Size-mediated response to public cues of predation risk in a tropical stream fish

In order to investigate any size-dependent differences between behavioural patterns, wild-caught Hart's rivulus Rivulus hartii of varying sizes were exposed to chemical alarm cues extracted from the skin of conspecifics or heterospecific Poecilia reticulata, or a tank water control, in a series of laboratory trials. In response to conspecific alarm cues, R. hartii subjects of the range of body sizes tested exhibited consistent, size-independent antipredator behaviours that were characterized by decreased locomotory activity and foraging levels and increased refuging behaviour. Conversely, focal R. hartii demonstrated significant size-dependent trends in response to heterospecific alarm cues, with smaller individuals exhibiting antipredator responses and larger individuals shifting their behaviour to increased levels of activity consistent with a foraging, or predatory, response. These results show that the behavioural responses of individual R. hartii to publicly available chemical alarm cues from heterospecifics are mediated by the size of the receiver.     

Larval Konosirus punctatus(Clupeidae) in a brackish river mouth on the Pacific coast of central Japan

Eggs of Konosirus punctatus in early developmental stages were collected from the eastern part of the mouth of Sagami Bay on the Pacific coast of central Japan. Advanced‐stage eggs and early larvae with notochord length ( L _N) of <7·5 mm were collected from the inner bay near the mouth of the Sagami River. Feeding larvae of >8·4 mm L _N were distributed in the mouth of the river, and juveniles of 24–90 mm standard length ( L _S) were collected from the lower reaches of the river between the river mouth and c . 3 km upstream of the river mouth. Hatch dates of larvae and juveniles collected in 2001 ( n  = 158) and in 2002 ( n  = 109) extended from late March to late July. The relationship between the otolith radius ( R _O) and L _N or L _S changed during the metamorphosis stage as characterized by 320 μm R _O and 22 mm L _S. Otolith growth rate, as an index of somatic growth rates in larval and early juvenile stages, was higher in cohorts that hatched later in the spawning season, i.e . from March to July. Konosirus punctatus that were spawned in the bay mouth area survived with different growth histories in the bay and lower reaches of the river, and recruited to the young‐of‐year population in the Pacific coastal waters of central Japan.     

Innate responses to conspecific and heterospecific alarm cues in the endangered eastern cape redfin Pseudobarbus afer

We examined innate responses to conspecific and heterospecific alarm cues in a small cyprinid minnow, the Eastern Cape redfin Pseudobarbus afer. We found that redfins respond to conspecific skin extract, which contains alarm chemicals, and showed that their preferred response is to hide in refugia. Redfins also respond to skin extract from an allopatric, distantly related minnow species, the chubbyhead barb Enteromius anoplus indicating that neither sympatry nor close phylogenetic relationships are necessary for recognition of heterospecific alarm cues. Although both conspecific and heterospecific alarm cues induced similar responses, the response to heterospecific cues was less intense. This may be explained by a trade-off between selection to maximise threat recognition and selection to avoid the costs of responding to irrelevant cues, or by differences in chemical structures of alarm cues between species. These findings have implications for the conservation of this Endangered fish species and for freshwater fishes throughout Africa.     

Consumption and gut evacuation rate of laboratory‐reared spotted seatrout (Sciaenidae) larvae and juveniles

The temperature and mass dependence of maximum consumption rate was measured for larval and early juvenile spotted seatrout Cynoscion nebulosus . Maximum consumption ( C _MAX) estimates were obtained from feeding and gut evacuation experiments on larvae (3·8–19 mm standard length, L _S) at three temperatures (24, 28 and 32° C), and maximum consumption experiments on juveniles at three temperatures (20, 26 and 32° C). Feeding levels were determined for larvae fed live prey ( Brachionus plicatilis and Artemia salina ) ad libitum . The midgut and total evacuation times were estimated for fish feeding continuously and discontinuously using alternate meals of tagged and untagged live prey. Temperature and fish size had significant effects on gut evacuation and consumption. The gut evacuation time increased with increasing fish size, and decreased with increasing temperatures. Mass‐specific midgut contents increased for small larvae <0·156 mg dry mass ( M _D)( c . 4 mm L _S), and decreased for larger larvae and juveniles. Maximum consumption was modelled by fitting a polynomial function to a reduced dataset of individuals feeding at high levels. The C _MAX model predicted an initial increase in specific feeding rate from 70 to 155% M _D day⁻¹ for small larvae, before declining for larger larvae and juveniles.     

Otolith description and age‐and‐growth of Kurtus gulliveri from northern Australia

The sagitta of Kurtus gulliveri was ovate, moderately thick with the following attributes: lateral surface convex, mesial surface flat; dorsal margin sinuate, posterior margin rounded ventrally, ventral margin rounded and irregular; sulcus divided into ostium and cauda by constriction of dorsal and ventral margins, heterosulcoid, colliculum heteromorph; dorsal depression large and distinct, ventral groove close to margin in larger otoliths; rostrum broad and antirostrum small, separated by wide, shallow excisural notch. Otolith size was moderate, average 4·6% standard length ( L _S), typical for a perciform. Annuli on 78 whole sagittae were read, and 15% of these were transversely sectioned for verification of the annuli. Males ranged from 94 to 235 mm L _S and females from 95 to 284 mm L _S. There was little difference in size distribution of the sample between the sexes, perhaps due to a 6 month spawning season over which young were continually added to the population. Some sexual dimorphism was noted, however, as age 2 year females were significantly larger than males of the same age. The largest fish aged was a 284 mm L _S, 3 year‐old female, and the oldest age reached was 4 years by two males. It appears likely that most spawning females are ≥2 years old, but some larger 1 year old fish may attain sexual maturity.     

Effects of delayed first feeding on the nutritional condition and mortality of California halibut larvae

The effect of the timing of first feeding (3, 4, 5, 6, 7 and 8 days post‐hatch, dph) on laboratory‐reared California halibut Paralichthys californicus larvae was evaluated by means of morphologic, morphometric and histological criteria. Larvae began to feed exogenously at 3 dph (2·7 ± 0·01 mm standard length, L _S) at 18° C. Eye pigmentation, rather than mouth opening was the most distinctive trait of California halibut larvae at first feeding. Larval growth was significantly affected by the time of first exogenous feeding. At notochord flexion (21 dph), the L _S of larvae fed for the first time at 3 dph was significantly larger (5·1 ± 0·1 mm) than that of those fed at 4 and 5 dph (4·9 ± 0·1 mm), although the latter fish had a more uniform size distribution. The point of no return was reached at 7 dph. Survival of larvae initially fed at 3, 4 and 5 dph was similar (58·4–60%), while no larvae were able to survive when food was offered for the first time between 6 and 8 dph. Food deprivation resulted in a progressive deterioration of the larval digestive system and atrophy of skeletal muscle fibres. Significant changes in the anterior and posterior enterocyte height were detected after 2 days of food deprivation. Similarly, tail height: L _S and trunk length: L _S ratios were the most sensitive morphometric indices to detect the effect of fasting on larval condition. Present results show that a combination of morphometric and histological variables can be used to evaluate the nutritional condition of California halibut larvae.     

Ontogenetic variability in external morphology and microhabitat use of spirlin Alburnoides bipunctatus from the River Rudava (Danube catchment)

The external morphometry of spirlin Alburnoides bipunctatus , a threatened species in parts of its native range, was re‐examined in specimens from the River Rudava, Slovakia, using geometrical shape analysis, and the relationships between morphometry and habitat use were evaluated. Spirlin <40 mm standard length ( L _S)(early and 'middle' juveniles) were found to differ significantly from those >51 mm (adults), whereas individuals between 41 and 50 mm L _S(late juveniles) represented an intermediate interval during which major changes in shape appeared to occur. Adults had proportionally smaller eyes, deeper body, and longer preanal part of the body than the early and 'middle' juveniles. These differences tended to coincide with the differences observed in microhabitat use of spirlin, namely a generally increasing preference for high velocity areas with increasing age of the fish. Thus, changes in external morphometry occurring during the ontogeny of spirlin might reflect an increasing affinity for more complex, lotic microhabitats as well as developments associated with sexual maturation.     

The feeding ecology of Morone americana larvae in the Chesapeake Bay estuarine turbidity maximum: the influence of physical conditions and prey concentrations

Feeding ecology of white perch Morone americana larvae, a major component of the ichthyoplankton community in the estuarine turbidity maximum (ETM) region of upper Chesapeake Bay, was evaluated in years of contrasting physical conditions. In 1998, high river flow led to an ETM with high turbidity and pronounced salinity stratification. In 1999, under low river flow conditions, turbidities in the ETM region were lower and the salt front was located 15 km up‐stream of its location in 1998. Copepodites and adults of Eurytemora affinis were the predominant prey in guts of all length classes of larvae (3·2–9·8 mm standard length, L _S) that were examined. Repeated‐measures, multiple regression analyses were conducted to evaluate the influence of several factors on feeding success by determining if physical conditions (temperature, current velocity, salinity, turbidity and light) and prey concentrations explained variability in mean gut fullness (gut contents mass per body mass) of small (<5 mm L _S) and large (>5 mm L _S) white perch larvae. Eurytemora affinis concentrations were significant in the statistical models for both small and large larvae. High concentrations of E. affinis , which enhanced encounter rates of white perch larvae with prey, may have been the most important factor determining feeding success in the ETM region. Larval feeding incidence (percentage of guts with food) was higher in 1998, suggesting that annual variability in river flow influenced larval feeding success by controlling physical and biological conditions in the ETM region.     

The ontogeny of habitat associations in the tropical tiger tail seahorse Hippocampus comes Cantor, 1850

This study examined how habitat associations changed with ontogeny in the tiger tail seahorse Hippocampus comes Cantor, 1850, over four reef zones in a coral reef ecosystem. Hippocampus comes showed ontogenetic differences in their use of habitat at the scale of reef zones (macrohabitat) and holdfasts (microhabitat). Across reef zones, juvenile size classes (25–105 mm standard length, L _S) were most abundant in wild macroalgal beds ( Sargassum spp.) (55·7%), while adults (>105 mm L _S) occupied both coral reefs (39·7%) and macroalgal beds (42·7%). Microhabitat use also varied with ontogeny. Juveniles generally used macroalgal holdfasts, while adults >135 mm L _S used a greater diversity of specialized microhabitats that included branching sponges, branching corals and tall seagrass. Ontogenetic changes in habitat association, as well as size-related shifts in crypsis and aggregation, suggest that H. comes experiences fitness trade-offs that vary with size; juveniles may associate with habitat that reduces predation, while larger individuals may use distinct microhabitat in reef zones to optimize reproductive success. Results are discussed in the context of targeted exploitation, expanding artisanal mariculture, habitat damage from illegal fishing and reserve design.     

Larval development of five species of blenny (Teleostei: Blenniidae) from the western central North Atlantic, with a synopsis of blennioid family characters

Light trap collections on oil and gas platforms in the northern Gulf of Mexico off Louisiana from 1995 to 1997 contained young of tessellated blenny Hypsoblennius invemar , freckled blenny Hypsoblennius ionthas , featherduster blenny Hypleurochilus multifilis , molly miller Scartella cristata and seaweed blenny Parablennius marmoreus , which are described. Interspecific differences were: number of dorsal, anal, pelvic and caudal fin elements; number of mandibular pores; presence or absence of canine teeth, hypural 5 and pectoral fin pigment; width of gill openings; length of preopercular spines (in larvae). Size at settlement differed among some taxa, but all five species settled within a narrow size window of c . 1·5 mm standard length ( L _S). Hypsoblennius invemar , H. ionthas , H. multifilis and S. cristata settled at mean sizes between 11·3 and 12·1 mm L _S, whereas P. marmoreus settled at a mean size of 19·3 mm L _S. Sexually dimorphic differences were consistently evident by 17–18 mm L _S in all species but P. marmoreus . The two smallest blennies with external characters normally associated with sexual maturity were a 20 mm male and 21 mm female H. multifilis . Primary caudal fin rays began to bifurcate between 17·0 and 18·3 mm L _S in H. invemar , H. ionthas , H. multifilis and S. cristata . Hypleurochilus multifilis displayed the external characteristics of being sexually mature at 20–21 mm L _S. Thus, bifurcation of primary caudal fin rays was an indicator of approaching sexual maturity in H. multifilis and this may also be true in the other species studied.     

Opercular beat rate for rainbow darters Etheostoma caeruleum exposed to chemical stimuli from conspecific and heterospecific fishes

Opercular beat rates of rainbow darters Etheostoma caeruleum were used as a measure of a physiological response to chemical stimulation. Rainbow darters responded significantly to some chemical cues (active and ambush predators, competitors, novel stimuli and to conspecific and heterospecific alarm cues) with increases in opercular movements; neutral cues and novel alarm cues did not elicit significant changes. Changes in opercular movements may be a good bioassay for determining detection of chemical stimuli by rainbow darters.     

Avoidance behavior in two sympatric planaria species: effects of conspecific and heterospecific chemical alarm cues

In many aquatic animals, predator avoidance can be stimulated by chemical cues, including those released by injured prey (alarm cues). Alarm cues of both conspecific and heterospecific origin have been identified within several fish taxa, where phylogenetic conservation of the cue-response complex is common. Turbellarian flatworms (planaria) are among the simplest animals known to respond to chemical cues released by injured conspecifics. We examined how two locally sympatric planaria species respond to conspecific and heterospecific chemical cues using macerated tissue suspensions. Brown (Girardia tigrina) and black (Dugesia dorotocephala) planaria both exhibited avoidance behavior when presented conspecific cues. Despite a significant twofold difference in body size (black > brown), stimulus prepared from a single (1×) individual of either species elicited avoidance. Increasing brown planaria cue concentration by macerating two individuals (2×) produced a significant increase in conspecific avoidance. Heterospecific stimuli produced asymmetric results. Black planaria avoided the brown planaria stimulus, but only in the higher concentration (2×) trials. Brown planaria did not consistently exhibit avoidance of the black planaria stimulus and some brown subjects approached and consumed black planarian tissues. Our results expand the demonstrated occurrence of alarm cues among planaria and suggest that avoidance behavior can be mediated by multiple environmental and intrinsic factors in freshwater Turbellaria.     

Detection of conspecific and heterospecific alarm signals by juvenile pumpkinseed under weak acidic conditions

Under neutral (pH 7·0) conditions, juvenile pumpkinseed Lepomis gibbosus exhibited significant antipredator responses, of similar intensities, to the chemical alarm cues of conspecifics, an allopatric congener the green sunfish Lepomis cyanellus and the artificial alarm cue of a sympatric prey guild member (Cyprinidae, hypoxanthine-3- N -oxide). Under weakly acidic conditions (pH 6·0), however, no increase in antipredator behaviour was seen in response to hypoxanthine-3- N -oxide and a quantitatively weaker response was found in response to conspecific and congener cues, suggesting that the use of chemical alarm cues by some prey fishes may be impaired by acid precipitation.     

Chemical Alarm Cues Are Conserved within the Coral Reef Fish Family Pomacentridae

Fishes are known to use chemical alarm cues from both conspecifics and heterospecifics to assess local predation risks and enhance predator detection. Yet it is unknown how recognition of heterospecific cues arises for coral reef fishes. Here, we test if naïve juvenile fish have an innate recognition of heterospecific alarm cues. We also examine if there is a relationship between the intensity of the antipredator response to these cues and the degree to which species are related to each other. Naïve juvenile anemone fish, Amphiprion percula, were tested to see if they displayed antipredator responses to chemical alarm cues from four closely related heterospecific species (family Pomacentridae), a distantly related sympatric species (Asterropteryx semipunctatus) and a saltwater (control). Juveniles displayed significant reductions in foraging rate when exposed to all four confamilial heterospecific species but they did not respond to the distantly related sympatric species or the saltwater control. There was also a strong relationship between the intensity of the antipredator response and the extent to which species were related, with responses weakening as species became more distantly related. These findings demonstrate that chemical alarm cues are conserved within the pomacentrid family, providing juveniles with an innate recognition of heterospecific alarm cues as predicted by the phylogenetic relatedness hypothesis.     

Growth variation in larval Makaira nigricans

The Atlantic blue marlin Makaira nigricans larvae were collected from Exuma Sound, Bahamas and the Straits of Florida over three summers (2000–2002). Sagittal otoliths were extracted and read under light microscopy to determine relationships between standard length ( L _S) and age for larvae from each year and location. Otolith growth trajectories were significantly different between locations: after the first 5–6 days of life, larvae from Exuma Sound grew significantly faster than larvae from the Straits of Florida. Exponential regression coefficients were similar among years for Exuma Sound larvae (mean instantaneous growth rate, G _L = 0·125), but differed between years for larvae from the Straits of Florida ( G _L = 0·086–0·089). Differences in larval growth rates between locations resulted in a 4–6 mm difference in L _S by day 15 of larval life. These differences in growth appeared to be unrelated to mean ambient water temperatures, and may have been caused by location‐specific differences in prey composition or availability. Alternatively, population‐specific differences in maternal condition may have contributed to these differences in early larval growth.     

Daily ration and prey size of juvenile piscivore Japanese Spanish mackerel

Daily ration of juvenile Japanese Spanish mackerel Scomberomorus niphonius (32·1–33·1 mm standard length, L _S) was estimated at three temperatures (18·6, 19·5 and 21·2° C) in the laboratory. Gastric evacuation rate ranged between 0·398 (18·6° C) and 0·431 (21·2° C). Diel change in instantaneous consumption rate indicated that juvenile Japanese Spanish mackerel are daylight feeders. The estimated values of the daily ration ranged between 66·1%(18·6° C) and 82·6%(21·2° C) of body mass. These per cent values of daily ration were converted to daily consumption in mg (28 mg at 18·6° C to 34 mg at 21·2° C) using the mean dry body mass of juvenile Japanese Spanish mackerel of 30 mm (42·1 mg). Stomach content analysis of wild specimens collected in the Seto Inland Sea, south‐western Japan, revealed that the majority of wild Japanese Spanish mackerel larvae and juveniles ingested fish larvae with a body size >50% of the predator L _S. Based on the predator‐prey size relationship, the daily consumption of a Japanese Spanish mackerel juvenile of 30 mm was equivalent to 5·1 (18·6° C) to 6·4 (21·2° C) Japanese anchovy Engraulis japonicus larvae which was the dominant prey organism in stomachs of the wild Japanese Spanish mackerel.     

The ontogeny of chemically mediated antipredator responses of fathead minnows Pimephales promelas

The antipredator responses of adult and larval fathead minnows Pimephales promelas to chemical alarm cues prepared throughout ontogeny were tested using various behavioural assays. Larval epidermis was also examined during ontogeny using standard haematoxylin and eosin staining techniques. Adults elicited an antipredator response to chemical alarm cue made from larvae as young as 8–17 days post‐hatch. Interestingly, larvae did not possess visible club cells until 28–37 days post‐hatch and did not respond to conspecific chemical alarm cue until 48–57 days post‐hatch. These results suggest that chemical alarm cue may not be contained within club cells and that the components of larval and adult chemical alarm cue may be similar throughout ontogeny.     

Responses of larval dragonflies to conspecific and heterospecific predator cues

Abstract.  1. In cannibalistic populations, smaller individuals are subject to predation by larger conspecifics, and small individuals commonly alter their behaviour in response to cannibals. Little is known, however, about the underlying cues that trigger such responses and how the behavioural responses to conspecific cannibals differ from heterospecific predators.
2. This study tests which cues are used for the detection of conspecific predators in the larva of the dragonfly Plathemis lydia and how the behavioural response to cannibals differed from the response to heterospecific predators.
3. Individuals were exposed to chemical cues, visual cues, and a combination of both cues from conspecifics as well as no predator and heterospecific predator controls during which their activity and feeding rates were observed.
4. Individuals increased their activity, spatial movement and feeding behaviour in response to either visual or chemical cues from conspecific predators, which was opposite to responses displayed with cues from heterospecific predators. Interestingly, the responses to visual and chemical cues from conspecifics combined were weaker than to either cue in isolation and similar to the no cue control.
5. The results clearly indicate that individuals are able to use chemical and visual cues to detect even very subtle differences in phenotype of conspecific predators.
6. The opposite response in behaviour when exposed to conspecific cannibals vs. heterospecific predators suggests that the presence of cannibals will increase the mortality risk of small individuals due to heterospecific predation. This risk-enhancement is likely to have important consequences for the dynamics of predator–prey interactions.     

Development of the pectoral, pelvic, dorsal and anal fins in cultured sea bream

The pectoral fin girdle was the first element of the fins to develop in Sparus aurata. By 3·1mm L _N (notochord length) the cleithrum was ossified and the cartilaginous caracoid-scapula was present. The fin was fully developed at 11·6 mm L _S (standard length) and by 16·0 mm L _S most elements of the fin were ossified. The pelvic fins were the last pair to develop and rudiments of these were first detected at 7·9 mm L _S. The pelvic fin and girdle were completely formed and ossified at 16·0 mm L _S. The development of dorsal and anal fins began at c. 6·5–7·0 mm L _S with the formation of 10 cartilaginous dorsal proximal radials and eight cartilaginous ventral proximal radials. The three cartilaginous predorsals (supraneurals) appeared at 7·7 mm L _S and the ossification of dorsal and anal proximal and distal radials began, respectively, at 10·5 mm L _S and 11·3 mm L _S. Ossified structures in the fins were also classified according to their origin, as being either dermal or endochondral. Finally the chronology of appearance of fin structures in S. aurata was compared with that reported for other Sparidae, Engraulidae and Haemulidae.     

Anti‐predator response of naïve and experienced common bully to chemical alarm cues

Histological analysis of the skin of common bully Gobiomorphus cotidianus , a New Zealand native eleotrid fish, revealed the presence of club cells in the epidermis. Epidermal club cells are frequently associated with the production of alarm substance (Schreckstoff). The behavioural responses of perch‐naïve and perch‐experienced common bullies to either conspecific skin extract or chemical cues from an introduced predator, perch Perca fluviatilis , were then examined. Both perch‐naïve and perch‐experienced common bullies exhibited a behavioural response when exposed to conspecific skin extract, indicating the probable presence of an alarm substance. In contrast, only perch‐experienced common bullies recognized and exhibited a subsequent behavioural response to the odour of perch. This study is the first to document the presence of epidermal club cells and a behavioural response to a conspecific chemical alarm signal for fishes in the Eleotridae. The results indicate that common bully can learn to recognize perch odour as a threat, and that this ability may be a result of previous predator labelling involving a conspecific alarm substance.