182.

NOTE monofilament line around the test attached to a dive weight

NOTE with the almost total demise of sunflower stars along the coast owing to wasting disease in 2013-14, this might not be too difficult to arrange

Research study 8.2

graph showing effect of upstream lobsters Panulirus interruptus that are consuming purple urchins Strongylocentrotus purpuratus, on grazing rate of downstream conspecific urchinsgraph showing effect of upstream damaged purple urchins Strongylocentrotus purpuratuson on grazing rate of downstream conspecificsIn a similar way, research at the Wrigley Marine Science Center, California shows that the downstream scent of predatory spiny lobsters Panulirus interruptus will significantly disrupt grazing by purple urchins Strongylocentrotus purpuratus. Three types of experiments are done in the laboratory. In the first, the scent of upstream urchin-fed lobsters reduces feeding by downstream urchins by 43% over control urchins not exposed to lobster scent (5d duration of experiment; see graph on Left). In the second, the scent of upstream damaged conspecific urchins produces no significant reduction in feeding, mainly owing to a strong response from larger-sized urchins (7d duration; see upper graph on Right). Finally, the scent of upstream mackerel-fed lobsters leads to a surprising 44% reduction in feeding (7d duration; see lower graph on Right). Why urchins would respond to mackerel-induced lobster-cues in this way is addressed by the author, but only briefly and without resolution. The study adds to our knowledge of the mainly understudied importance of predation risk in urchin-kelp interactions. However, in her study the author provides no data on lobster abundances in southern California kelp forests and thus the reader is unclear on the significance of the effects shown. Matassa 2010 Mar Ecol Progr Ser 400: 283.

graph showing effect of upstream lobsters Panulirus interruptus that are consuming mackerel fishes, on grazing rate of downstream conspecific urchinsNOTE the food is weighed portions of kelp Macrocystis pyrifera





Research study 9

schematic showing quadrat array for study of kelp and purple sea-urchin Strongylocentrotus purpuratus distributionsDoes the close coupling of sea urchins and kelp forests as witnessed in the northern parts of their ranges extend also to their southern range limits? This is examined for purple sea-urchins Strongylocentrotus purpuratus and a favoured kelp food Macrocystis pyrifera at 4 sites along the Pacific Coast of Baja California, Mexico. Two of the sites are just south of the California border and 2 are in the region of Bahia Tortugas at latitude 27.7oN (see map). These last sites are in a biogeographic transition zone between the temperate waters of the California Current and the subtropical waters further south. The authors establish transect lines (30 x 2m) at each site and sample 11 one-m2 quadrats along each line for density of kelp plants and their fronds, and numbers of sea urchins at 2 times of year (winter: Dec-Jan and spring: Mar-Apr; see schematic on Right). Sea urchins are more dense in the southern populations than in the northern ones. Overall, the authors conclude that at all 4 sites densities of kelps and sea urchins are positively correlated, but some significant seasonal and latitudinal variations also exist. Where the community disappears south of the transition zone is not mentioned by the authors. Beas-Luna & Ladah Bot Mar 2014 57 (2): 73.photograph of purple sea-urchins Strongylocentrotus purpuratus in barrens area near kelp bed

NOTE many species of shallow-water algae and invertebrates encounter their southern limits of distribution in this area. These include several large kelps, abalones, and red and purple sea urchins






The barrens area on the seaward side of
the kelp bed hosts many sea urchins, mainly
the purple Strongylocentrotus purpuratus

Research study 10

graph showing relationship between commercial fishing intensity and bioimass of sheepheads Semicossyphus pulcher A variant on the familiar story further north of sea otters preying on sea urchins leading to enhanced growth of kelps has, in the Channel Islands of California, the otters being replaced by sheephead fishes Semicossyphus pulcher. Studies by researchers from Moss Landing Marine Laboratories and Marine Science Institute UC find that predation on urchins by small-sized sheepheads is low (comprising only 3% of gut contents), but larger-sized sheepheads favour urchins (up to about 40% of gut contents), the larger the better, and their loss to these fish predators greatly benefits kelp forests. Sheephead-harvesing in marine-protected areas (established in 2003) is, in turn, regulated by California Fisheries legislation. Therefore, levels of sheephead predation on urchins and thus impacts on kelp forests are ultimately mediated by harvest policies instituted by fisheries management practices. Prior to the establishment of a system of no-fishing reserves in the islands in 2003, sheephead numbers were low, and kelp beds were in decline; by 10yr after being protected, however, sheephead numbers have increased significantly, and kelp beds have flourished. Additionally, sea urchins when abundant have direct negative effects on abundance of fleshy seaweeds, and indirect positive effects on abundance of coralline algae (in the absence of kelp canopy the corallines flourish). Hamilton & Caselle 2014 Proc Roy Soc B 282:20141817.

NOTE the researchers assay several marine reserves in the California Channel Islands, most notably in Catalina and San Nicolas, over several decades, collecting data on numbers of sheepheads, red and purple urchins Mesocentrotus franciscanus and Strongylocentrotus purpuratus, and giant kelps Macrocystis pyrifera (both numbers of plants and stipes per plant). Gut contents of sheepheads of different ages are also assayed

Research study 11

graph showing correlation between extent of sedentary behaviour in sea urchins Strongylocentrotus franciscanus and their drift-algae catching effectiveness Red sea-urchins Strongylocentrotus franciscanus often pack together like tinned sardines, and have little or no ability to move. In one assessment, researchers at Friday Harbor Laboratories, Washington note that 87% of individuals in study populations below the macroalgal zone do not move at all over a 3wk study period. Sedentary behaviour is especially noted in areas where availability of drift algae for capture is relatively high (see graph). At depths deeper than about 20m in the San Juan Archipelago region, macroalgae are scarce owing to diminished light for growth, and the sea urchins rely almost entirely on drift algae for their sustenance. At depths deeper than this, urchin density decreases as individuals spread out to search for food. One hypothesis in the present study is that red sea-urchins will migrate vertically from deeper to shallower habitats in winter when abundance of drift algae is low. The researchers also predict that differences in sea-urchin density will be accompanied by significant differences in composition of the benthic communities beneath them. In fact, seasonal monitoring of experimental and control quadrats at depths of 10, 20, and 30m using SCUBA at 3 sites reveals no significant seasonal change in density. However, the researchers do find that the benthos under the urchins is significantly more bare and has greater densities of coralline algae, snails, crabs, and shrimps photograph of red sea-urchins Stronglyocentrotus franciscanus dispersed below the kelp zonethan nearby control areas lacking sea urchins. The authors conclude that even when sedentary, urchins can exert a strong influence on benthic community structure. Lowe et al. 2015 Ecology 36: 129.

NOTE now known as Mesocentrotus franciscanus

NOTE the authors determine the significance of these data using linear-regression analysis but, as this is a correlation, wouldn’t testing of a correlation coefficient have been more appropriate?

Sea urchins Strongylocentrotus franciscanus in an algal-
barren area below the kelp level, about 15m depth

Research study 12

graph showing densities of purple urchins Strongylocentrotus purpuratus and sunflower stars Pycnopodia helianthoides in relation to annual water temperatures in the Channel Islands, CaliforniaAn interesting study by researchers at the Moss Landing Marine Laboratories, California assesses the effect of water temperature on predation of purple urchins Strongylocentrotus purpuratus by sunflower stars Pycnopodia helianthoides1 in the northern Channel Islands, California. The study has its roots in global climate change but, unlike most such studies on future effects of warming or acidification that are laboratory-based, this one uses a natural temperature gradient in the Islands in conjunction with laboratory assessments of temperature effects on predation rate. Field assessments2 show, firstly, that at sites with mean annual temperatures <14oC sunflower stars are common and purple urchins rare, while at sites >14oC the reverse is true (see graph). In laboratory tests with 10 urchins being placed into 120L tanks with single sea stars for 24h at temperatures of either 13 or 17oC3 (seasonal high temperature), predation by sunflower stars is significantly less at the high temperature, supporting the field observations. The significance of these findings with respect to future warming is the possible deleterious effects on kelp-forest integrity. With respect to this, the researchers show, predictably, a reciprocal relationship between urchin and kelp densities. Thus, where sunflower stars are abundant, urchins are sparse, and kelps flourish. However, while the authors write that "these results clearly demonstrate that...(the effect of this predation)...is strongly mediated by temperature", they should remind themselves that the field data are correlative only, not cause-and-effect. Also, as expected in such studies there is high variability in the data. There may be many other factors at play including substratum topography, food availability, presence of other predators and competitors, and so on. Bonaviri et al. 2017 J Anim Ecol 86: 490. For more on this subject see LEARNABOUT/ECHINODERMATA/seasEnvi.php#RS9

NOTE1 the species name helianthoides is misspelled "heliantoides" throughout the text, indicating careless proofreading on the part of the authors, the journal editor, and possibly others. The mistakes are minor in themselves, but one wonders what other errors may have been overlooked...easy to do with numerical data. However, the error is amusing as it may remind some (older) readers of the ditty that pokes fun at certain New York accents: ..."toity poiple boids, sittin on da koib, a-choipin an' a-boipin, an' eatin doity woims"...

NOTE2 the field part of the study is greatly aided by the authors having access to data on densities of P. helianthoides and S. purpuratus monitored by the US National Park Service at 16-17 sites in the Channel Islands over the past 4 decades

NOTE3 equivalent to summer temperatures at sites where P. helianthoides is rare

Research study 13

graphs showing feeding responses of spiny lobsters Panulirus interruptus in laboratory tests with varying densities of prey urchinsTop-down control of population numbers of sea urchins in kelp forests has long been thought to be be density-dependent. That is, numbers eaten by a predator, will change stepwise with changes in predator or prey densities. But, with non-predator-related changes in prey densities and with different types of predators, how does this relationship hold up? In the absence of sea otters along the southern California coast, the main predators of sea urchins Mesocentrotus franciscanus (red) and Strongylocentrotus purpuratus (purple) are sheephead fishes Semicossyphus pulcher and spiny lobsters Panulirus interruptus. On reefs in southern California where red urchins may be harvested, density may be highest in protected reserves, so the patterns of interaction are varied. The extent to which urchin densities are regulated top-down by predators in San Diego kelp forests is investigated by researchers at the Coastal & Marine Institute Laboratory in both laboratory and field experiments. Unexpectedly, laboratory tests with lobsters, with purple urchins alone and mixed with equal numbers of reds, reveal a saturating feeding response over time, with urchin mortality being inversely density-dependent (see graphs) . In other words, lobsters feed less with increasing density of prey. How this might work in the field is, of course, unknown, but overall the authors think fishes to be the major urchin predators in southern California kelp beds. However, field observations on fishes (mainly sheepheads) eating purple urchins in replicate kelp-forest plots reveal a highly variable pattern of mortality across all density levels. At low densities of urchins typical of kelp-forest habitat, mortality increases to a peak density of about 11 urchins/m2. At higher densities, typical of urchin "fronts", mortality from fishes is independent of prey density. Another variable is added in mixed urchin assemblages. When red and purple urchins are intermixed, predator effects on purple urchins may be modulated. Fish predators are now turned off, perhaps because of the spine protection conferred, and mortality rates becomeeven more variable. These and other observations suggest to the researchers that top-down control of urchin densities may be much less frequent than earlier imagined. Dunn & Hovel 2019 Ecology 100 (3): e02625; see also Nichols et al. 2015 Mar Biol 162: 1227 for a preliminary study on the same topic. Photograph courtesy Kevin Lee, Fullerton, California diverkevin

NOTE a situation presently exists (2018-19) in a coastal area of Haida Gwaii, British Columbia where kelp forests are being turned into barrens from over-grazing of red urchins. In addition to harvesting and marketing the top-quality urchins for local consumption, indigenous First Nations people in consultation with Canadian Federal Fisheries scientists have engaged in wholesale elimination of the urchins using commercial and volunteer divers with rakes and hammers. The urchins are faced with a new type of predator. Is this wise conservation strategy, or just further mis-management of marine stocks? Experience tells us that it is often wiser to leave such things alone. Kelp forests and sea-urchins have been around much longer than have humans. One suggestion in such a circumstance might be to let the urchins do their thing, but monitor the situation carefully at regular intervals. In the long run this information will be of much greater value than simply killing themphotograph of spiny lobsters Panulirus interruptus sheltering under an overhang

NOTE a functional explanation for this is unclear, although in human terms it is not surprising that a certain food becomes less appetising and/or less nutritionally beneficial when consumed on its own or in excess

Several spiny lobsters Panulirus interruptus packed into a protective
overhang. The yellow checker-boards so noticeable in this particular
lighting do not highlight the eyes. The real eyes are stalked and are
positioned immediately above the highlighted black "eyespots" . If
these were tropical fish one might be thinking of eyespot mimicry
...thus a predatory fish might strike at these rather than at the
actual eyes. Is it possible to test this idea in the laboratory?