The role of synchronized calling,ambient light,and ambient noise,in anti-bat-predator behavior of a treefrog

Summary Male treefrogs, Smilisca sila (Hylidae), produce calls of varying complexity and demonstrate a remarkable ability to synchronize their calls with those of neighbors. The bat Trachops cirrhosus eats frogs and uses the frogs' advertisement calls as locational cues. The bats are less likely to respond to synchronous calls than to asynchronous calls, and when given a choice prefer complex calls to simple calls.Experiments with bat models indicate that, like other frogs, S. sila probably uses visual cues to detect hunting bats. In response to bat models the frogs decreased both the number and the complexity of their calls. The calling behavior of the frogs was sampled in the field during periods with and without artificial illumination. The frogs produced fewer and less complex calls, and they tended to call from more concealed sites, during the period without illumination, when presumably it would have been more difficult for the frogs to detect hunting bats. S. sila tended to call from sites with higher ambient noise level, the noise primarily originating from waterfalls. The frequencies of the dominant energies in the waterfall sounds completely overlapped the frequency range of the S. sila call; thus waterfalls might mask the frog calls. When given a choice between calls produced near and away from waterfall sounds, bats preferred the latter.     

The relative value of call embellishment in túngara frogs

Facultative traits that have evolved under sexual selection, such as the acoustic ornaments present in the advertisement signals of male túngara frogs (Physalaemus pustulosus), offer a unique opportunity to examine selection for trait exaggeration with a focus on individual differences amongst signalers. By contrast, many studies of mate choice use experimental designs that obscure the inter-individual variation amongst signalers available for selection to act on—through the use of “typical” or average signals from the population. Here, we use dichotomous female phonotaxis choice tests to determine how the value of male call embellishment varies across 20 individual males frogs recorded from the wild—a sample which captures the acoustic diversity present in the population. We tested 20 females for each male call pair (i.e., 400 females). The results show widespread preference amongst females for ornamented calls (“whine–chucks”) over simple calls (“whines”), yet also demonstrate substantial variation in the relative benefits for individual male frogs—some males enjoy appreciable benefits by using ornaments while others (30% of males in this study) do not. We also show that the relative amplitude of the chuck to the whine correlates positively with the value of call elaborations across these 20 males. Finally, by manipulating the relative amplitude of whines and chucks using both natural and synthetic calls, we demonstrate directly that this single call parameter is key to determining the relative value of call elaborations across males.     

Female preferences for spectral call properties in the western genetic lineage of Cope's gray treefrog (Hyla chrysoscelis)

Female frogs discriminate among potential mates based on individual variation in male advertisement calls. While considerable data have accumulated allowing comparisons of female preference functions among species, we still lack fundamental knowledge about how and why the shapes of preference functions for particular call properties vary among populations within all but a few species. Here, we report results from a study aimed at describing female preference functions for spectral call properties in Cope's gray treefrog (Hyla chrysoscelis). Widespread throughout the eastern half of North America, Cope's gray treefrog is the diploid member of the cryptic diploid–tetraploid Hyla versicolor species complex, and its populations are divided into two distinct genetic lineages (eastern and western). In this study of a western lineage population, we recorded and analyzed the spectral properties of 1,000 advertisement calls from 50 males and conducted two-stimulus phonotaxis experiments to estimate a population-level preference function. Females preferred calls with average frequencies over calls with frequencies that were 2 or 3 semitones (1.4 or 2.1 standard deviations, respectively) lower than the population mean. We observed no behavioral discrimination between calls with average and higher-than-average frequencies. Preferences discriminating against low-frequency calls were weak and were abolished by attenuating the preferred average call by 3?dB. We discuss these results in light of previous studies of eastern lineage populations, geographic variation in female preference functions, and the potential adaptive value of discriminating against calls with low frequencies.     

Species-specificity and individual variation in the song of male Nathusius’ pipistrelles (<Emphasis Type="Italic">Pipistrellus nathusii</Emphasis>)

Sequences of the advertisement calls produced by male Nathusius’ pipistrelles (Pipistrellus nathusii) during the autumn mating period were recorded from individuals at two separate sites in Antrim, Northern Ireland, in August 2004. Several male roosts were found at these sites in close proximity to a single maternity roost, each containing approximately 200 adult females and their young. Analysis of measured parameters of four identified call types revealed that there were significant differences in call structure between sites and between individuals. Playback experiments, performed outside the adult female and juvenile roost sites, comprised of experimental advertisement call sequences of P. nathusii, Pipistrellus pygmaeus and Pipistrellus pipistrellus and control sound recorded without bats present (silence). Response was measured by simultaneously recording ultrasound during playbacks and counting the number of echolocation pulses identified as those of P. nathusii above a predetermined amplitude threshold. Significantly greater numbers of P. nathusii echolocation pulses were recorded during playback of male P. nathusii advertisement calls than during playback of congeners’ advertisement calls and control sound. The number of echolocation pulses recorded was similar during playback of P. pipistrellus and P. pygmaeus advertisement calls and silence. We suggest that, due to call complexity, male P. nathusii advertisement calls should be classified as ‘song’. Species-specificity and individual variation suggests that the songs of male P. nathusii have the potential to play a role in mate attraction and mate assessment.     

Grazing of phytoplankton by copepods in eastern Antarctic coastal waters

Chlorophyll a, primary productivity and grazing by copepods on phytoplankton were measured in the upper water column during the summer of 1994/1995 at a coastal site near Davis Station, East Antarctica. Chlorophyll a was at a maximum in mid-December, then dropped markedly as the coastal fast ice melted and broke‐out. Phytoplankton biomass increased again from mid‐ to late‐February. Copepods accounted for at least 65% of zooplankton biomass in the water column before sea ice break‐out, whereas larval polychaetes and ctenophores dominated after ice break‐out. Oncaeacurvata was the numerically dominant species throughout the study. The highest grazing rate (8.7 mg C␣m⁻³␣d⁻¹) was recorded on 21 December when O.␣curvata accounted for 64% of the total. Grazing had decreased markedly by 28 December (0.9 mg C m⁻³ d⁻¹); again O. curvata accounted for over 50% of the total ingested. Copepod grazing increased after ice break-out until the last experiment on 20 February (⋍5 mg C␣m⁻³␣d⁻¹). The main species responsible for grazing during this period were O. curvata, Oithonasimilis, Calanoidesacutus and unidentified copepod nauplii. It was estimated that copepods removed between 1 and 5% of primary productivity. Received: 11 October 1996 / Accepted: 22 October 1996     

A possible phylogenetically conserved urgency response of great tits (<Emphasis Type="Italic">Parus major</Emphasis>) towards allopatric mobbing calls

Black-capped chickadees Poecile atricapillus alter the number of D notes of their chick-a-dee call to reflect urgency and threat. Here, I tested whether heterospecific responses of an allopatric species to these mobbing calls occur. Heterospecific chickadee mobbing calls and songs from North America were broadcast to European great tits (Parus major) and compared with conspecific mobbing calls. During conspecific mobbing playbacks, all great tits approached the speaker, during the heterospecific “chick-a-dee” playbacks, 63.3% individuals approached the speaker, while during the song playback, only 31.3% of the great tits approached the speaker. Minimum distances of great tits were lower during conspecific mobbing calls compared to allopatric chick-a-dee calls and to allopatric chickadee song. Also, minimum distances were lower when comparing allopatric chick-a-dee calls and chickadee song. Great tits approached the speaker on average down to (mean ± SE) 20.0 ± 1.8 m during playbacks of 1–4 D elements, to 17.7 ± 2.0 m during playbacks of 5–7 D elements and down to 11.5 ± 2.0 m during playbacks of 8–11 D elements. The number of D notes was inversely related to minimum distance. Thus, the urgency message encoded in the D notes was perceived also by an allopatric but phylogenetically related European species, suggesting that the heterospecific response is possibly phylogenetically conserved.     

Habitat utilization and alteration by the invasive burrowing isopod, Sphaeroma quoyanum, in California salt marshes

In recent years the pace of exotic species introduction and invasion has accelerated, particularly in estuaries and wetlands. Species invasions may affect coastal ecosystems in many ways. Alteration of sedimentary environments, through structure formation and burrowing, has particularly dramatic effects on coastal habitats. This study examines modification of channel bank and marsh edge habitat by the burrowing Australasian isopod Sphaeroma quoyanum Milne Edwards, in created and natural salt marshes of San Diego Bay and San Francisco Bay. Abundance and distribution patterns of this isopod species, its relationships with habitat characteristics, and its effects on sediment properties and bank erosion were examined seasonally, and in several marsh microhabitats. Mean isopod densities were 1541 and 2936 individuals per 0.25 m² in San Francisco Bay, and 361 and 1153 individuals per 0.25 m² in San Diego Bay study sites during December and July 1998, respectively. This isopod forms dense, anastomosing burrow networks. S. quoyanum densities did not differ as a function of location within creeks or location in natural versus created marshes. Burrows, which are on average 6 mm wide and 2 cm long, were associated with firm sediments containing high detrital biomass. Although erosion is a natural process along salt marsh banks, enclosure experiments demonstrated that isopod activities can enhance sediment loss from banks. In areas infested with S. quoyanum, losses may exceed 100 cm of marsh edge per year. The effects of habitat alteration by this invading species are likely to increase in severity in the coastal zone as these ecosystems become degraded. Received: 30 March 2000 / Accepted: 21 September 2000     

Interspecific acoustic interactions of the neotropical treefrog Hyla ebraccata

Summary Hyla ebraccata, H. microcephala and H. phlebodes commonly occur together in Panama. The three species have calls exhibiting broad frequency overlap and call during the same season and time of day from similar microhabitats, frequently in close proximity. The vocal repertoires of the three species are structurally and functionally similar. All employ multi-part advertisement and aggressive calls which consist of a primary note followed by a variable number of clicks. H. ebraccata males often responded to heterospecific calls with multi-note synchronized responses, and calls with primary notes greater than 150–200 ms were most effective in eliciting synchrony. Playback experiments with synthetic 1-note advertisement calls of different durations and both synthetic 1-note advertisement calls and 200 ms tones of different frequencies demonstrated that H. ebraccata males will synchronize with stimuli which are similar in frequency and duration to conspecific calls. Data from a two-choice experiment with female H. ebraccata demonstrate that calls of individual H. microcephala can reduce the attractiveness of a H. ebraccata male's calls if primary notes overlap. By synchronizing response calls to those of H. microcephala, a H. ebraccata may reduce the chances that his calls are rendered less attractive to potential mates.Aggressive calls of these species are graded and are characterized by higher pulse repetition rates and often longer durations than advertisement calls. H. ebraccata males respond to aggressive calls of H. microcephala and H. phlebodes as they do to their own calls. Heterospecific aggressive interactions probably occur because the species interfere acoustically. Our results demonstrate that H. ebraccata males behave in ways which enhance their ability to communicate in a noisy assemblage of conspecific and heterospecific males.     

Feeding and predation impact of two chaetognath species, Eukrohnia hamata and Sagitta gazellae, in the vicinity of Marion Island (southern ocean)

The predation impact of the two chaetognaths Eukrohnia hamata and Sagitta gazellae on mesozooplankton standing stock were investigated in three depth layers during two 24 h stations occupied in the vicinity of Marion Island in late austral summer (April/May) 1986. The zooplankton community at both stations was dominated by small copepods (Oithona spp., Microcalanus spp.), which accounted for >95% of total zooplankton abundance. Chaetognaths comprised <2% of total zooplankton abundance. E. hamata constituted >95% of the total chaetognath stock. The general trend in both species was decreasing abundance with increasing depth, which appeared to be correlated to the distribution of copepods (r ² = 0.45; P <0.05). Gut-content analysis showed that copepods (mainly Oithona spp., Calanus spp. and Rhincalanus gigas) and ostracods were the main prey of both species, accounting for 87 and 61% of the total number of prey in E. hamata and S.␣gazellae stomachs, respectively. In the guts of S.␣gazellae, pteropods (Limacina spp.) and chaetognaths were also well represented. The mean number of prey items (NPC) for E. hamata ranged from 0.02 to 0.06 prey individual⁻¹ which corresponds to an individual feeding rate (Fr) of between 0.05 and 0.12 prey d⁻¹. For S.␣gazellae, the NPC values were higher, varying between 0.04␣and 0.20 prey individual⁻¹, or between 0.15 and 0.76 prey d⁻¹. The daily predation impact of the two chaetognaths was estimated at between 0.3 and 1.2% of the copepod standing stock or between 7 and 16% of the daily copepod production. Predation by S. gazellae on chaetognaths accounted for up to 1.6% of the chaetognath standing stock per day. Received: 26 November 1996 / Accepted: 31 October 1997     

Non-indigenous ascidians in southern California harbors and marinas

Southern California's many large harbors form an important coastal ecosystem, yet they are also a␣major destination for thousands of pleasure craft and cargo vessels that have often traveled great distances. Many groups of marine organisms, including ascidians, have now been documented as undergoing range extensions as a consequence of rapid ship-transport between distant harbors phenomenon. This has resulted in a rapid increase in the rate of introductions of non-indigenous species worldwide, yet these effects of boat traffic remain largely unstudied in southern California. Ascidians are sessile marine filter-feeders, hermaphroditic, and often self-fertilizing; many species are tolerant of a wide range of environmental conditions, can reach sexual maturity in just a few weeks, and have a long breeding season. This paper documents the arrival of 14␣non-indigenous species in southern California harbors␣during this century, 13 of which have persisted:␣four prior to the 1960s (Cionaintestinalis, Styelaclava, S.␣plicata, Botryllusschlosseri), another by 1972 (S.␣canopus, formerly S. partita), and 8 since 1983 [C.␣savignyi, Ascidia zara, Ascidia sp., Polyandrocarpa zorritensis, Symplegma brakenhielmi (formerly S. oceania, and S. reptans, Microcosmus squamiger, and Molgula␣manhattensis)]. We estimate the relative abundance and seasonal fluctuations of both non-indigenous and native ascidians in all harbors in southern California from San Diego to Santa Barbara based upon the historical record, our 35 yr of field notes, and our recent surveys carried out during fall 1994, spring and fall 1995, fall 1996 and spring 1997. Possible points of origin of the exotics and predictions on further U.S. Pacific coast range-extensions are included. The concomitant decline in numbers and species of native ascidians in the harbors of southern California during this century is also reviewed. Received: 4 March 1997 / Accepted: 26 September 1997     

Aggressive thresholds in Dendropsophus ebraccatus: habituation and sensitization to different call types

Males in many chorusing anuran species use aggressive calls during defense of calling spaces from other males. The minimal intensity of another male’s vocalizations that elicits an aggressive call response has been termed the aggressive threshold. Previous studies of aggressive thresholds have shown that they are plastic: males habituated (increased their aggressive thresholds) in response to repeated presentation of stimuli above initial threshold levels. Habituation likely contributes to the stable chorus structure of these species, in which aggressive calling is rare compared to advertisement calls. I have observed high levels of aggressive calling in the treefrog Dendropsophus ebraccatus, suggesting that males of this species do not habituate. In this study, I investigated the plasticity of aggressive thresholds in D. ebraccatus. I measured the aggressive thresholds of males before and after suprathreshold stimulation by both advertisement and aggressive calls. I found that the different call types had different effects: males habituated to advertisement calls but lowered their aggressive thresholds in response to aggressive calls. I consider the latter response to be an example of sensitization, a behavior that has been documented infrequently in vocalizing anurans. Sensitization is a plausible mechanism responsible for the high levels of aggressive calling observed in this species. Given the high costs of aggressive calling, however, it is unclear why a mechanism that increases aggressive call output would be maintained.     

Habituation in the green frog, Rana clamitans

Habituation and its role in the dear enemy effect was investigated in a population of green frogs, Rana clamitans. Green frogs have a prolonged breeding season, and males defend territories centered around suitable oviposition sites. We tested the prediction that male green frogs will habituate to broadcasts of synthetic conspecific stimuli. Our results indicate that male green frogs can discriminate familiar from unfamiliar stimuli. We suggest that habituation helps to mediate the territorial interactions between male green frogs. Strangers present a greater threat than familiar neighbors. By habituating to the advertisement vocalizations of their near neighbors, males avoid costly interactions with individuals that are not a major threat to their territories. Received: 20 July 1998 / Accepted after revision: 6 September 1998     

Greater spear-nosed bats give group-distinctive calls

Individually distinctive vocalizations are ubiquitous; however, group distinctive calls have rarely been demonstrated. Under some conditions, selection should favor calls indicating social group membership in animals that forage in groups. Greater spear-nosed bats (Phyllostomus hastatus) give calls that appear to facilitate recognition of social group mates who are unrelated. Females give loud broadband (4–18 kHz) vocalizations termed screech calls when departing on foraging trips and at foraging sites. Screech calls help to establish foraging groups among social group members, and to maintain contact over the long distances they travel while foraging. I test two hypotheses about how screech calls may be structured to convey caller identity. Individual calls may be distinct and group members may learn to recognize each individual's calls and to associate the individual with the social group. Alternatively, groups may give distinct calls and individuals within groups may share call characteristics. To test these hypotheses I conducted multivariate acoustic analysis of multiple calls from 28 bats from three social groups. Although the ubiquity of individually distinctive calls in other taxa makes this result more likely, the results reveal that group calls are highly distinctive. Individual bats within groups are statistically indistinguishable. Calls appear to decrease slightly in frequency as bats age. Call convergence among unrelated group mates implies vocal learning in this species. Received: 28 March 1996 / Accepted after revision: 6 October 1996     

Zooplankton responses to hypoxia: behavioral patterns and survival of three species of calanoid copepods

Seasonally recurrent and persistent hypoxic events in semi-enclosed coastal waters are characterized by bottom-water dissolved oxygen (d.o.) concentrations of < 2.0 ml l⁻¹. Shifts in the distribution patterns of zooplankters in association with these events have been documented, but the mechanisms responsible for these shifts have not been investigated. This study assessed interspecific differences in responses to hypoxia by several species of calanoid copepods common off Turkey Point, Florida, USA: Labidocera aestiva (Wheeler) (a summer/fall species), Acartia tonsa (Dana) (a ubiquitous year-round species), and Centropages hamatus (Lilljeborg) (a winter/spring species). Under conditions of moderate to severe hypoxia 24-h survival experiments were conducted for adults and nauplii of these species from August 1994 to October 1995. Experiments on adults used a flow-through system to maintain constant d.o. concentrations. Adults of A. tonsa showed no decline in survival with d.o. as low as 1.0 ml l⁻¹, sharp declines in survival at d.o. = 0.9 to 0.6 ml l⁻¹, and 100% mortality with d.o. = 0.5 ml l⁻¹. Adults of L. aestiva and C. hamatus were more sensitive to oxygen depletion: both species experienced significant decreases in survival for d.o. = 1.0 ml l⁻¹. Nauplii of L. aestiva and A. tonsa showed no significant mortality with d.o. = 1.1 to 1.5 ml␣l⁻¹ and d.o. = 0.24 to 0.5 ml l⁻¹, respectively. In addition, experiments investigating behavioral avoidance of moderate to severe hypoxia were carried out for adults of all three species. None of the three species effectively avoided either severely hypoxic (d.o. < 0.5 ml l⁻¹) or moderately hypoxic (d.o. ≈ 1.0 ml l⁻¹) bottom layers in stratified columns. These results suggest that in␣nearshore areas where development of zones of d.o. < 1.0 ml l⁻¹ may be sudden, widespread, or unpredictable, patterns of reduced copepod abundance in bottom waters may be due primarily to mortality rather than avoidance. Received: 31 August 1996 / Accepted: 24 September 1996     

Acoustically mediated neighbor recognition in the North American bullfrog,Rana catesbeiana

Summary Playback experiments were conducted to determine whether territorial male bullfrogs (Rana catesbeiana) were capable of discriminating between advertisement calls of neighbors and strangers. Territorial males adjacent to subject males were removed and replaced with a speaker which broadcast calls of a stranger or the removed male. Ten of 11 males responded more strongly to calls of strangers than to calls of neighbors. When advertisement calls of neighbors were broadcast to subject males from positions within the removed neighbor's territory and from positions opposite the subject male, all 9 males tested responded more strongly to calls of a neighbor broadcast from a new position than to playbacks of the same call broadcast within the removed neighbor's territory. Because male bullfrogs recognize familiar calls and associate these calls with a particular direction or location, these data provide the first evidence for acoustically mediated neighbor recognition in frogs.     

Size assessment in simulated territorial encounters between male green frogs (Rana clamitans)

We examined the ability of male green frogs to assess the size of an opponent based on the dominant frequency of their advertisement call, which is negatively correlated with size, using synthetic stimuli to simulate intruders of different sizes. In one field playback experiment, we broadcast a pair of stimuli representing a small and a large male; in a second experiment, we broadcast calls of a medium and a large male. In both experiments, males produced calls with significantly lower dominant frequencies in response to each stimulus. Contrast analyses revealed that males lowered the dominant frequency of their calls more in response to the large-male stimulus than in responses to the small- and medium-male stimuli. In the second experiment, males also responded to the large-male stimulus by calling at higher rates. There were no differences in mean note duration or the number of moves made toward or around the playback speaker in response to any stimulus. Thus, the frequency of an opponent's calls elicits a differential modification of calling behavior, primarily in the form of differential dominant frequency alteration, suggesting that males use dominant frequency to assess the size of opponents during aggressive encounters. Received: 17 April 1998 / Accepted after revision: 7 October 1998     

Female preferences for multiple attributes in the acoustic signals of the Italian treefrog, <Emphasis Type="Italic">Hyla intermedia</Emphasis>

The advertisement call of frogs and toads is an example of multiple message signal because different acoustic properties encode different kinds of biologically significant information. In the Italian treefrog, Hyla intermedia, pulse rate and frequency have been found to be under stabilizing female preferences and to encode information important for mate recognition, whereas the number of calls per call group have been found to be under directional preferences and, thus, to be important for mate quality assessment. In this study, we investigate preferences for calls that differ simultaneously in frequency, pulse rate, and number of calls per call group, and we ask how these properties interact with each other in influencing female mating decisions. Results of two-choice phonotaxis experiments provide no evidence to support the hypothesis that females process multi-attribute signals in a hierarchical way. In contrast, the pattern of preferences is consistent with the ‘preference function’ hypothesis, that is, with the hypothesis that females rank signals along an ordinal scale of values and choose accordingly. Pulse rate and frequency influence mating preferences more than does the number of calls per call group. The interaction between pulse rate and frequency is not additive but multiplicative: small differences in either pulse rate or frequency that, alone, have no effects on female choice, interact synergistically so that their combination has strong influence on female preferences. A preference repeatability test shows strong among-female differences in preference for multi-attribute signals. We suggest that this result reveals not only a variation in attribute values among females, but also a variation in the way females weight and combine attribute values into a single preference score.     

Genetic divergence and phylogenetic inferences in five species of Mugilidae (Pisces: Perciformes)

Allozyme electrophoresis was used to compare the genetic divergence of global populations of Mugil cephalus Linnaeus, 1758 and two congeneric [M.␣curema Cuvier & Valenciennes, 1836; M. gyrans (Jordan & Gilbert, 1885)] and two more distantly related [Liza ramada (Risso, 1826); Xenomugil thoburni (Jordan & Starks, 1896)] species on the basis of 18 enzyme loci. The amount of genetic divergence among the species examined is in agreement with their present systematic status, the differences being larger among non-congeneric species than among species of the same genus. Intraspecific genetic distances in M. cephalus (average Nei's D= 0.154), although higher than those reported for conspecific populations of fish, appear to be small when compared to the interspecific values among mugilid species (0.821 ≤ Nei's D ≤ 1.744). Phylogenetic trees obtained by genetic distance methods and discrete character parsimony analysis were of similar topology, except for the relationships within the genus Mugil and for the arrangement of M. cephalus populations. Received: 7 April 1997 / Accepted: 4 February 1998     

Effects of municipal piers on the growth of juvenile fishes in the Hudson River estuary: a study across a pier edge

The growth rates of two fish species, the winter flounder Pseudopleuronectes americanus (Walbaum) (19.3 to 42.6 mm total length, TL) and the tautog Tautogaonitis (Linnaeus) (23.9 to 55.9 mm TL), were used to evaluate habitat quality under and around municipal piers in the Hudson River estuary, USA. Growth rates were measured in a series of 10 d field caging-experiments conducted at two large piers in the summers of 1996 and 1997. Cages (0.64 m²) were deployed along␣transects that stretched from underneath the piers to beyond them, encompassing the pier edge (the transitional zone between the pier interior and the outside). Growth in weight (G _w ) was determined at five locations along the transect, 40 m beneath the pier, 20 m beneath the pier, at the pier edge, 20 m beyond the pier edge, and 40 m beyond. Under piers, mean growth rates of winter flounder and tautogs were negative (xˉG _W = −0.02 d⁻¹), and rates were comparable to laboratory-starved control fishes (xˉG _W = −0.02 d⁻¹). In contrast, mean growth rates at pier edges and in open waters beyond piers were generally positive (xˉG _W ranged from −0.001 to +0.05 d⁻¹), with growth at pier edges often being more variable and less rapid than at open-water sites. Analyses of stomach contents upon retrieval of caged fishes revealed that dry weights of food were generally higher among fishes caged at open-water stations (xˉ range = 0.02 to 0.72 mg dry wt) than at pier-edge (xˉ range = 0.01 to 0.54 mg) or under-pier (xˉ range = 0.03 to 0.11 mg) stations, although it was apparent that benthic prey were available at all stations on the transect. Our results indicate poor feeding conditions among fishes caged under piers, and suboptimal foraging among fishes caged at pier edges. Inadequate growth rates can lead to higher rates of mortality, and, based on these and other earlier experiments, we conclude that under-pier environments are poor-quality habitats for some species of juvenile fishes. Received: 12 March 1998 / Accepted: 9 November 1998     

Low molecular-weight factor from Plesiastrea versipora (Scleractinia) that modifies release and glycerol metabolism of isolated symbiotic algae

When symbiotic dinoflagellate algae (Symbiodinium sp., isolated from the coral Plesiastrea versipora) were incubated with NaH¹⁴CO₃ in the light in seawater, they released 22.69±9.16 nmol carbon/10⁶ algae. Release of photosynthetically fixed carbon was stimulated more than six-fold for algae incubated in host-tissue homogenate (148.54±97.03 nmol C/10⁶ algae) and more than four-fold (102.00±49.16 nmol C/10⁶ algae) for algae incubated in a low molecular weight fraction (≤1 000 M _r ) prepared from host homogenate. Soluble released ¹⁴C-labelled products, as determined by chromatography and autoradiography, were the same when algae were incubated in either host homogenate or the low molecular weight fraction. After 4 h incubation in the light (300 mol photons m⁻² s⁻¹),␣intracellular␣glycerol increased in algae incubated with the low molecular weight fraction (an increase of 0.39 to␣0.67 nmol glycerol/10⁶ algae) compared with little or no increase in algae incubated in seawater (0 to 0.12 nmol glycerol/10⁶ algae). Partial inhibition of triglyceride synthesis (up to 51%) was also observed when algae were incubated in the low molecular weight fraction. All these effects are the same as those observed when algae were incubated in host homogenate. These data indicate that the “host release-factor” activity of P.␣versipora is a compound of low molecular weight. Received: 13 February 1997 / Accepted: 24 October 1997