Differential ingestion of zooplankton by four species of bivalves (Mollusca) in the Mali Ston Bay,Croatia

This study provides information about differences in composition of ingested zooplankton amongst bivalve species coexisting in the same area in a period from May 2009 to December 2010. The study was conducted at the Mali Ston Bay (42°51′ N, 17°40′ E)—the most important bivalve aquaculture area in the eastern Adriatic Sea. Stomach content analysis was performed on cultured species—Ostrea edulis and Mytilus galloprovincialis, and commercially important bivalve species from their natural environment—Modiolus barbatus and Arca noae. Results confirmed carnivory in bivalves, both from natural and cultured populations, but cultured species had higher numbers of zooplankters than those living on the seabed. The most abundant taxa were bivalve larvae, followed by tintinnids, copepods, unidentified eggs and gastropod larvae. Recorded numbers of bivalve larvae in M. galloprovincialis stomach were the highest so far reported and show that mussels impact the availability of natural spat.     

<Emphasis Type="Italic">Cochlodinium polykrikoides</Emphasis> blooms and clonal isolates from the northwest Atlantic coast cause rapid mortality in larvae of multiple bivalve species

Globally, many commercial bivalve populations have declined in recent decades. In addition to overharvesting and habitat loss, the increasing frequency and intensity of harmful algal blooms (HABs) are likely to contribute to bivalve losses, particularly in cases where blooms negatively impact larval stages. This paper reports on the lethal effects of clonal cultures and blooms of Cochlodinium polykrikoides from the US Atlantic coast on the larvae of three species of commercially and ecologically valuable bivalves: the Eastern oyster (Crassostrea virginica), the bay scallop (Argopecten irradians), and the Northern quahog (hard clam; Mercenaria mercenaria). Both cultures and blooms of C. polykrikoides were highly toxic to all three species of bivalve larvae causing 80–100% mortality during 24- to 72-h exposures at concentrations of 1–2 × 10³ cells ml⁻¹. Toxicity was dependent on cell densities, growth stage of C. polykrikoides (i.e. cultures in exponential stage growth were more toxic than later stages), exposure time of larvae to cells (i.e. longer exposure caused higher mortality), the age of larvae (i.e. younger larvae were more sensitive), and the relative abundance of C. polykrikoides (i.e. the presence of other microalgae decreased toxicity). Free radical-scavenging enzymes (peroxidase and catalase) and the removal of C. polykrikoides cells (i.e. culture filtrate) significantly increased larval survival suggesting toxicity is maximized by contact with live cells and may involve labile toxins bound by these compounds including e.g. reactive oxygen species. The toxicity of C. polykrikoides to bivalve larvae was generally more severe than other HAB species (e.g. Karenia brevis, Karlodinium veneficum, Alexandrium tamarense, Prorocentrum minimum). Since the bivalves in this study spawn in the months when C. polykrikoides blooms on the east coast of North America, these results suggest that these blooms may have detrimental effects on efforts to restore these already diminished populations.     

Assimilation efficiencies and turnover rates of trace elements in marine bivalves: a comparison of oysters, clams and mussels

Assimilation efficiencies (AEs) and physiological turnover-rate constants (k) of six trace elements (Ag, Am, Cd, Co, Se, Zn) in four marine bivalves (Crassostrea virginica Gmelin, Macoma balthica Linnaeus, Mercenaria mercenaria Linnaeus, and Mytilus edulis Linnaeus) were measured in radiotracer-depuration experiments. Egestion rates of unassimilated elements were highest during the first 24 h of depuration and declined thereafter. Significant egestion of unassimilated Co, however, continued for up to 5 d in Macoma balthica, Mercenaria mercenaria and Mytilus edulis. With the exception of the extremely low values for ^110 mAg, ¹⁰⁹Cd, and ⁶⁵Zn in C. virginica, physiological turnover-rate constants (k) showed no general pattern of variation among elements, bivalve species or food types, and were relatively invariant. Values from  ≤0.001 to 0.1 d⁻¹ were observed, but excluding those for Co, most values were  ≤0.04 d⁻¹. In all four species, the AEs of Ag, Am, and Co were generally lower than those of Cd, Se, and Zn. The AEs of Ag, Cd, Se, and Zn in these bivalves are directly related to the proportion of each element in the cytoplasmic fraction of ingested phytoplankton, indicating that >80% of elements in a prey alga's cytoplasm was assimilated. C. virginica, Macoma balthica, and Mercenaria mercenaria assimilated ∼36% of the Ag and Cd associated with the non-cytoplasmic (membrane/organelle) fraction of ingested cells in addition to the cytoplasmic fraction. The ratio of AE:k, which is proportional to the consumer–prey trace-element bioaccumulation factor (concentration in consumer:concentration in prey) was generally greater for Cd, Se, and Zn than for Ag, Am, and Co. This ratio was lowest in Mytilus edulis, suggesting that this bivalve, the most widely employed organism in global biomonitoring, is relatively inefficient at accumulating important elements such as Ag, Cd, and Zn from ingested phytoplankton. Received: 7 February 1997 / Accepted: 24 February 1997     

Preservation methods alter stable isotope values in gelatinous zooplankton: implications for interpreting trophic ecology

Jellyfish are increasingly topical within studies of marine food webs. Stable isotope analysis represents a valuable technique to unravel the complex trophic role of these long-overlooked species. In other taxa, sample preservation has been shown to alter the isotopic values of species under consideration, potentially leading to misinterpretation of trophic ecology. To identify potential preservation effects in jellyfish, we collected Aurelia aurita from Strangford Lough (54^o22′44.73″N, 5^o32′53.44″W) during May 2009 and processed them using three different methods prior to isotopic analysis (unpreserved, frozen and preserved in ethanol). A distinct preservation effect was found on δ¹⁵N values: furthermore, preservation also influenced the positive allometric relationship between individual size and δ¹⁵N values. Conversely, δ¹³C values remained consistent between the three preservation methods, conflicting with previous findings for other invertebrate, fish and mammalian species. These findings have implications for incorporation of jellyfish into marine food webs and remote sampling regimes where preservation of samples is unavoidable.     

Divergent foraging strategies of three co-occurring north Pacific flatfishes

Despite facing similar constraints imposed by the environment, significant variation in life history traits frequently exists among species generally considered to comprise a single ecological guild. For juvenile flatfishes, constraints on foraging activity include variation in light and prey availability, as well as predation risk. This paper describes the visual constraints on, and divergent foraging strategies of three co-occurring north Pacific flatfish species, northern rock sole (Lepidopsetta polyxystra), Pacific halibut (Hippoglossus stenolepis), and English sole (Pleuronectes vetulus). Visual foraging abilities measured in the laboratory decreased rapidly below 10⁻⁴ μmol photons·m⁻² s⁻¹, and were similar among species. Despite similar sensory constraints, field sampling in August 2004 at a Kodiak Island nursery site (Holiday Beach, 57^o41.2′ N, 152^o27.7′ W) identified species differences in diets, diel foraging patterns, and within-nursery depth distributions. Northern rock sole and English sole fed primarily on bivalve siphons and polychaetes, whereas mysids dominated the diets of Pacific halibut. Northern rock sole were geographically the most widespread but feeding activity was temporally restricted to the dusk period. Pacific halibut were rare in shallow depths (<5 m) and fed most intensively prior to dusk. English sole fed throughout the daylight hours and were abundant only in the shallowest (<5 m) habitats. These differences in diets, foraging times, and habitat use appear related to previously documented species-specific behavioral characteristics as well as general spatial (increasing with depth) and temporal (increasing during foraging activity) variations in predation risk. At one extreme, the conservative behavioral strategy of northern rock sole may permit use of a broader range of foraging habitats, whereas English sole may be restricted to shallow water by limited behavioral responses to predation threat. These observations demonstrate that the appearance of habitat partitioning is not due to differences in sensory ability, but reflects multi-faceted, species-specific responses to the ecological tradeoffs between foraging and predation risks.     

Recovery after trampling disturbance in a canopy-forming seaweed population

The recovery of Ascophyllum nodosum, and the associated benthic community, was followed after repeated trampling disturbance at different intensities, from 2005 to 2010, at its southern limit (41°41′27″N; 8°50′57″W). The cover of A. nodosum and associated organisms and the demography of A. nodosum individuals in the most disturbed areas were followed over a recovery period of 54 months. Recovery of A. nodosum assemblages was negatively affected by increasing trampling intensity. After 54 months, the highest intensity plots were still significantly different from control plots. Growth and mortality rates of A. nodosum increased shortly after disturbance but progressively recovered to levels close to control plots. The results of this study demonstrate the sensitivity of this habitat-building species to human trampling and also show its plasticity to cope with disturbance events.     

Energy relations of the bivalve Mercenaria mercenaria on an intertidal mudflat

Measurements of the filtration rate and oxygen consumption of the bivalve Mercenaria mercenaria (L.) have been made using flowing water systems, and then combined with growth and survivorship data to construct an energy budget for a tidal-flat population in Southampton Water. Estimated consumption (1292 kcal m^-2 year^-1) is mainly deposited as faeces and pseudofaeces (759 kcal) or excreted (160 kcal); 29% (374 kcal) is assimilated, of which 241 kcal are used for respiration, 72 kcal for flesh production and 61 kcal for gamete production. M. mercenaria contributes significantly to other trophic groups in Southampton Water; predators remove an estimated 55 kcal m^-2 year^-1, and 829 kcal pass to the scavenger/decomposer chain.     

Single-step species identification of bivalve larvae using multiplex polymerase chain reaction

One of the biggest obstacles to studying recruitment variation in marine bivalves is the need to collect and process large numbers of plankton samples. Larval bivalves are notoriously difficult, if not impossible, to identify to species using morphological criteria alone. Remote time-series collections could satisfy the sampling challenge, but efficient identification techniques must be developed to obtain species-specific data. Thus, we have developed a multiplex polymerase chain reaction (PCR) identification assay in which a single reaction is capable of accurate and efficient discrimination of five target bivalve species based on the size of cytochrome oxidase I products. The assay was tested with cultured and field-sampled larvae as well as adult genomic DNAs. Using a single whole larva as template, multiplex PCR reactions were capable of discriminating among the commercially important bivalves: Mercenaria mercenaria, Argopecten irradians, Mulinia lateralis, Spisula solidissima and Mya arenaria. Overall accuracy was 92%, including very few false positives. The efficiency of this assay stems from its ability to discriminate multiple target species with a single molecular step that ultimately can be automated to process large numbers of larvae. Received: 30 March 2000 / Accepted: 10 July 2000     

Diel cycles of activity,metabolism, and ammonium concentration in tropical holothurians

Movement rate, oxygen consumption, and respiratory tree ammonium concentration were measured in situ in the holothurians Pearsonothuria graeffei and Holothuria edulis in the Agan-an Marine Reserve, Sibulan, Philippines (9°20′30″N, 123°18′31″E). Measurements were made both day and night for both species during April–July 2005. P. graeffei had significantly higher movement rate during the day than at night (1.14 and 0.27 m h⁻¹, respectively; three-way ANOVA, P < 0.05) while H. edulis had higher movement rate at night compared to the day (0.83 and 0.07 m h⁻¹, respectively), spending the daylight hours sheltering under coral. More than 80% of H. edulis had movement rate of zero during the day. Oxygen consumption of P. graeffei was significantly higher during the day than at night (1.61 and 0.83 μmol O₂ g⁻¹ h⁻¹, respectively; two-way ANCOVA, P < 0.05), but the reduction at night was not as pronounced as the reduction in movement. H. edulis had a 75% reduction in oxygen consumption during the day compared to night (0.51 and 1.96 μmol O₂ g⁻¹ h⁻¹, respectively), matching this species’ reduced movement rates during the day. Ammonium concentration in water withdrawn from the respiratory trees of P. graeffei during the day (12.0 μM) was three times higher than in respiratory tree water sampled at night (4.3 μM) and 15 times higher than ambient seawater (0.8 μM; three-way ANOVA, P < 0.05). Ammonium concentration in the respiratory tree water of H. edulis was six times higher at night (14.6 μM) than during the day (2.2 μM) and 16 times higher than that of ambient seawater (0.9 μM). Even though H. edulis and P. graeffei are found within the same coral reef environment, they may affect different substrates and reef organisms due to their different habitats and distinct but opposite diel cycles.     

Domoic acid in benthic flatfish on the continental shelf of Monterey Bay,California, USA

Within Monterey Bay, California, USA, the food web transfer of domoic acid (DA), a neurotoxin produced by diatoms of the genus Pseudo-nitzschia, has led to major mortality events of marine mammals and birds. Less visible, and less well known, is whether invertebrates and fish associated with the benthos are also affected by blooms of DA-producing Pseudo-nitzschia spp. This study examines the presence of DA in benthic flatfish offshore of Davenport, California, (37°0′36″N, 122°13′12″W) and within Monterey Bay, California (36°45′0″N, 122°1′48″W), including species that feed primarily in the sediment (benthic-feeding) and species that feed primarily in the water column (benthopelagic-feeding). Flatfish caught between 10 December 2002 and 17 November 2003 at depths of 30–180 m had concentrations of DA in the viscera ranging from 3 to 26 μg DA g⁻¹ of viscera. Although the DA values reported are relatively low, benthic-feeding flatfish were frequently contaminated with DA, especially as compared with the frequency of contamination of flatfish species that feed in the water column. Furthermore, on days in which both benthic-feeding and benthopelagic-feeding flatfish were collected, the former had significantly higher concentrations of DA in the viscera. Curlfin turbot, Pleuronicthys decurrens, the flatfish with both the highest level and frequency of DA contamination, are reported to feed exclusively on polychaetes, suggesting that these invertebrates may be an important vector of the toxin in benthic communities and may pose a risk to other benthic-feeding organisms. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sea otter foraging on deep-burrowing bivalves in a California coastal lagoon

Sea otter, Enhydra lutris, predation had no detectable effect on abundance and size distribution of deep-burrowing bivalve prey in the Elkhorn Slough, California, USA. Up to 23 otters were present for 6 mo of the study period (March 1984 through April 1985). This is in contrast to previous studies of sea otter predation, especially on the shallow-burrowing Pismo clam Tivela stultorum, which can be found along the wave-exposed coast near the slough. The deep-burrowing clams Tresus nuttallii and Saxidomus nuttalli made up 61% of the prey taken in the slough, and are more difficult for otters to excavate than Pismo clams. The occurrence of foraging otters was highest in an area where the two bivalve prey were extremely abundant (18 individuals m^–2). However, the otters did not selectively prey on the largest clams available within the study sight, but foraged preferentially in a patch of smaller individuals where bivalve burrow depth was restricted by the presence of a dense clay layer. This foraging strategy maximized the amount of prey biomass obtained per unit volume of sediment excavated. Our findings suggest that in soft-sediment habitats deep-burrowing bivalves may be more resistant to otter predation than shallower burrowers.     

Metabolic disposition of lactate in the horseshoe crab Limulus polyphemus and the stone crab Menippe mercenaria

The metabolism of ¹⁴C-labelled lactate was investigated in the horseshoe crab Limulus polyphemus and the stone crab Menippe mercenaria. When a bolus of (¹⁴C-U)-D-lactate was injected into L. polyphemus, there was substantial release pf ¹⁴CO₂ into the medium. In the case of M. mercenaria, ¹⁴CO₂ release was also observed after injection of (¹⁴C-U)-L-lactate into experimental individuals. Analysis of the distribution of radioactivity in whole body extracts of both species revealed easily detectable amounts of radioactivity in the glycogen fraction, although the bulk of the radioactivity was in the cation, anion/neutral and CO₂ fractions. To investigate the metabolism of lactate further, ¹⁴C-labelled lactate was injected into large individuals of L. polyphemus and M. mercenaria, and the distribution of radioactivity was determined in the hemolymph, muscle and hepatopancreas. Utilization of (¹⁴C-U)-D-lactate by L. polyphemus resulted in the accumulation of significant amounts of labelled glucose in all three body compartments as well as the production of labelled glycogen in the telson levator muscle and hepatopancreas. Utilization of (¹⁴C-U)-L-lactate resulted in a similar pattern of glucose and glycogen labelling in the hemolymph, cheliped muscle and hepatopancreas of M. mercenaria. These studies demonstrate that both L. polyphemus and M. mercenaria have the capacity for glyco- and gluconeogenesis using lactate as the substrate.     

The reproductive ecology of the invasive ascidian, <Emphasis Type="Italic">Styela clava,</Emphasis> in Auckland Harbour,New Zealand

The ascidian Styela clava, native to the north-west Pacific, is an invasive species affecting New Zealand’s marine ecosystems, biodiversity and aquaculture operations. To provide detailed information on the reproductive biology of S. clava in New Zealand for post-border biosecurity management, long-term seasonal patterns of gametogenesis were determined from May 2006 to May 2008 in Auckland’s Waitemata Harbour (36°49′20″S, 174°45′85″E). Of particular interest was whether the critical 15°C threshold spawning temperature for reproduction observed in the Northern Hemisphere applied here to the first Southern Hemisphere study. S. clava gametogenesis followed a regular seasonal cycle with ripe gametes appearing as early as September and persisting to June; this time frame corresponds to the period when sea surface temperatures in the region first reach 15°C and with spawning occurring mainly during late summer to early autumn. From photoperiod manipulation, it was determined that spawning occurred at approximately 18:20. The extended reproductive period and a short generation time in the Waitemata Harbour provides a lengthy opportunity for S. clava to spread. Findings are discussed in relation to S. clava’s post-border management.     

Genetics and shell morphology in a hybrid zone between the hard clams Mercenaria mercenaria and M. campechiensis

Populations of Mercenaria mercenaria (L.) from South Carolina, USA, and M. campechiensis (Gmelin) from the Gulf of Mexico, Florida, USA, were sampled in 1987. The two species differed at all of seven enzyme loci tested, as well as in the thickness of shell ridges and nacre color. The difference in lunule shape was not great, although differences in relative shell width, shell weight, and lunule size make morphometric discrimination between the species possible. Shell ridges, nacre color, and multivariate morphometrics in a sample of clams collected from the Indian River Lagoon on the Atlantic coast of Florida in 1985 do not assort independently. Individuals with thick ridges, white nacre, and/or campechiensis-like morphometrics have significantly different allele frequencies at most enzyme loci from individuals with thin ridges, purple nacre, and/or mercenaria-like morphometrics. The deviations are in the direction predicted from the analysis of the allopatric populations of M. mercenaria and M. campechiensis. M. mercenaria outnumber M. campechiensis in the Indian River sample, but the majority of the clams seem to be hybrids.Contribution No. 235 from South Carolina Marine Resources Center     

Shifting abundance of the ctenophore Mnemiopsis leidyi and the implications for larval bivalve mortality

Along the mid-Atlantic coast of the US, the ctenophore Mnemiopsis leidyi (Agassiz) appears to be increasing in abundance and undergoing shifts in its historical seasonal distribution. We provide new data on shifting ctenophore abundance in Long Island estuaries and its implications for top-down control of the plankton community. Peak mean biovolume estimates of M. leidyi in Long Island estuaries in 2006 revealed ctenophore abundance values that were a factor of two to five times greater than previous studies conducted two decades ago. Furthermore, peak M. leidyi densities in 2006 occurred 2–3 months earlier than previously documented, suggesting a shift in the seasonal maxima of M. leidyi. Application of daily ingestion rates to zooplankton abundance indicates that, at its highest densities M. leidyi can remove an overall average of 20–89% per day of bivalve veligers and other zooplankton taxa, including adult copepods, nauplii, and tintinnids. Increasing ctenophore abundance, especially during a period when they were not historically abundant (i.e., June) may have significant consequences for species which spawn at this time. For example, current populations of M. leidyi represent a major source of larval mortality for bivalves which may inhibit recovery of shellfish populations and reinforce their low abundance state in Long Island estuaries.     

Does spawning behavior minimize exposure to environmental stressors for encapsulated gastropod embryos on rocky shores?

Adults of motile intertidal invertebrates are able to seek shelter to avoid environmental stress associated with low tides, but embryos within egg masses are effectively sessile for the duration of their encapsulation. Gastropod egg masses from 34 taxa on two rocky shores in SE Australia (34°37′08″S, 150°92′03″E and 34°35′45″S, 150°53′20″E) were surveyed over 2 years (June 2002–May 2004) to test the hypothesis that eggs are deposited in patterns that minimize exposure to environmental stress. Egg masses were expected to be predominantly deposited in shaded habitats not prone to environmental extremes. It was also anticipated that the deposition of egg masses in habitats exposed to UVR, desiccation, and/or extremes in temperature would occur when exposure to these abiotic factors was minimized. Among the taxa investigated, only four species spawned in full sun (Bembicium nanum, Nerita morio, Siphonaria zelandica and S. denticulata). Summer had the highest UVR index, water temperature, and air temperature as well as the lowest daytime tides. Univariate and multivariate analyses confirmed that egg mass abundance was highest during summer, with no change in egg mass size. This study shows that those species depositing egg masses on the surfaces of rock platforms do not adjust the seasonal timing or macrohabitat location of their spawning to avoid physiologically stressful conditions, particularly UVR. Alternate reasons for the evolution of egg mass deposition behavior in apparently sub-optimal habitats are discussed, and it is almost certainly the complex interplay of a variety of highly species-specific factors that is responsible for the patterns observed.     

Effects of Lunatia heros predation on the population dynamics of Mya arenaria and Macoma balthica in Maine,USA

The interaction between the naticid snail predator Lunatia heros and 2 iteroparous, infaunal, intertidal bivalves was investigated in Lubec, Maine, USA. The Mya arenaria population consists primarily of young, small individuals. M. arenaria survivorship is low when young (3.5% yr^-1 for the first 5 yr), then increases. M. arenaria can attain a length of 60 mm, but it is susceptible to L. heros attack only until it is 30 mm long. It delays reproduction until it is 20 mm long (3.8 yr) and diverts its resources instead into rapid early growth (4.9 mm yr^-1 for the first 5 yr). The Macoma balthica population has a larger proportion of older individuals than does that of M. arenaria. Survivorship is higher for M. balthica than for M. arenaria (76.3% yr^-1 for the first 5 yr). Unlike M. arenaria, M. balthica attains a final length of only 25 mm and all sizes are susceptible to L. heros attack. M. balthica grows slowly (2.7 mm yr^-1 for the first 5 yr) and diverts its resources into earlier reproduction at a length of 10 mm (2.9 yr). These contrasting life-history patterns and the possible relationship between bivalve resource allocation and refuges from predation are discussed.     

Feeding habits and phenotypic changes in proboscis length in the southern oyster drill, Stramonita haemastoma (Gastropoda: Muricidae), on Florida sabellariid worm reefs

The southern oyster drill, Stramonita (= Thais, Kool 1987) haemastoma, is a common intertidal and subtidal predator in the southeastern United States. It uses specialized feeding structures and foraging strategies to bore holes through the shell of its bivalve prey. However, on the east coast of Florida, S. haemastoma, is common on sabellariid worm reefs constructed by the polychaete Phragmatopoma lapidosa (Walton Rocks Beach, Florida, 27°17′N, 80°12′W), a habitat where the snail’s typical prey are scarce. From 1999 to 2001, we examined the feeding habits of S. haemastoma on sabellariid reefs and the behavioral and morphological responses of S. haemastoma that accompanied switching from a diet of bivalves to sabellariids. On worm reefs S. haemastoma feeds on P. lapidosa by inserting the proboscis deep into a worm’s tube. Worm-feeding snails had longer proboscises (~3.7 times shell height) than bivalve-feeding conspecifics (~2.0 times shell height). Snails raised on different diets showed significant differences in proboscis length suggesting that the proboscis length is phenotypically plastic. Whereas typical oyster drills must bore holes for days before ingesting prey, S. haemastoma on worm reefs avoids boring and attacks and consumes worms in 15–50 min. In the laboratory, oyster drills consumed 1.7 worms per day, spending <1 h each day feeding. On sabellariid reefs, differences in feeding, handling costs, and prey value, are likely to have a significant effect on the ecology and life history of S. haemastoma in this habitat.     

An experimental field test of host-finding mechanisms in a Caribbean gnathiid isopod

Field experiments were conducted from dusk to dawn off St. John (18° 18′ 59.32″ N, 64° 43′ 24.5″ W) and Guana Island (18° 28″ 28.31″ N, 64° 34′ 30.83″ W), Virgin Islands from June through August 2008-2010 to assess the sensory cues used by the nocturnal/crepuscular fish-parasitic gnathiid isopod, Gnathia marleyi, to locate fish hosts. Experimental traps providing both visual and olfactory cues from live French grunts (Haemulon flavioliniatum) attracted significantly more gnathiids than traps providing only visual cues or control traps (empty or with a rock), which were not significantly different from each other. In another experiment, traps providing both cues and only olfactory cues attracted significantly more gnathiids than empty control traps, but were not significantly different from each other. Our findings suggest that during nocturnal and crepuscular periods, visual cues provided by resting or slow-moving fish are not alone sufficient to attract gnathiids, while olfactory cues alone are. The traps designed for this study offer a new method of sampling free-living gnathiid isopods.