Facultative mutualism between an herbivorous crab and a coralline alga: advantages of eating noxious seaweeds

Biology, Ecology and Management of Aquatic Macrophytes and Algae (Volume I)

Aquatic macrophytes and algae constitute essential components of aquatic ecosystems, fulfilling diverse and critical roles in sustaining ecological integrity and equilibrium [...].

The Specialist Marine Herbivore Elysia papillosa Grows Faster on a Less Utilized Algal Diet

AbstractMany small specialist herbivores utilize their food resources both for nutrition and as a structural refuge or resource. Trophic linkage cannot solely be inferred from physical association of herbivores with a potential food item, because herbivores may temporarily inhabit algae or plants on which they do not feed. Elysia papillosa, a small sacoglossan sea slug, consumes and sequesters chloroplasts from the siphonaceous, chlorophytic alga Penicillus capitatus; it also maintains moderate densities on this alga. Recently, E. papillosa was also infrequently found in association with the alga Penicillus lamourouxii, which displays density similar to that of P. capitatus. After collecting E. papillosa from each of the two algal species from a shallow-water site along the west central coast of Florida, we used DNA barcoding of the rbcL gene sequences in order to determine whether the slug was consuming both algal species. The molecular data indicated that E. papillosa consumed and sequestered chloroplasts from the same algal species from which they were collected. A laboratory feeding experiment tested whether algal diet (P. capitatus or P. lamourouxii) had an impact on slug growth rate as measured by change in body size (mm). After 3 weeks E. papillosa fed P. lamourouxii achieved a mean body length that was 1.5-2 times that recorded for slugs fed P. capitatus, but maximum growth depended on the original field host. Thus, while the highest densities of E. papillosa in the field occurred on P. capitatus, slugs grew much faster on P. lamourouxii in the laboratory. The observed association of E. papillosa with P. capitatus must be related to other factors, such as foraging efficiency, algal morphology, algal biochemistry, or algal suitability as a refuge.

Mediterranean Red Macro Algae Mats as Habitat for High Abundances of Serpulid Polychaetes

The Mediterranean Sea harbors more than 17,000 eukaryotic marine species, with several ecosystems recognized as biodiversity hotspots, such as Posidonia oceanica meadows. Recent research indicates that benthic mats formed by the fleshy red alga Phyllophora crispa are also associated with high species richness. Among key groups found in these mats are sessile polychaetes, which live as epiphytes on the red algae thalli. Knowledge of abundance, species richness, and spatial variation of polychaetes associated with these habitats is still scarce. We carried out a comparative assessment focusing on serpulid polychaetes within samples from P. crispa mats and neighboring P. oceanica meadows at six different sampling sites around Giglio Island (Tyrrhenian Sea, Italy). A total of 17 serpulid taxa were identified. The abundance of serpulids (5665 individuals m−2 of P. crispa mat) were similar to neighboring P. oceanica meadows (2304 individuals m−2 leaves and 5890 individuals m−2 shoots). The number of serpulid taxa was significantly higher in P. crispa mats (average 6.63 ± 1.32 taxa) compared to P. oceanica beds (average 1.56 ± 0.63 and 1.84 ± 1.04 taxa in leaves and shoots, respectively). Within habitat type, there were no significant differences in species richness between sites. The most abundant species found was Josephella marenzelleri (61% of individuals), while Vermiliopsis spp. and Bathyvermilia sp. were exclusively found in P. crispa samples. Our results highlight that P. crispa mats host an exceptional diversity and that these habitats should be included in conservation strategies. Further research should focus on the significance of other important taxonomic groups within these mats and evaluate the distribution of P. crispa in different regions of the Mediterranean Sea.

Environmental drivers of rhodolith beds and epiphytes community along the South Western Atlantic coast

Environmental conditions shape the occurrence and abundance of habitat-building organisms at global scales. Rhodolith beds structure important hard substrate habitats for a large number of marine benthic organisms. These organisms can benefit local biodiversity levels, but also compete with rhodoliths for essential resources. Therefore, understanding the factors shaping the distribution of rhodoliths and their associated communities along entire distributional ranges is of much relevance for conservational biology, particularly in the scope of future environmental changes. Here we predict suitable habitat areas and identify the main environmental drivers of rhodoliths' variability and of associated epiphytes along a large-scale latitudinal gradient. Occurrence and abundance data were collected throughout the South-western Atlantic coast (SWA) and modelled against high resolution environmental predictors extracted from Bio-Oracle. The main drivers for rhodolith occurrence were light availability and temperature at the bottom of the ocean, while abundance was explained by nitrate, temperature and current velocity. Tropical regions showed the highest abundance of rhodoliths. No latitudinal pattern was detected in the variability of epiphytes abundance. However, significant differences were found between sampled sites regarding the composition of predominant taxa. The predictors influencing such differences were temperature and nitrate. The Tropical region is abundant in species with warm-water affinities, decreasing toward warm temperate region. The expressive occurrence of tropical species not referred before for warm temperate beds indicate a plausible tropicalization event.

Patchy spread patterns in three-species bistable systems with facultative mutualism

A three-species population system under a facultative mutualistic relationship of one of the species is studied. The considered interactions are as follows: facultative between the first species and the second species, obligatory mutualism between the second species and the first one, and the third species is a predator of the first species. For this purpose, we extend the model proposed by Morozov et al., originally used to describe obligatory mutualism, to consider obligatory and facultative mutualism and prove that under adequately selected parameters this system produces a spatial patchy spread of populations or continuous wave fronts. Since the analytical treatment of a three-species model is often prohibitive, we first analyze the interaction between two mutualist species without diffusion and without the presence of the predator. Some parameters are fixed in the bistable regime of the mutualistic species to further consider the influence of the third species. The remaining parameters are then selected to produce patchy patterns under different mortality rates. Finally, the equations of the final three-species system are numerically solved to test the influence of different initial conditions in the formation of patchy populations. It is confirmed that the velocity and the profile of a traveling front are independent on the initial conditions. Our approach opens the way to study more general biological situations.

Diversity and abundance of conspicuous macrocrustaceans on coral reefs differing in level of degradation

Coral reefs sustain abundant and diverse macrocrustaceans that perform multiple ecological roles, but coral reefs are undergoing massive degradation that may be driving changes in the species composition and abundance of reef-associated macrocrustaceans. To provide insight into this issue, we used non-destructive visual census techniques to compare the diversity and abundance of conspicuous macrocrustaceans (i.e., those >1 cm and visible without disturbance) between two shallow Caribbean coral reefs similar in size (∼1.5 km in length) and close to each other, but one (“Limones”) characterized by extensive stands of the branching coral Acropora palmata, and the other (“Bonanza”) dominated by macroalgae and relic coral skeletons and rubble (i.e., degraded). We also assessed the structural complexity of each reef and the percent cover of various benthic community components. Given the type of growth of A. palmata, we expected to find a greater structural complexity, a higher cover of live coral, and a lower cover of macroalgae on Limones, and hence a more diverse and abundant macrocrustacean community on this reef compared with Bonanza. Overall, we identified 63 macrocrustacean species (61 Decapoda and two Stomatopoda). Contrary to our expectations, structural complexity did not differ significantly between the back-reef zones of these reefs but varied more broadly on Limones, and the diversity and abundance of macrocrustaceans were higher on Bonanza than on Limones despite live coral cover being higher on Limones and macroalgal cover higher on Bonanza. However, the use of various types of microhabitats by macrocrustaceans differed substantially between reefs. On both reefs, the dominant species were the clinging crab Mithraculus coryphe and the hermit crab Calcinus tibicen, but the former was more abundant on Bonanza and the latter on Limones. M. coryphe occupied a diverse array of microhabitats but mostly coral rubble and relic skeletons, whereas C. tibicen was often, but not always, found associated with colonies of Millepora spp. A small commensal crab of A. palmata, Domecia acanthophora, was far more abundant on Limones, emerging as the main discriminant species between reefs. Our results suggest that local diversity and abundance of reef-associated macrocrustaceans are partially modulated by habitat degradation, the diversity of microhabitat types, and the establishment of different commensal associations rather than by structural complexity alone.

The influence of species, density, and diversity of macroalgal aggregations on microphytobenthic settlement

Intertidal macroalgae can modulate their biophysical environment by ameliorating physical conditions and creating habitats. Exploring how seaweed aggregations made up of different species at different densities modify the local environment may help explain how associated organisms respond to the attenuation of extreme physical conditions. Using Silvetia compressa, Chondracanthus canaliculatus, and Pyropia perforata, we constructed monocultures representing the leathery, corticated and foliose functional forms as well as a mixed tri-culture assemblage including the former three, at four densities. Treatment quadrats were installed in the intertidal where we measured irradiance, temperature, particle retention, and water motion underneath the canopies. Additionally, we examined the abundance and richness of the understory microphytobenthos with settlement slides. We found that the density and species composition of the assemblages modulated the amelioration of extreme physical conditions, with macroalgal aggregations of greater structural complexity due to their form and density showing greater physical factor attenuation. However, increasing the number of species within a patch did not directly result in increased complexity and therefore, did not necessarily cause greater amelioration of the environment. Microphytobenthic composition was also affected by species composition and density, with higher abundances under S. compressa and C. canaliculatus canopies at high and mid densities. These results support the idea that the environmental modifications driven by these macroalgae have a significant effect on the dynamics of the intertidal environment by promoting distinct temporal and spatial patchiness in the microphytobenthos, with potentially significant effects on the overall productivity of these ecosystems.

Declines in plant palatability from polar to tropical latitudes depend on herbivore and plant identity

Long-standing theory predicts that the intensity of consumer-prey interactions declines with increasing latitude, yet for plant-herbivore interactions, latitudinal changes in herbivory rates and plant palatability have received variable support. The topic is of growing interest given that lower-latitude species are moving poleward at an accelerating rate due to climate change, and predicting local interactions will depend partly on whether latitudinal gradients occur in these critical biotic interactions. Here, we assayed the palatability of 50 seaweeds collected from polar (Antarctica), temperate (northeastern Pacific; California), and tropical (central Pacific; Fiji) locations to two herbivores native to the tropical and subtropical Atlantic, the generalist crab Mithraculus sculptus and sea urchin Echinometra lucunter. Red seaweeds (Rhodophyta) of polar and temperate origin were more readily consumed by urchins than were tropical reds. The decline in palatability with decreasing latitude is explained by shifts in tissue organic content along with the quantity and quality of secondary metabolites, degree of calcification or both. We detected no latitudinal shift in palatability of red seaweeds to crabs, nor any latitudinal shifts in palatability of brown seaweeds (Phaeophyta) to either crabs or urchins. Our results suggest that evolutionary pressure from tropical herbivores favored red seaweeds with lower palatability, either through the production of greater levels of chemical defenses, calcification, or both. Moreover, our results tentatively suggest that the "tropicalization" of temperate habitats is facilitated by the migration of tropical herbivores into temperate areas dominated by weakly defended and more nutritious foods, and that the removal of these competing seaweeds may facilitate the invasion of better-defended tropical seaweeds.

Ontogenetic variability in the feeding behavior of a marine amphipod in response to ocean acidification

Global stressors like ocean acidification (OA) are expected to influence the quality or palatability of primary producers like algae. Such changes can trigger a response on algal consumers' feeding strategies, and this response may not necessarily be the same for the consumers during the ontogeny. We used a mesocosm's system to expose algae to current and projected OA conditions (390 and 1000ppm, respectively) and then compared the feeding behavior and absorption efficiency of juvenile and adult stages of the amphipod Orchestoidea tuberculata. Specifically, we measured consumption rates (with and without a choice) and absorption efficiency on algae exposed and not exposed to OA. Our results show that OA affect the amphipod's consumption and feeding preferences, and that these effects were related with the analyzed ontogenetic stage (juveniles versus adults). These results support the existence of an ontogenetic change in the response of this species and others similar marine invertebrates to OA, which highlight the need to incorporate different life stages in the study of OA or others global stressors.

Plant defences on land and in water: why are they so different?

BACKGROUND

Plants (attached photosynthesizing organisms) are eaten by a wide variety of herbivorous animals. Despite a vast literature on plant defence, contrasting patterns of antiherbivore adaptation among marine, freshwater and land plants have been little noticed, documented or understood.

SCOPE

Here I show how the surrounding medium (water or air) affects not only the plants themselves, but also the sensory and locomotor capacities of herbivores and their predators, and I discuss patterns of defence and host specialization of plants and herbivores on land and in water. I analysed the literature on herbivory with special reference to mechanical defences and sensory cues emitted by plants. Spines, hairs, asymmetrically oriented features on plant surfaces, and visual and olfactory signals that confuse or repel herbivores are common in land plants but rare or absent in water-dwelling plants. Small terrestrial herbivores are more often host-specific than their aquatic counterparts. I propose that patterns of selection on terrestrial herbivores and plants differ from those on aquatic species. Land plants must often attract animal dispersers and pollinators that, like their herbivorous counterparts, require sophisticated locomotor and sensory abilities. Plants counter their attractiveness to animal helpers by evolving effective contact defences and long-distance cues that mislead or warn herbivores. The locomotor and sensory world of small aquatic herbivores is more limited. These characteristics result from the lower viscosity and density of air compared with water as well as from limitations on plant physiology and signal transmission in water. Evolutionary innovations have not eliminated the contrasts in the conditions of life between water and land.

CONCLUSION

Plant defence can be understood fully when herbivores and their victims are considered in the broader context of other interactions among coexisting species and of the medium in which these interactions occur.

Brachyuran and anomuran crabs associated with Schizoporella unicornis (Ectoprocta, Cheilostomata) from southeastern Brazil

The main goals of this investigation were to describe the community structure of anomuran and brachyuran crabs inhabiting reefs constituted by colonies of Schizoporella unicornis, and to provide a species importance ranking for this community. Collections were carried out on S. unicornis reefs at two-month intervals from May 2003 to May 2004, in the rocky sublittoral of the southeastern Brazilian coast. Relative abundance and occurrence were used to rank these species in the hierarchy importance. A total of 2,018 individuals were obtained, in 11 families, 22 genera and 31 species. Porcellanidae and Pilumnidae were the most abundant families, comprising respectively almost 60% and 15% of individuals sampled. The species ranking indicated four main groups A, B, C and D, with group A subdivided. Subgroup A1 contained 9 species, including the species of greatest ecological importance for community regarding abundance and occurrence. The great abundance of crabs associated with S. unicornis seems to be the result of its recognized importance during the crab developmental cycle, and as shelter and food for some Decapod species. These observations reveal the importance of conserving the areas occupied by these reef colonies, which appear to be an important environment for maintaining local biodiversity.

Activated chemical defenses suppress herbivory on freshwater red algae

The rapid life cycles of freshwater algae are hypothesized to suppress selection for chemical defenses against herbivores, but this notion remains untested. Investigations of chemical defenses are rare for freshwater macrophytes and absent for freshwater red algae. We used crayfish to assess the palatability of five freshwater red algae relative to a palatable green alga and a chemically defended aquatic moss. We then assessed the roles of structural, nutritional, and chemical traits in reducing palatability. Both native and non-native crayfish preferred the green alga Cladophora glomerata to four of the five red algae. Batrachospermum helminthosum, Kumanoa holtonii, and Tuomeya americana employed activated chemical defenses that suppressed feeding by 30-60 % following damage to algal tissues. Paralemanea annulata was defended by its cartilaginous structure, while Boldia erythrosiphon was palatable. Activated defenses are thought to reduce ecological costs by expressing potent defenses only when actually needed; thus, activation might be favored in freshwater red algae whose short-lived gametophytes must grow and reproduce rapidly over a brief growing season. The frequency of activated chemical defenses found here (three of five species) is 3-20× higher than for surveys of marine algae or aquatic vascular plants. If typical for freshwater red algae, this suggests that (1) their chemical defenses may go undetected if chemical activation is not considered and (2) herbivory has been an important selective force in the evolution of freshwater Rhodophyta. Investigations of defenses in freshwater rhodophytes contribute to among-system comparisons and provide insights into the generality of plant-herbivore interactions and their evolution.

The fundamental role of competition in the ecology and evolution of mutualisms

Mutualisms are interspecific interactions that yield reciprocal benefits. Here, by adopting a consumer-resource perspective, we show how considering competition is necessary in order to understand the evolutionary and ecological dynamics of mutualism. We first review the ways in which competition shapes the ecology of mutualisms, using a graphical framework based on resource flows rather than net effects to highlight the opportunities for competition. We then describe the known mechanisms of competition and show how it is a critical driver of the evolutionary dynamics, persistence, and diversification of mutualism. We argue that empirical and theoretical research on the ecology and evolution of mutualisms will jointly progress by addressing four key points: (i) the existence and shape of physiological trade-offs among cooperation, competition, and other life-history and functional traits; (ii) the capacity for individuals to express conditional responses to variation in their mutualistic and competitive environment; (iii) the existence of heritable variation for mutualistic and competitive traits and their potentially conditional expression; and (iv) the structure of the network of consumer-resource interactions in which individuals are embedded.

Herbivore preference for native vs. exotic plants: generalist herbivores from multiple continents prefer exotic plants that are evolutionarily naïve

Enemy release and biotic resistance are competing, but not mutually exclusive, hypotheses addressing the success or failure of non-native plants entering a new region. Enemy release predicts that exotic plants become invasive by escaping their co-adapted herbivores and by being unrecognized or unpalatable to native herbivores that have not been selected to consume them. In contrast, biotic resistance predicts that native generalist herbivores will suppress exotic plants that will not have been selected to deter these herbivores. We tested these hypotheses using five generalist herbivores from North or South America and nine confamilial pairs of native and exotic aquatic plants. Four of five herbivores showed 2.4-17.3 fold preferences for exotic over native plants. Three species of South American apple snails (Pomacea sp.) preferred North American over South American macrophytes, while a North American crayfish Procambarus spiculifer preferred South American, Asian, and Australian macrophytes over North American relatives. Apple snails have their center of diversity in South America, but a single species (Pomacea paludosa) occurs in North America. This species, with a South American lineage but a North American distribution, did not differentiate between South American and North American plants. Its preferences correlated with preferences of its South American relatives rather than with preferences of the North American crayfish, consistent with evolutionary inertia due to its South American lineage. Tests of plant traits indicated that the crayfish responded primarily to plant structure, the apple snails primarily to plant chemistry, and that plant protein concentration played no detectable role. Generalist herbivores preferred non-native plants, suggesting that intact guilds of native, generalist herbivores may provide biotic resistance to plant invasions. Past invasions may have been facilitated by removal of native herbivores, introduction of non-native herbivores (which commonly prefer native plants), or both.

Induced chemical defenses in a freshwater macrophyte suppress herbivore fitness and the growth of associated microbes

The freshwater macrophyte Cabomba caroliniana induces a chemical defense when attacked by either the crayfish Procambrus clarkii or the snail Pomacea canaliculata. Induction by either consumer lowers the palatability of the plant to both consumers. When offered food ad libitum, snails feeding on non-induced C. caroliniana grew 2.6-2.7 times more than those feeding on induced C. caroliniana. Because snails fed less on induced plants, this could be a behavioral effect (reduced feeding), a physiological effect of the induced metabolites on the consumer, or both. To assess these possibilities, we made artificial diets with lipid extracts of induced versus non-induced C. caroliniana and restricted control snails to consuming only as much as treatment snails consumed. Growth measured as shell diameter was significantly lower on the diet containing extract from induced, as opposed to non-induced, plants; change in snail mass was more variable and showed a similar, but non-significant, trend. Thus, snails may reduce feeding on induced plants to avoid suppression of fitness. The induced defenses also suppressed growth of co-occurring microbes that might attack the plant through herbivore-generated feeding scars. When two bacteria and three fungi isolated from C. caroliniana surfaces were cultured with the lipid extract from induced and non-induced C. caroliniana, both extracts inhibited the microbes, but the induced extract was more potent against three of the five potential pathogens. Thus, induced plant defenses can act against both direct consumers and microbes that might invade the plant indirectly through herbivore-generated wounds.

Stable isotope analysis of production and trophic relationships in a tropical marine hard-bottom community

Seagrasses produce much of the organic carbon in the shallow waters of the Caribbean and it has long been assumed that a substantial portion of this carbon is exported to nearby habitats, contributing substantially to their food webs. In the shallow coastal waters of the Florida Keys (USA), seagrass intersperses with hard-bottom habitat where bushy, red macroalgae are the most prominent primary producers. However, the relative importance of seagrass-derived carbon versus autochthonous algal production or phytoplankton in supporting higher trophic levels within hard-bottom communities has never been investigated. We compared the carbon and nitrogen isotopic values of potential primary producers and representative higher trophic level taxa from hard-bottom sites on the bay-side and ocean-side of the Florida Keys. We also included in our study a set of bay-side sites that experienced significant ecological disturbances over the past decade (e.g., cyanobacteria blooms, seagrass die-off, and sponge die-offs) that may have altered trophic relationships in those regions. We did not detect any differences among regions in the trophic status of hard-bottom taxa that might be associated with ecosystem disturbance. However, our results suggest that autochthonous production of algal detritus is an important source of secondary production in these hard-bottom communities, with seagrass and phytoplankton contributing smaller fractions.

Algal growth and species composition under experimental control of herbivory, phosphorus and coral abundance in Glovers Reef, Belize

The proliferation of algae on disturbed coral reefs has often been attributed to (1) a loss of large-bodied herbivorous fishes, (2) increases in sea water nutrient concentrations, particularly phosphorus, and (3) a loss of hard coral cover or a combination of these and other factors. We performed replicated small-scale caging experiments in the offshore lagoon of Glovers Reef atoll, Belize where three treatments had closed-top (no large-bodied herbivores) and one treatment had open-top cages (grazing by large-bodied herbivores). Closed-top treatments simulated a reduced-herbivory situation, excluding large fishes but including small herbivorous fishes such as damselfishes and small parrotfishes. Treatments in the closed-top cages included the addition of high phosphorus fertilizer, live branches of Acropora cervicornis and a third unmanipulated control treatment. Colonization, algal biomass and species composition on dead A. palmata "plates" were studied weekly for 50 days in each of the four treatments. Fertilization doubled the concentration of phosphorus from 0.35 to 0.77 microM. Closed-top cages, particularly the fertilizer and A. cervicornis additions, attracted more small-bodied parrotfish and damselfish than the open-top cages such that there was moderate levels of herbivory in closed-top cages. The open-top cages did, however, have a higher abundance of the chemically and morphologically defended erect algal species including Caulerpa cupressoides, Laurencia obtusa, Dictyota menstrualis and Lobophora variegata. The most herbivore-resistant calcareous green algae (i.e. Halimeda) were, however, uncommon in all treatments. Algal biomass increased and fluctuated simultaneously in all treatments over time, but algal biomass, as measured by wet, dry and decalcified weight, did not differ greatly between the treatments with only marginally higher biomass (p < 0.06) in the fertilized compared to open-top cages. Algal species composition was influenced by all treatments with a maximum between-treatment Bray-Curtis similarity of only 29%. The fertilized cages showed rapid colonization by a mixed turf community largely composed of the filamentous brown (Hincksia mitchelliae) and green (Enteromorpha prolifera) species. Algal cover in the fertilized cages leveled at 80% after 20 days compared to less than 50% in the other treatments. There was no evidence that A. cervicornis suppressed algal colonization compared to the unmanipulated controls. Instead, the herbivore susceptible Padina sanctae-crucis was the most abundant algae followed by Jania capillacea in this treatment in contrast to the more chemically defended Dictyota menstrualis that dominated the unmanipulated controls. We conclude that A. cervicornis was not suppressing algae as a group and its loss cannot account for the observed changes in algal abundance in most reefs except for creating space. In contrast, A. cervicornis appears to attract aggressive damselfish that may reduce herbivory by larger herbivores. Phosphorus enrichment can lead to rapid colonization of space by filamentous turf communities but not high biomass and dominance of erect frondose algae within 50 days. Moderate levels of herbivory by large-bodied herbivores promoted moderately herbivore-resistant erect brown and green algae that are commonly reported on disturbed reefs. Consequently, all the studied factors influenced algal communities but seldom as commonly predicted.

Induced indirect defence in a lycaenid-ant association: the regulation of a resource in a mutualism

Indirect defences involve the protection of a host organism by a mutualistic partner. Threat of predation to the host organism may induce the production of rewards and/or signals that attract the mutualistic partner. In laboratory and field experiments we show that threatened lycaenid butterfly larvae (Plebejus acmon) produce more nectar rewards from their gland and were tended by protective ants twice as much as controls. Ant attendance did not affect the leaf consumption or feeding behaviour of larvae in the absence of predators. Inducible nectar production and indirect defence in this system may be a mechanism by which larvae provide rewards for services when they are needed the most. Such a system may stabilize the mutualistic association between lycaenid larvae and ants by preventing exploitation by either partner.

Geographic Variation in Camouflage Specialization by a Decorator Crab

In North Carolina, the decorator crab Libinia dubia camouflages almost exclusively with the chemically noxious alga Dictyota menstrualis. By placing this alga on its carapace, the crab behaviorally sequesters the defensive chemicals of the plant and gains protection from omnivorous consumers. However, Dictyota is absent north of North Carolina, whereas Libinia occurs as far north as New England. Crabs from three northern locations where Dictyota is absent (Rhode Island, Connecticut, and New Jersey) camouflaged to match their environment, rather than selectively accumulating any one species. When D. menstrualis was offered to crabs from northern sites, they did not distinguish between it and other seaweeds for camouflage, whereas crabs from Alabama and two locations in North Carolina used D. menstrualis almost exclusively. In addition, in winter and spring, when Dictyota was seasonally absent in North Carolina, Libinia selectively camouflaged with the sun sponge Hymeniacidon heliophila, which was chemically unpalatable to local fishes. Thus, southern crabs were consistent specialists on chemically defended species for camouflage, while northern crabs were more generalized. The geographic shift in crab behavior away from specialization coincides with a reported decrease in both total predation pressure and the frequency of omnivorous consumers. These shifts in the nature and intensity of predation pressure may favor different camouflage strategies (generalist vs. specialist), contributing to the observed geographic differences in camouflage behavior.

Mass Spawning by Green Algae on Coral Reefs

Predawn episodes of mass spawning by green algae (up to nine species in five genera on a single morning) intermittently cloud Caribbean waters. Species- and sex-specific bouts of anisogamous gamete release occurred synchronously and predictably on a given morning, with closely related species spawning at different times. Algal sexual reproduction was seasonal, but, unlike the mass-spawning behavior of other sessile marine organisms, showed no lunar or tidal cycling. The discovery of mass-spawning behavior by these algae has important implications for future studies of the reproductive ecology and speciation of a vital, yet poorly understood, component of the coral reef community.