Small Predatory Fish Research Articles

Movement of an imperiled esocid fish in an agricultural drain

Animal movement is increasingly affected by human alterations to habitat and climate change. In wetland systems, widespread hydrologic alterations from agriculture have changed the shape, function, and stability of shallow streams and wetland habitats. These changes in habitat quality and quantity may be especially consequential for freshwater fishes such as Grass Pickerel (Esox americanus vermiculatus), a small predatory fish found in disjunct populations across southern Ontario and listed as Special Concern under Canada’s Species at Risk Act. To characterize Grass Pickerel movement response to stream-channel alterations, Fisheries and Oceans Canada implemented a tracking study to monitor the movements of a Grass Pickerel population in an agricultural drain on the Niagara Peninsula (Ontario, Canada). From 2009 to 2013, 2007 Grass Pickerel were tagged and tracked in the 37.3 km2 Beaver Creek watershed using a combination of mark-recapture surveys and eight fully automated passive integrated transponder tag antennas. Most individuals moved within 500 m (i.e., stationary fish) while 16% of the fish moved > 500 m (i.e., mobile fish), with a maximum median movement distance of 1.89 km and a maximum movement distance of 13.5 km (a long-tail distribution). Most movements occurred near the largest confluence where only a few were long-distance upstream or downstream movements. Mobile fish were larger than their stationary counterparts. Grass Pickerel in sites with higher abundance had more mobile fish, implying potential density dependence. Our results highlight that, while a long-distance dispersal ability exists in extant Grass Pickerel populations, the current conditions of riverscapes may prevent these dispersals from occurring. For declining Grass Pickerel populations, limitations to their movement ecology may substantially increase the likelihood of local extirpations.

Effects of seasonal spawning closures on pike (Esox lucius L.) and perch (Perca fluviatilis L.) catches and coastal food webs in the western Baltic Sea

Marine protected areas have become one of the main tools in the battle to curb marine biodiversity loss and habitat degradation. Yet, implementation of permanent fishery closures has often generated resource user conflicts that ultimately undermine conservation goals. Here we assessed the influence of an alternative and often more accepted measure – seasonal fish spawning closures – on large predatory fish and coastal food webs in the western Baltic Sea (Sweden). In spring 2017, we conducted a multivariable field survey in 11 seasonal closures and 11 paired references areas open to fishing. In each area, pike was sampled through angling, and perch and mesopredators through gillnet surveys. To assess trophic cascades, we measured zooplankton abundance and loss of tethered gammarids from predation. Catches per unit effort of northern pike (Esox lucius) – the main target species in recreational fisheries – were ca. 2.5 times higher per unit effort in closures than reference areas; an effect that may be caused by higher abundance and/or higher catchability of pike in the absence of fishing. Catch and weight per unit effort of the more common predator European perch (Perca fluviatilus), and the mesopredators roach (Rutilus rutilus) and three-spined stickleback (Gasterosteus aculeatus) in survey nets were, however, unaffected by closures. Moreover, a previously hypothesized trophic cascade from perch to zooplankton via three-spined stickleback was supported by the analyses, but appeared independent of closures. Yet, predation risk for tethered gammarid amphipods (a prey of stickleback and an important grazer on macroalgae) was three times higher in fished areas than in closures; a cascading closure effect that may potentially be caused by small predatory fish being less active in protected areas to avoid pike predation. Overall, our results suggest that spawning closures impact pike abundance and/or behavior and could help limit the effects of fishing, but that more research is needed to disentangle i) what mechanism(s) that underlie the protection effect on pike catches, ii) the apparently weaker closure impacts on other fish species, as well as iii) the potential for cascading effects on lower trophic levels. Therefore, new seasonal spawning closures should be implemented in addition to (and not instead of) much-needed permanent closures, which have well-known effects on the wider ecosystem.

Higher reproductive performance of a piscivorous avian predator feeding on lower trophic-level diets on ponds with shorter food chains

Variation in food-chain length may influence a predator’s trophic position. In aquatic food webs, the energy value of prey typically increases with its trophic rank; hence a higher trophic-level diet is often assumed to indicate better habitat quality. We related the body and health condition of pre-fledged Red-necked Grebes Podiceps grisegena to their dietary trophic level (estimated using stable nitrogen isotope signals of feathers) in two managed pond habitats with contrasting prey availability due to different fish population structures. Ponds stocked with young, small common carp Cyprinus carpio provided abundant fish and non-fish (insects and amphibians) resources for chicks. In ponds stocked with large carp, which also supported populations of small predatory fish, the breeding success of grebes was comparatively poor, because carp exceeded the size suitable for chicks and adversely affected non-fish prey. Pre-fledged grebes were in better condition (greater body mass, lower heterophil/lymphocyte ratio) in the food-rich small-fish ponds than in the food-poor ponds dominated by large fish. Values for δ15N suggested shorter food chains and a lower trophic-level diet for grebes in the food-rich ponds. Bayesian carbon and nitrogen isotope mixing models demonstrated the dietary prominence of small fish, both carp and predatory species. Between-habitat differences in food-chain length and grebe trophic position resulted from the higher trophic rank of small predatory fish in the food-poor ponds compared to the omnivorous carp in the food-rich ponds. Our results suggest that in aquatic food webs, feeding at higher trophic levels by strongly size-limited generalist avian predators can be associated with overall food scarcity due to the impact of fish, and thus trophic status cannot be used uncritically as a proxy for aquatic habitat quality.

An experimental test of how parasites of predators can influence trophic cascades and ecosystem functioning.

Parasites can shape the structure and function of ecosystems by influencing both the density and traits of their hosts. Such changes in ecosystems are particularly likely when the host is a predator that mediates the dynamics of trophic cascades. Here, we experimentally tested how parasite load of a small predatory fish, the threespine stickleback, can affect the occurrence and strength of trophic cascades and ecosystem functioning. In a factorial mesocosm experiment, we manipulated the density of stickleback (low vs. high), and the level of parasite load (natural vs. reduced). In addition, we used two stickleback populations from different lineages: an eastern European lineage with a more pelagic phenotype (Lake Constance) and a western European lineage with a more benthic phenotype (Lake Geneva). We found that stickleback caused trophic cascades in the pelagic but not the benthic food chain. Evidence for pelagic trophic cascades was stronger in treatments where parasite load of stickleback was reduced with an antihelmintic medication, and where fish originated from Lake Constance (i.e., the more pelagic lineage). A structural equationmodel revealed that differences in stickleback lineage and parasite load were most likely to impact trophic cascades via changes in the composition, rather than overall biomass, of zooplankton communities. Overall, our results provide experimental evidence that parasites of predators can influence the cascading effects of fish on lower trophic levels with consequences on ecosystem functioning.

Cuttlefish

In this quick guide, Darmaillacq and Osorio introduce the reader to the fascinating biology of the cuttlefish.

Habitat Selectivity and Reliance on Live Corals for Indo-Pacific Hawkfishes (Family: Cirrhitidae).

Hawkfishes (family: Cirrhitidae) are small conspicuous reef predators that commonly perch on, or shelter within, the branches of coral colonies. This study examined habitat associations of hawkfishes, and explicitly tested whether hawkfishes associate with specific types of live coral. Live coral use and habitat selectivity of hawkfishes was explored at six locations from Chagos in the central Indian Ocean extending east to Fiji in the Pacific Ocean. A total of 529 hawkfishes from seven species were recorded across all locations with 63% of individuals observed perching on, or sheltering within, live coral colonies. Five species (all except Cirrhitus pinnulatus and Cirrhitichthys oxycephalus) associated with live coral habitats. Cirrhitichthys falco selected for species of Pocillopora while Paracirrhites arcatus and P. forsteri selected for both Pocillopora and Acropora, revealing that these habitats are used disproportionately more than expected based on the local cover of these coral genera. Habitat selection was consistent across geographic locations, and species of Pocillopora were the most frequently used and most consistently selected even though this coral genus never comprised more than 6% of the total coral cover at any of the locations. Across locations, Paracirrhites arcatus and P. forsteri were the most abundant species and variation in their abundance corresponded with local patterns of live coral cover and abundance of Pocilloporid corals, respectively. These findings demonstrate the link between small predatory fishes and live coral habitats adding to the growing body of literature highlighting that live corals (especially erect branching corals) are critically important for sustaining high abundance and diversity of fishes on coral reefs.

Phytoplankton fuel the energy flow from zooplankton to small nekton in turbid mangrove waters

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 469:7-24 (2012) - DOI: https://doi.org/10.3354/meps09997 Phytoplankton fuel the energy flow from zooplankton to small nekton in turbid mangrove waters L. L. Chew1,2, V. C. Chong1,2,*, K. Tanaka3, A. Sasekumar1,2 1Institute of Biological Sciences, Faculty of Science, and 2Institute of Ocean and Earth Sciences, University of Malaya, 50603 Kuala Lumpur, Malaysia 3Japan International Research Center for Agricultural Sciences, 1-1 Ohwashi, Tsukuba, Ibaraki 350-8686, Japan *Corresponding author. Email: chong@um.edu.my ABSTRACT: Fish, zooplankton, seston, benthic microalgae and mangrove leaves were examined to investigate the trophic role of zooplankton in the food web of Matang estuaries. Despite the high turbidity and large amounts of detrital material in the water column, the study reveals that phytoplankton fuel the energy flow to zooplankton and small nekton in mangrove-fringed estuaries. The stable carbon isotope (δ13C) values and C/N ratios (7.2 to 8.2) of fine seston (<63 µm) in estuaries indicate the importance of phytoplankton (δ13C: −22.8 ± 0.6‰) to zooplankton (−23.4 to −18.2‰) nutrition, with a trophic contribution of 70 to 84%, whereas mangroves contributed <11%. In adjacent coastal waters, zooplankton (−19.2 to −15.1‰) grazed on both phytoplankton and benthic diatoms (−17.3 ± 1.24‰). Aggregated or mucilage-secreting diatoms (giving depleted δ13C values) were abundant in the estuarine seston, but did not appear to be consumed or assimilated by zooplankton. Stomach content analysis showed significant consumption of zooplankton, especially copepods (mainly Pseudodiaptomus annandalei), sergestids (Acetes spp.) and mysids by young and small nekton (<14 cm standard length) in mangrove estuaries, while δ13C values indicate the increasing importance of mangrove carbon to juvenile fish nutrition (8 to 44%). The range of δ15N values from primary producers to small predatory fish indicates 4 trophic levels (excluding true piscivores) in Matang estuaries, with zooplankton at the second and third trophic level. KEY WORDS: Stable isotopes · Stomach contents · Phytoplankton · Zooplankton · Nekton · Turbid mangrove waters Full text in pdf format PreviousNextCite this article as: Chew LL, Chong VC, Tanaka K, Sasekumar A (2012) Phytoplankton fuel the energy flow from zooplankton to small nekton in turbid mangrove waters. Mar Ecol Prog Ser 469:7-24. https://doi.org/10.3354/meps09997 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 469. Online publication date: November 26, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Trophic cascades in seagrass meadows depend on mesograzer variation in feeding rates, predation susceptibility, and abundance

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 456:29-42 (2012) - DOI: https://doi.org/10.3354/meps09678 Trophic cascades in seagrass meadows depend on mesograzer variation in feeding rates, predation susceptibility, and abundance Rebecca J. Best*, John J. Stachowicz Evolution and Ecology, University of California Davis, One Shields Avenue, Davis, California 95616, USA *Email: rjbest@ucdavis.edu ABSTRACT: Seagrasses provide important habitat for fishes and invertebrates but are declining around the globe, often due to overgrowth by algae. One hypothesis for this overgrowth is that overfishing of top consumers has led to greater numbers of small predatory fishes that reduce the abundance of mesograzers. This trophic cascade hypothesis requires that the same species that control algal biomass are also susceptible to fish predation. While mesograzers are known to vary in their feeding rates on algae and seagrasses, much less is known about variation in predation susceptibility and how this is related to grazing abilities. For 6 common mesograzers from Bodega Harbor, California, USA, we assessed feeding rates on macroalgae (Ulva spp.), epiphytic microalgae, and eelgrass. We then assessed predation susceptibility using juvenile cabezon Scorpaenichthys marmoratus in tanks of eelgrass habitat with and without Ulva. We found that the fastest consumers of all 3 primary producers were the least susceptible to predation. This appeared to be due to predator avoidance strategies; fish consumed visible caprellids at a higher rate than the larger consumers, which were either better camouflaged or able to avoid predation by building tubes within the macroalgae. Using our feeding and predation rates, along with relative abundances from field surveys, we calculated the expected trophic cascade effect with and without grazer species differences. Because fish predation was skewed towards the most abundant but least important (per capita) grazers, incorporating trait variation led to a 50 to 80% reduction in expected trophic cascade effects. Examining other seagrass communities for either similar grazer species or a similar mismatch between feeding rates and predation susceptibility may improve our understanding of the variation in trophic cascade effects across systems. KEY WORDS: Epiphytes · Overgrowth · Amphipod · Eelgrass Full text in pdf format PreviousNextCite this article as: Best RJ, Stachowicz JJ (2012) Trophic cascades in seagrass meadows depend on mesograzer variation in feeding rates, predation susceptibility, and abundance. Mar Ecol Prog Ser 456:29-42. https://doi.org/10.3354/meps09678 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 456. Online publication date: June 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

High rate of prey consumption in a small predatory fish on coral reefs

Small piscivores are regarded as important regulators of the composition of coral reef fish communities, but few studies have investigated their predatory ecology or impact on assemblages of juvenile fishes. This study investigated the foraging ecology of a common coral reef predator, the dottyback Pseudochromis fuscus, using underwater focal animal observations. Observations were conducted at two times of year: the summer, when recruit fishes were an available food item and winter, when remaining juveniles had outgrown vulnerability to P. fuscus. During the summer, P. fuscus directed 76% of its strikes at invertebrates and 24% at recruiting juvenile fishes. When striking at fishes, P. fuscus exhibited two distinct feeding modes: an ambush (26% successful) and a pursuit mode (5% successful). Predator activity in the field peaked at midday, averaging 2.5 captures h−1 of juvenile fishes. Monitoring of activity and foraging in the laboratory over 24-h periods found that P. fuscus was a diurnal predator and was active for 13 h d−1 during the summer. The number of hours during which foraging was recorded differed greatly among individuals (n = 10), ranging from 4 to 13 h. The number of predatory strikes did not increase with standard length, but the success rate and consumption rate of juvenile fishes did increase with size. Estimated hourly mortality on juvenile fish ranged from 0.49 fish h−1 in small P. fuscus individuals (30–39 mm standard length, SL; equating to 6.3 per 13 h day) to 2.4 fish h−1 in large P. fuscus individuals (55–65 mm SL; 30.6 per 13 h day). During the winter, P. fuscus struck at invertebrates with a similar rate to the summer period. These observations of the predatory ecology of P. fuscus support the hypothesis that in coral reef systems, small piscivores, because of their high metabolism and activity, are probably important regulators of coral reef fish community composition.

Trophic interactions in Zostera marina beds along the Swedish coast

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 369:63-76 (2008) - DOI: https://doi.org/10.3354/meps07646 Trophic interactions in Zostera marina beds along the Swedish coast Therese Jephson1,*, Per Nyström2, Per-Olav Moksnes3, Susanne P. Baden4 1Department of Ecology, Marine Biology, Campus Helsingborg, Lund University, Box 882, 25108 Helsingborg, Sweden 2Department of Ecology, Limnology, Ecology Building, Lund University, 22362 Lund, Sweden 3Department of Marine Ecology, University of Gothenburg, Box 461, 40530 Göteborg, Sweden 4Department of Marine Ecology, University of Gothenburg, The Sven Loven Centre for Marine Sciences—Kristineberg, 45034 Fiskebäckskil, Sweden *Email: therese.jephson@limnol.lu.se ABSTRACT: We compared eelgrass Zostera marina communities in 3 regions in Sweden believed to be affected by eutrophication and overfishing, to determine whether bottom-up or top-down processes control the biomass of epiphytic macroalgae and grazers. Nitrogen and carbon isotope signatures were analyzed to explore the food webs and to identify the grazing species feeding on filamentous algae and/or eelgrass. Mixing model (IsoSource version 1.3.1) analysis of the isotope signatures indicated that the amphipods Gammarus locusta and Microdeutopus gryllotalpa fed primarily on filamentous algae and that only 2 small gastropod species consumed eelgrass. Moreover, the grass shrimp Palaemon elegans and P. adspersus were ca. 1 trophic level above amphipods and algae, but according to the mixing model played different trophic roles in the different areas. The highest biomass of filamentous algae was found in the west coast beds housing grazers with the lowest biomass and mean size (predominantly G. locusta and M. gryllotalpa, 0.5 to 3 mm). In contrast, the Baltic Sea beds had low algal biomass, but the grazers (mostly G. locusta and Idotea baltica) had higher biomass and were significantly larger (mean size ca. 10 mm). An overall negative correlation was found between grazer biomass and biomass of filamentous algae. The significantly smaller grazers and absence of isopod grazers on the west coast may be due to substantial consumption by small predatory fish. This supports the suggestions that, in Swedish eelgrass beds, grazers are top-down controlled, and overexploitation of large predators and eutrophication play an important role in the recent increases in algal biomass. KEY WORDS: Seagrass · Eelgrass · Zostera · Stable isotopes · Mixing model · Trophic interactions Full text in pdf format PreviousNextCite this article as: Jephson T, Nyström P, Moksnes PO, Baden SP (2008) Trophic interactions in Zostera marina beds along the Swedish coast. Mar Ecol Prog Ser 369:63-76. https://doi.org/10.3354/meps07646 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 369. Online publication date: October 13, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Proximity to reef influences density of small predatory fishes, while type of seagrass influences intensity of their predation on crabs

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 340:235-243 (2007) - doi:10.3354/meps340235 Proximity to reef influences density of small predatory fishes, while type of seagrass influences intensity of their predation on crabs Mathew A. Vanderklift1,2,*, Jason How2, Thomas Wernberg2, Lachlan D. MacArthur2, Kenneth L. Heck Jr.3, John F. Valentine3 1CSIRO Marine and Atmospheric Research, Private Bag 5, Wembley, Western Australia 6913, Australia 2Centre for Ecosystem Management, Edith Cowan University, 100 Joondalup Drive, Joondalup, Western Australia 6027, Australia 3Dauphin Island Sea Lab and University of South Alabama, 101 Bienville Boulevard, Dauphin Island, Alabama 36528-0369, USA *Email: mat.vanderklift@csiro.au ABSTRACT: The magnitude of the influence that predators exert on their prey is partly determined by factors that influence the density and behaviour of predators, and partly by factors that influence the survival of prey in the presence of predators. We tested whether the presence of reefs influences the density of small predatory fishes inhabiting seagrass meadows, and the intensity of predation by these fishes on tethered crabs, by taking measurements at increasing distances from 10 rocky reefs at 2 locations separated by several hundred km. We also tested whether these variables were influenced by type of seagrass by comparing patterns yielded in meadows dominated by Amphibolis spp. and Posidonia sinuosa, 2 seagrasses with different morphological and architectural characteristics. The densities of small predatory fishes declined rapidly with increasing distance from reefs, with significantly lower densities 30 m from reefs than immediately adjacent to reefs. Densities >300 m from reefs were generally not significantly different to densities 30 m from reefs. However, the survival of tethered crabs showed no consistent trend associated with distance from reefs. Type of seagrass was not a significant influence on the density of small predatory fishes, but did influence the survival of tethered crabs: those in Amphibolis spp. meadows were 3.1× more likely to be eaten than those in P. sinuosa meadows. These patterns were consistent between locations. Our results suggest that over landscape scales (hundreds of metres) reefs are an important influence on abundance of small predatory fishes, but not on the intensity of predation on crabs. Differences in the intensity of predation between the 2 seagrass species are likely due to their dissimilar morphological and architectural characteristics. KEY WORDS: Connectivity · Food webs · Habitat linkages · Seagrass morphology · Shelter · Wrasse Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 340. Online publication date: June 18, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Impact of differential predation potential on eelgrass (Zostera marina) faunal community structure

The trophic structure of fauna within eelgrass beds (Zostera marina) was assessed at two sites in Little Egg Harbor, New Jersey, USA during the summer of 1999. Although the sites were similar with respect to both Z. marina shoot density and plant biomass, they differed significantly in the relative distribution of large predatory fish (e.g., Cynoscion regalis, Paralichthys dentatus, Morone saxatilis). Site One, Marsh Elder, was characterized by a significantly greater catch per unit effort for large predators than Site Two, Shelter Island. Gut content analysis provided direct evidence of trophic linking and significant declines between these fish and four of the five most abundant organisms collected in throw traps used to analyze the density of large benthic prey/small predators. The densities of grass shrimp (Palaemonetes spp. Hippolyte zostericola), blue crab (Callinectes sapidus), and small predatory fish (e.g., Syngnathus fuscus, Opsanus spp., Tautoga onitus) were significantly reduced at Marsh Elder, potentially as a direct impact of large predatory fish. In turn, the differences in the density of small predators observed between sites produced either a significant positive or negative effect on the distribution of small benthic prey (e.g., polychaetes, amphipods), resulting in a two-step trophic cascade within the system. Additionally, an analysis of similarities defined each site independently for both large prey/small predators and small benthic prey community structure. Although the mechanism which produced the differences in the distribution of large predatory fish remains unknown, their impact on faunal community structure mediated not only the distribution of their potential prey, but also subsequent lower trophic levels.

The effects of predator abundance and habitat structural complexity on survival of juvenile sea urchins

We studied the effect of the abundance of predatory fishes and structural complexity of algal assemblages on the survival of juveniles of the sea urchin Paracentrotus lividus on Mediterranean infralittoral rocky bottoms. Post-settlement juveniles (2–10 mm) were placed on four distinct natural substrates with increasing structural complexity (coralline barren, algal turf, erect fleshy algal assemblages and small crevices) inside and outside the Medes Islands Marine Reserve. Predation on these sea urchins increased at greater abundance of predatory fishes, and decreased with greater structural complexity. The refuge provided by structural complexity, however, decreased with increasing size of sea urchin recruits. Predation on the smallest post-settlers was carried out almost exclusively by small fishes (<20 cm), mainly the labrid Coris julis, while the dominant predator of larger juveniles was the sparid Diplodus sargus. Our results demonstrate the cascading effects caused by the prohibition of fishing in marine reserves, and highlight the potential role of small predatory fishes in the control of sea urchin populations.

DOES VARIABLE COLORATION IN JUVENILE MARINE CRABS REDUCE RISK OF VISUAL PREDATION?

We discovered that newly settled marine rock crabs, Cancer irroratus, exhibit a variety of non-adult colors early in life. This color polymorphism predominates in populations of minute juvenile crabs living in polychromatic habitats where it apparently renders them inconspicuous to visual predators such as fish. Experiments revealed lower frequencies of non-adult color morphs in monochromatic three-dimensional habitats with predators and polychromatic habitats from which predators were excluded. These patterns result from selective predation on visually contrasting color morphs. Adult crabs are monochromatic, conspicuous, and not associated with shelters. Both polymorphic newly settled and monochromatic large individuals occur in environments dominated by small predatory fish where larger adult crabs are at low risk of predation. Behavioral and visual crypsis may only be important early in life when post-settlement mortality is high, and survival at that stage determines recruitment and ultimately population densities. The well-known examples of camouflage among insects usually apply to adults who, unlike these marine counterparts, are small relative to their predators and thus remain vulnerable throughout their lives. Many other large marine crustaceans are cryptic only early in life, suggesting that this early developmental color polymorphism might be an important difference between marine and terrestrial arthropods.

Does Variable Coloration in Juvenile Marine Crabs Reduce Risk of Visual Predation?

We discovered that newly settled marine rock crabs, Cancer irroratus, exhibit a variety of non-adult colors early in life. This color polymorphism predominates in populations of minute juvenile crabs living in polychromatic habitats where it apparently renders them inconspicuous to visual predators such as fish. Experiments revealed lower frequencies of non-adult color morphs in monochromatic three-dimensional habitats with predators and polychromatic habitats from which predators were excluded. These patterns result from selective predation on visually contrasting color morphs. Adult crabs are monochromatic, conspicuous, and not associated with shelters. Both polymorphic newly settled and monochromatic large individuals occur in environments dominated by small predatory fish where larger adult crabs are at low risk of predation. Behavioral and visual crypsis may only be important early in life when post-settlement mortality is high, and survival at that stage determines recruitment and ultimately population densities. The well-known examples of camouflage among insects usually apply to adults who, unlike these marine counterparts, are small relative to their predators and thus remain vulnerable throughout their lives. Many other large marine crustaceans are cryptic only early in life, suggesting that this early developmental color polymorphism might be an important difference between marine and terrestrial arthropods.

Is there safety‐in‐numbers for prey?

The abundance of prey affects the rate of predation, but little consensus exists on whether this enhances or reduces per capita mortality. Studies of aggregating prey in marine habitats generally emphasise that the probability of predation of any individual is the reciprocal of the number of prey within a school. A field experiment tested the alternative hypotheses that predation by predatory fish on schooling prey (1) increased with an increase in the number of prey per school and that this caused (2) survival to be lower in schools with more individuals. The number of prey (juvenile Acanthochromis polyacanthus) per school was manipulated in replicate treatments with natural densities of large predatory fish (open plots) and treatments without large predatory fish (exclusion cages). Large predatory fish preyed on juveniles in a density‐dependent manner and this was the key source of density‐dependent mortality in plots open to all predators. There was some suggestion that small predatory fish also prey on juveniles in a density‐dependent manner, but this was weak and did not translate into density‐dependent mortality of juveniles. It would appear that aggregation of prey may be a successful strategy against predation from some predators, but not always every predator, or all predators in combination.

Encounter Rates of a Juvenile Reef Fish with Small and Predatory Fishes

Previous studies have shown that predation by large fishes (> 200 mm TL) is responsible for greater mortality rates of juvenile Acanthochromis polyacanthus on continuous than on patch reefs. Yet 43% of mortality on continuous reefs cannot be explained by these piscivores. I hypothesized that small predatory fishes are a source of this unaccounted mortality and tested the prediction that juvenile A. polyacanthus encounter small predatory fishes at a greater rate on continuous than on patch reefs. Of all fishes encountered by A. polyacanthus, fishes known to be potential predators of A. polyacanthus were most frequently encountered. There was a trend for A. polyacanthus to encounter these species at a greater rate on continuous than on patch reefs, but this was not supported by the analyses. This result highlights current uncertainty about sources of mortality in juvenile reef fishes. Future research is needed to establish the relative contribution of alternate sources of mortality to overall mortality so that the predictive power of hypotheses about processes that control reef fish abundance can be improved.

Host-plant specialization by a non-herbivorous amphipod: advantages for the amphipod and costs for the seaweed.

Studies of factors affecting host plant specialization by herbivores commonly highlight the value of the plant as both food and habitat, but often cannot distinguish the relative importance of these plant traits. A different approach is to study non-herbivorous animals that specialize on particular plants but do not feed on tissue from these plants. Such animals will not be affected directly by the nutritional, chemical, or morphological traits that determine the value of the plant as a food. This study reports on a filter-feeding amphipod, Ericthoniusbrasiliensis, that lives in domiciles it constructs by curling terminal segments of the green, calcified, and chemically defended seaweed Halimedatuna. We examined the temporal (1850s-1990s) and spatial (Caribbean, Mediterranean, and Pacific regions) scale of the association, the factors that may select for specialization on H. tuna, and the effect of the amphipod on growth of its host. Sampling along 125 km of coral reefs in the Florida Keys (USA) indicated that almost all populations of H. tuna had been colonized by this amphipod. Infested plants occurred on nine of ten reefs that supported H. tuna populations, with between 8 and 75% of the plants on those reefs colonized by the amphipod. For infested plants, 2-23% of all segments on each plant had been curled by the amphipod. Common co-occurring congeners of H. tuna (H. opuntia and H. goreaui) were never used for domicile construction. A survey of 1498 Halimeda specimens collected during the last 140 years and archived in the U.S. National Museum of Natural History (Smithsonian Institution, Washington, D.C.) indicated that the association has existed for >100 years and occurs throughout the Caribbean region, never in the Indo-Pacific or Mediterranean, and only on H. tuna. Predation by fishes could select for amphipod specialization on H. tuna. Laboratory experiments demonstrated that amphipods inhabiting curled segments of H. tuna were relatively immune from fish predation while those on the exterior surface of the plant or in open water were rapidly eaten. Segments of H. tuna are large enough to provide full protection from predators, while those of the co-occurring congeners H. goreaui and H. opuntia are of a size that may provide only partial protection. Experimental addition of E. brasiliensis to H. tuna plants in the field significantly decreased segment accumulation on infested relative to uninfested control plants. Whether this negative effect was a direct or indirect consequence of amphipod occupancy is unclear. Rolling plant portions into domiciles could directly decrease host growth by increasing shading and decreasing exposure of plant surface area to water column nutrient flux. Amphipod occupancy could indirectly slow net host growth if fishes selectively feed on plant sections occupied by amphipods. Underwater video showed that herbivorous fishes did not graze infested plants more than uninfested plants, but small predatory fishes did prefer feeding from infested plants. These non-herbivorous fishes may slow host growth by damaging the terminal meristematic tissues of plants during attacks on amphipods. This study demonstrates that habitat specialists can negatively impact hosts without consuming them and that specialization on a plant can occur due to its habitat value alone (as opposed to its value as a food).

Rates at which post‐larval prawns are digested by a small predatory fish and the implications for predation studies

Juvenile Monocanthus chinensis digested most of the flesh of post‐larval Penaeus monodon within 60 min, and most exoskeletal components within 180 min. Rapid cooling of predator samples should improve estimates of consumption of young prawns.

Rates at which post-larval prawns are digested by a small predatory fish and the implications for predation studies

Juvenile Monocanthus chinensis digested most of the flesh of post-larval Penaeus monodon within 60 min, and most exoskeletal components within 180 min. Rapid cooling of predator samples should improve estimates of consumption of young prawns.