The relationship between lunar periodicity and activity of fish-parasitic gnathiid isopods in the Caribbean

Gnathiid isopod larvae are members of the marine demersal zooplankton community and are common ectoparasites of coral reef fishes, emerging from the substratum, mostly at night and crepuscular periods to feed on fish blood. Given that the activity of many marine organisms is often linked to changes in the phase of the moon, we examined the relationship between lunar phase and activity in gnathiid isopods on Caribbean reefs. We employed two sampling methods to quantify gnathiid activity: (1) Emergence traps set on reefs over a 24 h period; and (2) live fish placed in cages on reefs and retrieved during night and dawn peaks in gnathiid activity. These were compared during discrete phases as well as a continuous metric, the lunar “angle”, and an estimate of ambient light availability (luminance). Lunar phase and angle varied in their statistical effect on gnathiid activity patterns by sampling method and/or year. Luminance had a significant but inconsistent effect on measures of gnathiid activity. Our results suggest that changes in the lunar cycle are not a strong predictor of gnathiid activity at our shallow reef study sites.     

Infestation dynamics of gnathiid isopod juveniles parasitic on the coral-reef fish Hemigymnus melapterus (Labridae)

The dynamics of infestation by parasitic juveniles of gnathiid isopods were investigated at Lizard Island in the summer of 1997/1998 to determine when, and at what rate, they infest fishes. Variation in gnathiid abundance on wild-caught fish (Hemigymnus melapterus) between dawn and sunset was examined, and unparasitized H. melapterus in cages were exposed to gnathiids in the field for 4 h (8 h for fish sampled at 06:00 h) at five different times of the day and night (10:00, 14:00, 18:00, 22:00, 06:00 hrs) on three reefs. To control for any potential effect of the cleaner-fish Labroides dimidiatus, which consumes large numbers of gnathiids each day, sampling was carried out on reefs from which all cleaner-fish had been removed. Gnathiid abundance decreased during the day. Standardized abundance per wild-caught fish was 1.9 times higher at dawn than at sunset. Gnathiids successfully infested fish in cages. Sixty-one percent of the fish in cages were infested with ≥1 gnathiids, with 51% of the fish having between 1 and 4 gnathiids after 4 h exposure. Gnathiids infested fish both day and night. The log₁₀ (abundance +1) of gnathiids per caged fish varied significantly between time periods, with higher gnathiid numbers on fish sampled at 18:00, 22:00, and 06:00 hrs than on fish sampled at 10:00 and 14:00 hrs. In contrast to gnathiid abundance on wild-caught fish, the number of gnathiids on caged fish sampled at sunset was as high as that at dawn. The estimated mean (SE) cumulative number of gnathiids per caged fish [mean size = 13.7 cm (±0.25)] per day was 7.8 (1.1); this is similar to the estimated mean number of 7.3 gnathiids on similar-sized wild-caught fish at 6:00 hrs. The high infestation rate of gnathiids on caged fish in the late afternoon contrasted with the low numbers on wild-caught fish at this time, suggesting that factors other than infestation behaviour may be responsible for the low numbers of gnathiids on wild-caught fish in the afternoon. Received: 19 August 1998 / Accepted: 1 June 1999     

An experimental test of alternative population augmentation scenarios

Human land use is fragmenting habitats worldwide and inhibiting dispersal among previously connected populations of organisms, often leading to inbreeding depression and reduced evolutionary potential in the face of rapid environmental change. To combat this augmentation of isolated populations with immigrants is sometimes used to facilitate demographic and genetic rescue. Augmentation with immigrants that are genetically and adaptively similar to the target population effectively increases population fitness, but if immigrants are very genetically or adaptively divergent, augmentation can lead to outbreeding depression. Despite well‐cited guidelines for the best practice selection of immigrant sources, often only highly divergent populations remain, and experimental tests of these riskier augmentation scenarios are essentially nonexistent. We conducted a mesocosm experiment with Trinidadian guppies (Poecilia reticulata) to test the multigenerational demographic and genetic effects of augmenting 2 target populations with 3 types of divergent immigrants. We found no evidence of demographic rescue, but we did observe genetic rescue in one population. Divergent immigrant treatments tended to maintain greater genetic diversity, abundance, and hybrid fitness than controls that received immigrants from the source used to seed the mesocosms. In the second population, divergent immigrants had a slightly negative effect in one treatment, and the benefits of augmentation were less apparent overall, likely because this population started with higher genetic diversity and a lower reproductive rate that limited genetic admixture. Our results add to a growing consensus that gene flow can increase population fitness even when immigrants are more highly divergent and may help reduce uncertainty about the use of augmentation in conservation.     

The voluntary approach to externality problems: An experimental test

An experiment to test the two Coasian propositions, conducted at the University of Arizona, using students as subjects is reported. Experimental conditions included incomplete information and economic liability rules. The experimental results statistically support the efficiency proposition. Support for the neutrality proposition arises in the case of long-time negotiations. Behavioral findings such as altruism and other interesting results are discussed together with their practical implications.     

An experimental test of female choice relative to male structural coloration in eastern bluebirds

Several experimental studies have shown that female birds use ornamental melanin and carotenoid plumage coloration as criteria in mate choice. Whether females choose mates based on natural variation in structural coloration, however, has not been well established. Male eastern bluebirds (Sialia sialis) display brilliant ultraviolet (UV)-blue plumage coloration on their head, back, wings, and tail, which is positively correlated with condition, reproductive effort, and reproductive success. We experimentally tested the hypothesis that female eastern bluebirds prefer as mates males that display brighter structural coloration by presenting breeding-condition females with males of variable coloration. We conducted two types of mate-choice experiments. First, females chose between males whose coloration was manipulated within the natural range of variation in the population; feathers were either brightened with violet marker or dulled with black marker. Second, females chose between males with naturally dull or bright plumage coloration. In both manipulated and unmanipulated coloration trials, female choice did not differ significantly from random with respect to structural coloration. We found no support for the hypothesis that the UV–blue coloration of male eastern bluebirds functions as a criterion in female mate choice.     

Inhibition of larval settlement to a soft bottom benthic community by drifting algal mats: An experimental test

The hypothesis that drifting red algal mats inhibit settlement of planktonic larvae was tested in a field experiment in 1986. Substratum free of algae (caged boxes) was compared with substratum covered with algae (natural substratum and open boxes). Whereas settling densities of 1500 to 5500 ind. m^-2 of the bivalves Macoma balthica, Cardium glaucum and Mya arenaria were observed in sediment without algal cover, no recruits of the same species were found beneath the algae during the period of peak settlement (June to July). The same difference was observed for the polychaete Nereis diversicolor, although in this case occasional individuals were found beneath the algae. The results demonstrate that algae mats may be efficient inhibitors of larval settlement to benthic soft-substratum communities. It is concluded that algae act as a larval filter.     

A test of reproductive skew models in a field population of a multiple-queen ant

Determining the evolutionary basis of variation in reproductive skew (degree of sharing of reproduction among coexisting individuals) is an important task both because skew varies widely across social taxa and because testing models of skew evolution permits tests of kin selection theory. Using parentage analyses based on microsatellite markers, we measured skew among female eggs (n=32.3 eggs per colony, range=20–68) in 17 polygynous colonies from a UK field population of the ant Leptothorax acervorum. We used skew among eggs as our principal measure of skew because of the high degree of queen turnover in the study population. Queens within colonies did not make significantly unequal contributions to queen and worker adult or pupal offspring, indicating that skew among female eggs reflected skew among daughter queens. On average, both skew among female eggs (measured by the B index) and queen–queen relatedness proved to be low (means±SE=0.06±0.02 and 0.28±0.08, respectively). However, contrary to current skew models, there was no significant association of skew with either relatedness or worker number (used as a measure of productivity). In L. acervorum, predictions of the concession model of skew may hold between but not within populations because queens are unable to assess their relatedness to other queens within colonies. Additional phenomena that may help maintain low skew in the study population include indiscriminate infanticide in the form of egg cannibalism and split sex ratios that penalize reproductive monopoly by single queens within polygynous colonies.     

Effects of copper on the faunas of marine soft-sediments: An experimental field study

Although laboratory experimental studies have shown that copper is toxic to marine organisms at concentrations found in contaminated sediments, there is little unequivocal evidence of undesirable ecological effects in the field, other than at extreme concentrations. We describe a study in Botany Bay, New South Wales, Australia, in which the concentrations of copper in marine sediments were experimentally enhanced. Changes in the abundance and taxonomic composition of the fauna of copper-treated sediments relative to those of two control treatments were monitored over a period of six months. Univariate (ANOVA) and multivariate (non-metric multidimensional scaling, MDS) analyses of the changes in the fauna showed that increased concentrations of copper (140 to 1200 g g^-1 compared with background concentrations of 29 to 40 g g^-1) had an impact on the fauna. The nature of the response varied among taxa. For example, in some taxa, numbers of individuals decreased through time relative to controls, whereas the abundance of another taxon remained fairly constant through time in the copper treatment while numbers of control individuals increased. Differences in the changes of the faunas through time among the control and copper treatments were not always consistent among replicate experimental units 5 m apart, nor were they consistent between replicate experimental sites 100 m apart. The magnitudes of the changes in the faunas caused by the copper treatment are considered in the context of the magnitude of previously measured natural temporal variation.     

Sensory cues associated with host detection in a marine parasitic isopod

Parasite population dynamics and the evolution of life history characteristics are strongly associated with the process of host infection. Parasites with free-living life stages have a narrow window to infect a host and have evolved a number of mechanisms to detect and locate a host. Cymothoa excisa is a parasitic isopod that is commonly found on the Atlantic croaker, Micropogonias undulatus, in the Gulf of Mexico. Here, we determined the infection window that constrains host-searching behavior in C. excisa and tested the behavioral response of the free-swimming larvae (mancae) to visual and chemical cues. Mancae were found to have an infection window of 7 days. Mancae were responsive to both visual and chemical cues. Our findings are the first to show that cymothoid isopods use visual and chemical cues to locate a host and that individuals display a host-locating strategy that maximizes host encounter rate, while reducing energy expenditure.     

An experimental test of the effect of the abalone Haliotis midae on benthic community composition

The South African abalone Haliotis midae is a commercially valuable species, but its numbers are seriously depleted due to illegal fishing. Overfishing not only affects the targeted species, but also potentially influences the integrity and functioning of the ecosystem through associated changes in community composition. We assessed the herbivorous effects of H. midae on the benthos through a 6-month field experiment involving exclusion/inclusion plots. This showed that benthic community composition remained largely unchanged whether H. midae was present or absent and specifically revealed no evidence that H. midae diminishes algal biomass, probably because in the region where we worked, it feeds mainly by trapping drift kelp rather than grazing on attached algae. The absence of responses to the removal of abalone was additionally evident in comparison with indices of abundance and diversity, functional groups and individual species. In the absence of abalone, there were subtle increases in the green alga Codium stephensiae and the crustose brown alga Zeacarpa leiomorpha and decreases in the four species of foliose macroalgae, but none of these effects were significant. The presence of H. midae thus had no negative grazing effects on any elements of the epilithic community composition. It does, however, have other influences, including occupation of space and provision of habitat in the form of its shell, which supports a community significantly different from the surrounding rocks.     

Reproductive biology of a small isopod symbiont living on a large isopod host: from the maternal marsupium to the protective grip of guarding males

Mating systems of many symbiotic crustaceans are characterised by a high degree of mate guarding. A peculiar case of mate guarding has been reported for small symbiotic janirid isopods where males mate with immature females. Field samples of individual hosts and laboratory experiments were conducted to reveal the mating behaviour of the symbiont in a natural environment, that is, on their hosts. Along the coast of the Magellan Strait, Chile, the janirid isopod Iais pubescens was frequently found on the shore-living isopod Exosphaeroma gigas. Symbiont prevalence (percent hosts occupied) was high at eight of the nine sampling sites. Mean symbiont intensity was very low at one site (<<1 individual host^-1), intermediate at two sites (1-10 individuals host^-1) and high at the other sites (10-40 individuals host^-1). The mean sex ratio (males:females) was male biased at most sampling sites (n=7). Females of I. pubescens reached substantially larger sizes (1.5-3.0 mm body length, BL) than males (1.1-1.9 mm BL). The majority of males were carrying small juveniles (66.15%), and males with juveniles were significantly larger than males without juveniles - this suggests that males prefer virgin juveniles to adult females and that they compete for small juveniles. In laboratory observations, males were seen to manipulate the marsupium of adult females that were about to release small juveniles. Males obtained virgin juveniles in this manner. Juveniles were carried for ~7 days, and they moulted shortly before being fertilised and released by males. The high proportion of juveniles carried by males in the field (68.2%) supports previous observations that males initially are not able to distinguish male and female juveniles. It is suggested that the mating system of symbiotic janirid isopods with long-term sperm storage and continuous receptivity in females and male mating with virgin females has evolved in response to highly unpredictable encounter probabilities between the sexes. Mate guarding and manipulation of small virgin juveniles may be favoured on the highly mobile hosts of symbiotic janirid isopods. Furthermore, adult females may gain by leaving their emerging offspring in the protective grip of guarding males, thereby reinforcing the maintenance of this peculiar mating system.     

Aspects of sodium regulation in a brackish-water and a marine species of the isopod genus Sphaeroma

Sodium and chloride concentrations of the haemolymph were determined in Sphaeroma rugicauda Leach and S. serratum Fabricius acclimatized to a range of salinities. Sodium loss and uptake (using ²²Na) were measured in salinities below 1.1 for S. rugicauda and 7.2 for S. serratum. Potential differences between haemolymph and medium indicate active uptake of both ions in certain salinities. The active-uptake component of total sodium-influx was found to be related non-linearly to the sodium concentration of the medium. Curves of were fitted to the activeuptake data. Km values indicate that, in S. rugicauda, the sodium uptake system has a greater affinity for sodium than in S. serratum. These findings are discussed in relation to the distribution of each species.     

Vertical swimming in wave-induced currents as a control mechanism of intertidal migration by a sand-beach isopod

Both the vertical and horizontal distributions ofEurydice pulchra (Leach) within the water column of the surf zone were recorded throughout complete tidal cycles on a sandy beach in North Wales during the summer of 1989. Upon emerging from the sand, the isopods tended to swim up in the water column, where transport onshore would be facilitated by the wave-induced, onshore currents which laboratory wave-tank experiments have confirmed to occur near the water surface. This combination of active and passive transport to the water's edge results in high numbers of individuals in the narrow swash zone. At and just after the time of high tide, individuals swim to the water/sediment interface where, as again confirmed by wave-tank experiments, the predominant water movement is offshore. Continued swimming near the bottom during the ebb tide before reburrowing in the sand ensures transport downshore and avoidance of stranding above the characteristic level of zonation on the shore. Vertical migrations ofE. pulchra in the water column permit differential exploitation of up and downshore currents to achieve horizontal migration.     

An experimental study of habitat selection by birds in a coffee plantation

Unique components of tropical habitats, such as abundant vascular epiphytes, influence the distribution of species and can contribute to the high diversity of many animal groups in the tropics. However, the role of such features in habitat selection and demography of individual species has not been established. Understanding the mechanisms of habitat selection requires both experimental manipulation of habitat structure and detailed estimation of the behavioral and demographic response of animals, e.g., changes in movement patterns and survival probabilities. Such studies have not been conducted in natural tropical forest, perhaps because of high habitat heterogeneity, high species diversity, and low abundances of potential target species. Agroforestry systems support a less diverse flora, with greater spatial homogeneity which, in turn, harbors lower overall species diversity with greater numerical dominance of common species, than natural forests. Furthermore, agroforestry systems are already extensively managed and lend themselves easily to larger scale habitat manipulations than protected natural forest. Thus, agroforestry systems provide a good model environment for beginning to understand processes underlying habitat selection in tropical forest animals. Here, we use multistate, capture-recapture models to investigate how the experimental removal of epiphytes affected monthly movement and survival probabilities of two resident bird species (Common Bush-Tanager [Chlorospingus ophthalmicus] and Golden-crowned Warbler [Basileuterus culicivorus]) in a Mexican shade coffee plantation. We established two paired plots of epiphyte removal and control. We found that Bush-Tanagers were at least five times more likely to emigrate from plots where epiphytes were removed compared to control plots. Habitat-specific movement patterns were not detected in the warbler. However, unlike the Golden-crowned Warbler, Common Bush-Tanagers depend upon epiphytes for nest sites and (seasonally) for foraging. These dispersal patterns imply that active habitat selection based on the presence or absence of epiphytes occurs in C. ophthalmicus on our study area. Survival rates did not vary with habitat in either species. Interestingly, in both species, survival was higher in the nonbreeding season, when birds were in mixed-species flocks. Movement by Common Bush-Tanagers into areas with epiphytes occurred mostly during the breeding season, when mortality-driven opportunity was greatest.     

An experimental evaluation of transgenerational isotope labelling in a coral reef grouper

Transgenerational isotope labelling (TRAIL) using enriched stable isotopes provides a novel means of mass-marking marine fish larvae and estimating larval dispersal. The technique, therefore, provides a new way of addressing questions about demographic population connectivity and larval export from no-take marine protected areas. However, successful field applications must be preceded by larval rearing studies that validate the geochemical marking technique, determine appropriate concentrations and demonstrate that larvae are not adversely affected. Here, we test whether injection of enriched stable barium isotopes (¹³⁵Ba and ¹³⁷Ba) at two dose rates produces unequivocal marks on the otoliths of the coral reef grouper Epinephelus fuscoguttatus. We also assess potential negative effects on reproductive performance, egg size, condition and larval growth due to injection of adult female fish. The injection of barium isotopes at both 0.5 and 2.0 mg Ba/kg body weight into the body cavities of gravid female fish was 100% successful in the geochemical tagging of the otoliths of larvae from the first spawning after injection. The low-dose rate produced no negative effects on eggs or larvae. However, the higher dose rate of 2 mg Ba/kg produced small reductions in yolk sac area, oil globule area, standard length and head depth of pre-feeding larvae. Given the success of the 0.5 mg Ba/kg dose rate, it is clearly possible to produce a reliable mark and keep the concentration below any level that could affect larval growth or survival. Hence, enriched Ba isotope injections will provide an effective means of mass-marking grouper larvae.     

Female resistance and male preference in a stream-dwelling isopod: effects of female molt characteristics

In the stream-dwelling isopod, Lirceus fontinalis, mating contests between males and females occur prior to pair formation. We examined the relative contribution of male preference and female resistance to contest outcomes. We first quantified male and female behavior during typical mating interactions and examined the relationship between time until molt (TTM) and mating outcomes. We then examined the role of male preference and female resistance in determining mating outcomes when females differed in molt type (growth, egg deposition) and appeared to differ in TTM (due to hormone applications). Both male preference and female resistance contributed to different components of the mating sequence but female resistance ultimately determined whether or not pair formation occurred. Males expressed a preference for females that appeared to be close to molt, using variation in levels of molt hormone as a cue. However, males did not discriminate between females based on molt type. Received: 5 March 1999 / Received in revised form: 10 August 1999 / Accepted: 16 October 1999