Selective consumption and facilitation by mesograzers in adult and colonizing macroalgal assemblages

Grazing pressure on macroalgae in littoral communities may vary with algal species, the age of an algal individual and grazer identity. Previous studies on alga–grazer interactions have shown that grazer preference for an algal species may release another one from interspecific competition. We measured the impacts of four common grazer taxa and the natural grazer guild on macroalgal communities at both their colonization and adult stages, and compared the impacts to grazer exclosures. The grazer effects were stronger on colonizing than on adult macroalgae; grazers did not reduce the total density of adult algae. Grazers both feed on propagules and indirectly facilitate other algae, depending on the grazer or algal species. Hydrobia species increased the settlement of spores of the red alga Ceramium tenuicorne. Similarly, the gastropod Theodoxus fluviatilis tended to facilitate one crustose algal species, but decreased the propagule density of annual filamentous algae, suggesting a preference for one species to the advantage of another. Effects of crustacean mesograzers on the studied macroalgae were weak. These results indicate that northern Baltic macroalgae are limited to grazing mainly during their colonization stage.     

Relation of food preference to fitness for the green sea urchin,Strongylocentrotus droebachiensis

We maintained 30 to 35 mm sea urchins (Strongylocentrotus droebachiensis) on 17 different macroalgal diets in the field over a 130-d period and correlated the resulting changes in somatic and gonadal tissues with measures of food preferences. The varions algae fell into there distinct categories according to their ability to support growth. The most striking effect of diet was on gonadal mass, there being a 14-fold increase for the best algal diet, compared to initial gonadal mass, and nearly a threefold decrease for the worst diet. The relation of food preference to somatic growth was nonlinear. A sharp increase in the growth of test diameter, total mass and test mass occurred between the nonpreferred and intermediately preferred algae, but growth rates were similar for intermediate and preferred algae. Gonadal growth tended to increase exponentially with an important measure of food preference, feeding rate (g ingested d^–1). These observations suggested that urchins only channel food resources into somatic growth up to a given point, additional reserves being used for producing gametes. The positive relationship of food preference to growth, and especially the strong correlation with gonadal production, indicates that the highly selective feeding of the urchin contributes to ils fitness.     

Microhabitat segregation of the amphipod genus Gammarus (Crustacea: Amphipoda) in the Northern Baltic Sea

Closely related species may occupy very similar niches but are often found to diverge by one or more traits when they share the same habitat. Five indigenous and sympatric Gammarus species are characteristic for the Baltic rocky littoral ecosystem. Yet, the species-specific distribution of these sympatric gammarids has not been well studied in the northern Baltic Sea. This study was undertaken to assess the spatial distribution of gammarid amphipods along wave exposure and depth gradients to study whether they show segregation in their microhabitat use. We sampled 12 rocky sublittoral shores along a wave exposure gradient over a period of 5 years. Samples differed with respect to depth and macroalgal type. Three of the five gammarid species occurred mainly in different depth zones and among different macroalgae at the exposed shores. In contrast, on protected shores, where algal zonation is weaker, a link to zonation and macroalgal type was almost absent. Moreover, the microhabitat use was strongest during the reproductive seasons of the species. The observed microhabitats of the three gammarid species fit well to their species-specific mean body sizes.     

Comparing macroalgal food and habitat choice in sympatric, tube-building amphipods, Ampithoe lacertosa and Peramphithoe humeralis

For small tube-building amphipods that live on the algae they consume, food and habitat are tightly linked. This study compared two closely related amphipods to determine whether the species’ algal preferences are based on the food value of the algae or on some other aspect of their algal habitat. Ampithoe lacertosa and Peramphithoe humeralis are both abundant on Shannon Point beach (Anacortes, Washington, USA; 48°30.542′ N, 122°41.070′ W) but specialize on different algae. In observations and laboratory experiments conducted July–September 1997, 2007, and 2008, the two species exhibited markedly different choices of food and habitat when offered six common macroalgae. Ampithoe lacertosa ate all algae offered, but preferentially built tubes on the green alga Ulva lactuca. Survival was relatively low among juveniles maintained on single species diets, except when they were fed Mazzaella splendens. Conversely, P. humeralis consumed primarily the brown kelp Saccharina latissima, Alaria marginata, and Desmarestia ligulata and preferred those species for tube building. Juvenile P. humeralis could not survive on a diet of U. lactuca or M. splendens. While A. lacertosa builds simple, temporary tubes and relocates frequently, P. humeralis is a highly thigmotactic species that builds long-term, complex tubes on the alga it prefers to eat. Feeding and habitat preferences of the two species were not clearly linked to nitrogen content of the algae, C:N ratio, or toughness of the algal tissue. Instead, preferences of the species may be related to their mobility and the permanence of the tubes they build. Ampithoe lacertosa and P. humeralis also use different feeding strategies; the former appears to mix algae to produce a high-quality diet, while the latter is more selective and has a capacity for compensatory feeding. The species are abundant on the same protected rocky shores, but specialize on different algae for habitat and food. Results suggest that the nutritional requirements of these sympatric mesograzers differ considerably and even closely related species can exhibit divergent behavioral strategies.     

Differences in herbivore preferences, phlorotannin production, and nutritional quality between juvenile and adult tissues from marine brown algae

 Juvenile and adult marine organisms differ in their morphology, chemistry, physiology, behavior, and ecology. Because juvenile algae are thinner, smaller, and have more delicate tissues than adults, they are often assumed to be more susceptible to grazers. We examined within-species food preferences of four common generalist herbivores for juvenile and adult tissues of eight common brown algae in two-choice laboratory food-preference experiments. Our results showed that juvenile algae did not tend to be a preferred food of herbivores. Juvenile tissues were significantly preferred over adult tissues in only four of the 32 combinations of algae and herbivores tested. In 12 experiments, adult tissues were preferred over juvenile tissues, and no choice occurred in the remaining 16 experiments. When sea urchins exhibited a preference, it was always for adult tissues. The other three herbivores, an isopod and two snails, were more variable in their choices, sometimes preferring juveniles, sometimes adults, and sometimes having no preference. We measured nitrogen and phlorotannin concentrations in adult and juvenile seaweeds to see whether these parameters were correlated with herbivore food preferences. Nitrogen levels were similar in juveniles and adults of three algal species and were higher in juveniles of two. Phlorotannin levels were higher in juveniles of four species and lower in juveniles of one. The other three species showed no differences in phlorotannin levels. Phlorotannin concentrations decreased with increasing juvenile size in three species and increased with increasing size in one species. Neither nitrogen nor phlorotannin concentrations explained overall herbivore food preferences for algae of different stages. Our results suggest that preferences of certain grazers for juvenile algae are not as strong as previously assumed and are dependent on herbivore species. Preferences between juveniles and adults are likely to be determined by a combination of morphological and chemical features of the tissues and the unique responses of herbivore species to those features. Received: 10 April 2000 / Accepted: 19 November 2000     

Fische als Feinde tropischer Seeigel

The techniques used by 9 species of fish (representing 3 families) to capture Diadema sea urchins were observed under water in the Red Sea at Eilat, Israel. Different species displayed different techniques. Fish belonging to the Lethrinidae and Labridae families devoured the entire urchin after capture; Labridae alone were also capable of breaking up large sea urchins by banging them on rocks. The fish did this by shaking its head while holding the urchin in its mouth. Under experimental conditions, fish can distinguish between large and small urchins, and show a preference for the small urchin. The fish also shun contact with some other species of urchins. The trigger fish Balistes fuscus exposed to a dummy sea urchin responds to optical stimuli (body shape and spines); when exposed to a living sea urchin it responds to both optical and behavioural stimuli (movements of the spines). Morphological and ethological preadaptation of the fish to alter its preference for other food is essential before it can develop differentiated capture techniques. The ecological significance of the behaviour of fish in the coral reef ecosystem and the alternating influences in the predator-prey-relationship are discussed.     

Effects of micro- and mesograzers on intertidal macroalgal recruitment

This study examined the effects of a guild of micrograzing harpacticoid copepods (dominated by two species of Paradactylopodia sp. nov. and one species of Scutellidium sp. nov.) and a mesograzing periwinkle, Afrolittorina praetermissa, on the early recruitment of intertidal macroalgae on a wave-exposed, rocky shore. This is the first study, to our knowledge, to examine the effects of micrograzers (<500 μm) on intertidal macroalgal recruitment. Data showed that microscopic harpacticoid copepods altered the assemblages and reduced the densities of several macroalgal taxa, while A. praetermissa changed the assemblages and reduced both the density and number of macroalgal taxa. Recruitment of encrusting coralline algae was actually higher in copepod inclusions than exclusions, suggesting that copepods may be beneficial to the recruitment of this algal group. These results contribute to the understanding of grazing as a factor causing high mortality of algal recruits, but also highlight the need for more studies that examine the effects of micro- and mesograzers on the distribution and abundance of macroalgae.     

Foods and predators of the green sea urchin Strongylocentrotus droebachiensis in Newfoundland waters

Gut analyses of the green sea urchin Strongylocentrotus droebachiensis (O. F. Müller) demonstrated that perennial phaeophytes, mostly fucoids and Alaria esculenta, were predominant in the diet. Ephemeral species, coralline algae and animals, were consumed in smaller amounts when available. Grazing by the urchins is evidently responsible for the dearth of non-coralline sublittoral algae in Newfoundland waters. Lobsters, rock crabs, purple sea stars, other urchins, and a variety of fishes and birds feed on S. droebachiensis, but predation is apparently not effective in limiting the abundance of the urchin.Studies in Biology from the Memorial University of Newfoundland No. 234.Contribution from the Marine Sciences Research Laboratory No. 66.     

Experimental evaluation of aggregation behavior in the sea urchin Strongylocentrotus droebachiensis

Defensive aggregation by Strongylocentrotus droebachiensis has been invoked as the fulcrum for the transformation of macroalgal beds into coralline barrens in the northwest Atlantic. We critically examined some of the mechanisms contributing to aggregation behavior by experimentally manipulating sea urchins, purported predators and food during autumn 1983 and spring 1984 both in the laboratory and field. We utilized several approaches to examine a range of sea urchin responses to the presence of food, tethered predators, caged predators, crushed con-specifics in the field and predators in laboratory tanks. Some of the field cages had the property of allowing free passage to sea urchins while retaining lobsters; this allowed distinctions to be made between artifacts caused by cage walls or topographic barriers and unrestricted behavioral responses of sea urchins. The results falsified the hypothesis that aggregations of S. droebachiensis are elicited by predators. Except in the presence of algae, sea urchins always avoided decapod predators (but not sea stars) and fled from the vicinity of injured conspecifics. However, avoidance behavior was subordinate to feeding responses, demonstrated by mass migrations of sea urchins into cages (with lobsters) to feed on algae. We reaffirmed by observation and manipulation previous studies which showed that sea urchins aggregate only in the presence of food. Two types of sea urchin groupings were delineated: (1) surficial two dimensional associations, often caused by topographic or other features which inhibited dispersal and (2) cohesive three-dimensional aggregations induced by food.     

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.Please address all requests for reprints to the senior author at his present address.     

Effects of nutrients, herbivory, and depth on the macroalgal community in the rocky sublittoral

We studied the interacting roles of nutrient availability and herbivory in determining the macroalgal community in a rocky littoral environment. We conducted a factorial field experiment where we manipulated nutrient levels and herbivory at two sublittoral depths and measured macroalgal colonization and the following young assemblage during the growing season. At the community level, grazing reduced algal colonization, though the effect varied with depth and its interaction with nutrient availability varied in time. In shallow water, the total density of macroalgae increased in response to nutrient enrichment, but the ability of grazers to reduce macroalgal density also increased with the nutrient enrichment, and thus, the community could not escape from the top-down control. In deep water, the algal density was lower, except in July when nutrient enrichment caused a very dense algal growth. Grazing at the greater depth, though effective, was generally of smaller magnitude, and in July it could not limit algal recruitment and growth. Species richness peaked at the intermediate nutrient level in deep but not in shallow water during most of the growing season. Grazing had no effect on diversity of the algal community at either depth and only a minor effect on species richness at the greater depth. Opportunistic and ephemeral algae benefited from the nutrient enrichment but were also grazed to very low densities. Slowly growing and/or perennial species colonized poorly in the nutrient enriched treatments, and depending on the species, either suffered or indirectly benefited from herbivory. For all species, effects of nutrients on colonization depended on depth; usually both nutrient and herbivory effects were more pronounced at the shallow depth. We conclude that grazers are able to reduce macroalgae over a large range of nutrient availabilities, up to 12-fold nutrient enrichment in the current experiment, and that the sublittoral depth gradient generates variation in the algal community control exerted by both herbivory and nutrient availability. Thus temporal and spatial variability in both top-down and bottom-up control and in their interaction, especially along the depth gradient, may be crucially important for producer diversity and for the successional dynamic in a rocky sublittoral environment.     

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.     

Chemoautotrophic symbiosis in the tropical clamSolemya occidentalis (Bivalvia: Protobranchia): ultrastructural and phylogenetic analysis

Patterns of algal seasonality, and their effect on the diet and feeding preferences of the herbivorous crab Grapsus albolineatus were investigated over an 18-mo period from March 1993 to August 1994 on an exposed tropical rocky shore (Hok Tsui Peninsula at Cape d' Aguilar, Hong Kong). Algal cover was greatest in the winter months, and lowest in the summer. Foliose algae such as Ulva fasciata, Porphyra suborbiculata, and Dermonema frappieri were dominant in the winter, but died off in the summer. During the hot summer months, perennial encrusting algae e.g., Ralfsia expansa, Hildenbrandia rubra, H. occidentalis, coralline crusts and the encrusting cyanobacteria Kyrtuthrix maculans, were the dominant algal species. Seasonal variation in algal abundance influenced the dietary selectivity of the herbivorous crab G. albolineatus. In the winter, the crab fed selectively on filamentous algae (e.g. Hincksia spp., Cladophora spp., Enteromorpha spp., and the cyanobacteria Lyngbya sp.). Foliose algae (e.g. U. fasciata, P. suborbiculata, Pterocladia tenuis) formed a small part of the diet, despite being the dominant species on the shore. Foliose and filamentous algae were virtually absent from the shore in the summer, and the crabs switched to feeding solely on encrusting algae. Electivity indices revealed preferences for green and brown turf species, and avoidance of foliose algae. Faecal analysis revealed that a greater proportion of the food is digested in the winter, suggesting that G. albolineatus is able to digest filamentous algae more efficiently than encrusting algae. Feeding preferences of G. albolineatus appear to be influenced by a number of factors, including the availability, digestibility and morphology of algae. The foraging behaviour and cheliped morphology of the crab also affect food choice. The monsoonal nature of Hong Kong's climate controls the diversity and abundance of intertidal algae and, therefore, indirectly influences the diet and subsequent growth and reproductive success of the herbivorous crab G. albolineatus.     

Diet, movement, and covering behavior of the sea urchin Toxopneustes roseus in rhodolith beds in the Gulf of California, México

The density, diet, movement, and covering behavior of Toxopneustes roseus (Agassiz) were investigated in rhodolith beds in the Gulf of California. Densities varied from a mean of 0.4 to 1.8/20 m² with most urchins occurring in aggregations. Spatial patterns of urchins varied with depth, with greatest abundance at intermediate depths (7.5–9.4 m) in the middle of the rhodolith bed. Urchins ate rhodoliths and nongeniculate coralline algal crusts almost exclusively, despite the availability of other algae. The mean amounts ingested were 3.87 and 7.96 g carbonate/individual per day. Even when food was abundant, animals were highly mobile, moving an average of 6.6–11.7 cm/h depending on site and time of day. Diel movement may be a behavioral adaptation to avoid surge, which is greatest during the day. Covering behavior may also be related to surge, as the ratio of covering material:body weight and the percent cover of material held were highest at the site with the most surge. While an urchin consumed rhodoliths, its movement spread the grazing impact over large areas. Bioturbation resulting from urchin feeding, movement, and covering activity probably benefits the rhodoliths by turning them, which maintains rhodolith integrity, prevents fouling, and contributes to bed persistence. Received: 22 February 1999 / Accepted: 3 July 2000     

Newly metamorphosed <Emphasis Type="Italic">Elysia clarki</Emphasis> juveniles feed on and sequester chloroplasts from algal species different from those utilized by adult slugs

The adult, sacoglossan sea slug, Elysia clarki (Pierce et al. in Molluscan Res 26, 2006), sequesters functional chloroplasts in cells of the digestive diverticula from four species of macroalgae in the order Bryopsidales (Penicillus capitatus, Penicillus lamourouxii, Halimeda incrassata, and Halimeda monile). Feeding experiments were conducted in December, 2003, using individuals raised in the laboratory from egg masses laid by E. clarki adults which had been collected from Grassy Key, Florida, USA, and 29 species of macroalgae collected from the Florida Keys, Tampa Bay, Tarpon Springs, Florida, or Woods Hole, Massachusetts or obtained from the Culture Collection of Algae at the University of Texas at Austin, Texas. For the first 14-day post-metamorphosis, juveniles ate only the thin filamentous species, Bryopsis plumosa or Derbesia tenuissima. Transmission electron microscopy showed that the chloroplasts from both algae were sequestered intracellularly in juvenile slugs. Individuals offered any other macroalga, including the four species fed on by adults, did not feed on or incorporate any chloroplasts, and soon died. Juveniles switched from B. plumosa to P. capitatus at a length of ∼ 1.0 cm, and fixed for microscopy 14 days later had intact intracellular chloroplasts from both algae. Nucleotide sequences of the chloroplast gene, rbcL, from DNA extracted from E. clarki, collected from Vaca Key, Florida, were aligned with 22 other available macroalgal rbcL sequences indicating that these E. clarki adults had fed on three species of algae in the Bryopsidales including Bryopsis pennata, and TEM results partially confirmed this conclusion.     

Larval settlement and metamorphosis of the mussel <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> on different macroalgae

Settlement of mussels is commonly associated with macroalgae. The effects of 19 macroalgal species on the settlement and metamorphosis of pediveliger larvae of the mussel Mytilus galloprovincialis were investigated in the laboratory. Settlement and metamorphosis inducing activities of macroalgae Chlorodesmis fastigiata and Ceramium tenerrimum collected each month during the period between January 2005 and April 2006 were also investigated. Furthermore, C. fastigiata and C. tenerrimum were subjected to various treatments to investigate the roles of bacteria and diatoms on the algal surface in the induction of larval settlement and metamorphosis of M. galloprovincialis and the characteristics of the cues in these two macroalgae. Pediveliger larvae of M. galloprovincialis settled and metamorphosed in high percentages on Cladophora sp., Chlorodesmis fastigiata, Centroceras clavulatum, and Ceramium tenerrimum, all of which were filamentous in morphology. Macroalgae that were cylindrical, phylloid, flabellate, palmate and pinnate all showed low (<8%) percentages of post-larvae but four other filamentous macroalgae also had low mussel larval settlement, suggesting that chemical factors may also be involved. Seasonal variation had no effect on inductive activities of C. fastigiata and C. tenerrimum. Treatment of C. fastigiata and C. tenerrimum with formalin, ethanol and heat resulted in the significant decrease or loss of their inductive activities. Survival of algal cells within treated macroalgae also decreased significantly. Treatment of the two macroalgae with antibiotics and GeO₂ reduced the numbers of bacteria and diatoms on their surface but did not affect their inductive activities, indicating that the cue was produced by macroalgae and not by coexisting bacteria and diatoms. However, conditioned water and crude extracts of these two macroalgae did not induce larval settlement and metamorphosis. Thus, larvae of M. galloprovincialis settled and metamorphosed on specific filamentous macroalgae. The chemical cues produced by C. fastigiata and C. tenerrimum were susceptible to ethanol and heat treatments and were not recovered in the conditioned water nor in the extracts. The finding that inactive C. tenerrimum can be produced from culturing its apical segments provides a new tool to elucidate the chemical cue(s) from macroalgae through manipulation of their culture conditions.     

Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses

Loss of macroalgae habitats has been widespread on rocky marine coastlines of the eastern Korean peninsula, and efforts for restoration and creation of macroalgal beds have increasingly been made to mitigate these habitat losses. Deploying artificial reefs of concrete pyramids with kelps attached has been commonly used and applied in this study. As a part of an effort to evaluate structural and functional recovery of created and restored habitat, the macroalgal community and food web structure were studied about a year after the establishment of the artificial macroalgal bed, making comparisons with nearby natural counterparts and barren ground communities. Dominant species, total abundance, and community structure of macroalgal assemblage at the restored macroalgal bed recovered to the neighboring natural bed levels during the study period. The main primary producers (phytoplankton and macroalgae) were isotopically well separated. δ¹³C and δ¹⁵N values of consumers were very similar between restored and natural beds but varied greatly among functional feeding groups. The range of consumer δ¹³C was as wide as that of primary producers, indicating the trophic importance of both producers. There was a stepwise trophic enrichment in δ¹⁵N with increasing trophic level. A comparison of isotope signatures between primary producers and consumers showed that, while suspension feeders are highly dependent on pelagic sources, invertebrates of other feeding guilds and fishes mainly use macroalgae-derived organic matter as their ultimate nutritional sources in both macroalgal beds, emphasizing the high equivalency of trophic structure between both beds. Isotopic signatures of a few mollusks and sea urchins showed that they use different dietary items in macroalgal-barren grounds compared with macroalgal beds, probably reflecting their feeding plasticity according to the low macroalgal biomass. However, isotopic signatures of most of the consumers at the barren ground were consistent with those at the macroalgal beds, supporting the important trophic role of drifting algae. Our results revealed the recoveries of the macroalgal community and trophic structure at the restored habitat. Further studies on colonization of early settlers and the following succession progress are needed to better understand the process and recovery rate in the developing benthic community.     

Effects of predators on sea urchin density and habitat use in a southern California kelp forest

Grazing sea urchins can reduce kelp abundance and therefore strongly affect kelp forest community structure. Despite the ecological importance of sea urchins, direct field studies on the role that urchin predators play in shaping urchin populations are rare for southern California. We conducted surveys and manipulative experiments within kelp forests near San Diego, CA, (32–51′28″N, 117–16′00″W) from 2006 to 2009 to determine whether predators such as sheephead (Semicossyphus pulcher) and spiny lobsters (Panulirus interruptus) may be linked to purple urchin (Strongylocentrotus purpuratus) and red urchin (Strongylocentrotus franciscanus) distribution and habitat use, as well as purple urchin density-dependent mortality. Purple urchins were less dense and more cryptic inside a local marine protected area (MPA) that contained high predator abundance than in nearby heavily fished areas, whereas red urchins rarely were found outside the MPA. Urchin proportional mortality was inversely density dependent during the day when sheephead were active, despite fish aggregations in plots of high urchin density, but was density independent during the night when lobsters were active. Urchin mortality was reduced under understory algal cover during the day, but not during the night. Examining whether urchin mortality from predation is density dependent and how habitat complexity influences this relationship is imperative because behavioral changes and increases in urchin populations can have vast ecological and economic consequences in kelp forest communities.     

The role of behavioral responses to predators in modifying urchins' (Strongylocentrotus droebachiensis) destructive grazing and seasonal foraging patterns

We documented spatial and temporal patterns of urchins (Strongylocentrotus droebachiensis) and periwinkles (Littorina littorea) in three habitats: a persistent Laminaria longicruris and L. digitata bed; an urchin dominated barrens, and the edge of the kelp bed that formed a boundary between the two. Urchins were rare in the kelp and, when present, always large and well hidden, a pattern we interpret as a response to crab and lobster predation. Urchins were abundant in the barrens, and, in the summer when predaceous fish were active during the day, foraged only at nigh. We observed the formation of a dense urchin feeding front along the kelp bed edge, and these urchins remained exposed and feeding even during the summer. Laboratory experiments demonstrated that aggregations are an effective defense against some predators, and that the presence of crabs increases the tendency of large urchins to aggregate. We hypothesize that healthy Laminaria spp. beds persist because kelp bed associated predators keep urchins at low densities and in hiding. A reduction in predation pressure permits urchin densities to increase to the point where they form aggregations, which provide better defense than hiding. These aggregations then graze destructively on Laminaria spp., forming barrens. These barrens seem to be a new, stable configuration of the system.     

Resource utilization for decorating in three intertidal majid crabs (Brachyura: Majidae)

 For three spider crabs (Tiarinia cornigera, Micippaplatipes and Pugettia quadridens quadridens), patterns of algal utilization for decorating were compared with the dynamics of algae on an intertidal rocky shore reef where the crabs co-occurred. T. cornigera and P. quadridens quadridens were most abundant from autumn to spring when the dominant algae (Boodleacoacta, Sargassum hemiphyllum, S. thunbergii and Corallina pilulifera) occurred in high coverage, while M. platipes was most abundant from spring to autumn. Monthly change of algae used for decorating was not correlated with algae growing in the crab habitat for T. cornigera, but for M. platipes, it was positively correlated for two algal species, and for P. quadridens quadridens, negatively correlated for one algal species. Each species of the spider crabs used some algal species preferentially for decoration. Decorating preference experiments conducted in the laboratory showed that M. platipes and P. quadridens quadridens exhibited similar preference to their algal utilization in the field, whereas for T. cornigera, algal preference in the experiment differed from utilization in the field. Comparisons between materials used for decoration and gut contents revealed that T. cornigera and M. platipes used algal species differently for decorating and feeding, while P. quadridens quadridens used the same algal species for both decorating and feeding. Different tactics for camouflage are discussed in terms of algal utilizations by the three majid species. Received: 25 September 1999 / Accepted: 22 June 2000