Evolution of larval size in cyclostome bryozoans

Cyclostomes are the only order of stenolaemate bryozoans living today. The non-feeding larvae of modern cyclostomes metamorphose on settlement to produce a calcified dome-shaped protoecium. Protoecial diameter provides a proxy for larval size. The sparse data available on living cyclostomes suggests that protoecial diameter is about one-and-a-half times greater than larval width. Here we use protoecial diameter to estimate larval sizes in fossil and Recent cyclostome species. A total of 233 protoecia were measured, 143 from Recent cyclostomes and 90 from fossil cyclostomes, of which 84 came from the Jurassic. Protoecial diameter ranged from 82.5 to 690 μm, with 89% of protoecia having diameters between 100 and 300 μm. A comparison of 30 Jurassic with 51 Recent taxa of tubuliporine cyclostomes showed a significant difference in size frequency. Although the Recent taxa have a larger size range (83–465 μm) than the Jurassic taxa (125–249 μm), Recent species have a lower mode (125–150 μm) than the Jurassic species (175–200 μm). Most Jurassic cyclostomes may therefore have had larger larvae than their extant relatives. Reduction in larval size may be a component of the previously hypothesized reduction in overall body size resulting from competitive displacement by cheilostome bryozoans.     

Influence of delayed metamorphosis on postsettlement survival and growth in the sipunculan Apionsoma misakianum

Abstract. Certain stresses experienced by marine larvae from many groups can dramatically reduce aspects of juvenile performance. This study reports the effects of delayed metamorphosis and nutritional stress on survival and growth of the deposit-feeding sipunculan Apionsoma (= Golfingia ) misakianum . Approximately 600 larvae collected from the Florida Current plankton were distributed among 3 treatment groups. Ninety larvae (controls) were offered sediment and adult-conditioned seawater 4 d after collection, to induce metamorphosis; larvae of this species could not be induced to metamorphose by increasing the K⁺ concentration of seawater. The remaining 500 larvae were kept swimming for either 2 or 4 weeks, with or without phyto-plankton (clone T-ISO). At the end of the periods of prolonged larval swimming, subsampled larvae (360) were induced to metamorphose as in the controls. Surviving individuals were retrieved 6 weeks after the addition of excess sediment in all treatments, and weighed to document growth. Neither delayed metamorphosis nor starvation influenced juvenile survival. However, starving larvae for 2 weeks significantly reduced mean juvenile growth rates relative to the mean growth rate of control individuals (p<0.0001), while prolonging larval life by 4 weeks significantly reduced mean juvenile growth rates (p<0.05) whether or not larvae were fed. Reduced juvenile growth rates may have been caused by nutritional stress experienced by larvae in both the starved and fed treatments. The rapid response of freshly collected larvae to sediment indicates that competent larvae of this species routinely delay metamorphosis in the field. The extent to which they also experience food limitation is not yet clear. If competent larvae are food limited while delaying metamorphosis in the field, our results suggest that juveniles will grow more slowly and may thus exhibit reduced fitness.     

Using KCl to determine size at competence for larvae of the marine gastropod Crepidula fornicata (L.)

Competent larvae of the marine gastropod Crepidula fornicata (L.) were induced to metamorphose (i.e., lose the velum) by elevating sea-water [KCl] by 5–50 mM. The response was optimal at 15–20-mM elevations, at which 50% metamorphosis was obtained in <4 h. Larvae that did not metamorphose during brief exposures (1–5 h) to elevated [KCl] generally maintained the larval form following transfer to control sea water, suggesting that competent larvae must be continuously immersed in the test solutions for metamorphosis to occur. The smallest larvae to respond to elevated [KCl] had shell lengths of ≈700–800 μm, the range of shell lengths within which larvae of this species become responsive to natural inducers. All larvae >≈1125 μm shell length metamorphosed in response to increased [KCl]. Rearing temperature may affect the size at which larvae of this species become responsive to K⁺. CaCl₂ (20-mM concentration elevations), GABA (4×10⁻⁷, 4×10⁻⁶ M), and NaCl (10–20-mM concentration elevations) generally failed to trigger metamorphosis. Twenty-mM elevations of [RbCl] and [CsCl] induced 100% metamorphosis but the juveniles were immobile and died after several days. Elevating [KCl] appears to be a reliable way to assess competence and trigger metamorphosis in larvae of C. fornicata.     

Effects of food availability on larval development in the slipper limpet Crepidula onyx (Sowerby)

Effects of food availability on the larval survival and development of Crepidula onyx were studied in four experiments by feeding the larvae with different concentrations of the chrysophyte Isochrysis galbana and by starving the larvae for different periods of time. Food concentration had a clear impact on the survival, growth and development time of C. onyx veligers. Larval development occurred only at 10⁴ cells ml⁻¹ and higher algal concentrations. No shell increment was detected in the veligers cultured for 12 days at 10² cells ml⁻¹I. galbana or the blank control. At 10³ cells ml⁻¹, there was only a slight increase in shell length over 12 days. At 10⁴ cells ml⁻¹, about 40% of the larvae became competent in 18 days. At 10⁵ and 10⁶ cells ml⁻¹, more than 90% of the larvae reached competence in 7 days. Initial starvation negatively affected the larval development, but the sensitivity differed among parameters measured on day 5: lower survivorship was detected only for larvae that had suffered 3 days or longer initial starvation, whereas one-day initial starvation caused shorter shells and lower percentage of competent larvae. Three days of continuous feeding was required for 50% of the larvae to reach competence. After feeding for 3 days, most larvae could become competent to metamorphose even under starvation. The time of starvation was also critical: larvae that suffered 1-day food deprivation in the first 2 days of larval release had shorter shells and lowered percent competent larvae than those that suffered the same length of food deprivation in later stages of development. Our study thus indicates that both food concentration and short-term starvation have detrimental effects on the larval development of this species, and that once the larva has consumed certain amount of food, starvation may induce metamorphosis.     

Relationships between larval nutritional experience, larval growth rates, juvenile growth rates, and juvenile feeding rates in the prosobranch gastropod Crepidula fornicata

Planktonic larvae experiencing short periods of starvation or reduced food supply often grow and develop more slowly, have poor survival, fail to metamorphose, metamorphose at smaller sizes, or grow slowly as juveniles. In this study, we examined the impact of short periods of food limitation at various stages of larval development on larval and juvenile growth in Crepidula fornicata. In addition, we considered whether juveniles that were stressed as larvae grew poorly because of reduced rates of food collection due to impaired gill function. For 5 experiments, larvae were either starved for several days beginning within 12 h of hatching or were starved for the same number of days following 1 or more days of feeding at full ration (cells of the naked flagellate Isochrysis galbana, clone T-ISO, at 18×10⁴ cells ml⁻¹). In one experiment, larvae were transferred for 2 or 4 days to seawater with extremely low phytoplankton concentration (1×10⁴ cells ml⁻¹). In all experiments, larvae were returned to full ration following treatment. Control larvae were fed full ration from hatching to metamorphosis. When larvae reached shell lengths of about 900 μm they were induced to metamorphose and then reared individually at full ration in glass bowls, with phytoplankton suspension replenished daily. Larval and juvenile growth rates were determined by measuring changes in shell length (longest dimension) over time. Juvenile feeding rates were determined by monitoring changes in phytoplankton concentration over 2–3 h at the end of the growth rate determinations. In general, larval growth rates for the first 2 days after the resumption of feeding were inversely proportional to the length of time that larvae were starved. However, larval growth rates ultimately recovered to control levels in most treatments. Starving the larvae caused a significant reduction in initial juvenile growth rates (first 3–4 days post-metamorphosis) in most experiments even when larval growth rates had recovered to control levels prior to metamorphosis. Juvenile growth rates were not significantly reduced when larvae were subjected to reduced food availability (1×10⁴ cells ml⁻¹), even for treatments in which larval growth rates were compromised. Mean weight-specific filtration rates for juveniles were significantly reduced (p<0.05) following larval feeding experience in only one or possibly 2 of the 4 experiments conducted. Our data suggest that although larvae of C. fornicata may fully recover from early nutritional stress, the resulting juveniles may exhibit poor initial growth due to impaired gill function, reduced digestive capability, or reduced assimilation efficiency.     

Larval size relative to larval feeding, cannibalism of larvae, egg or adult female size and larval–adult setal patterns among 13 phytoseiid mite species

Phytoseiid mite larvae vary in size and feeding type. We compared larval size to feeding by larvae, cannibalism of larvae by adult females, egg and adult female size and the setae lengths of larvae and adults among 13 species. There was no relationship between size of larvae and either feeding by larvae or cannibalism of larvae by adult female mites. Correlations were highest between larval size as measured by idiosoma plus extended leg lengths and adult female size of idiosoma plus extended leg lengths (r²=0.746), while next highest was larval idiosoma length and adult female idiosoma length (r²=0.662) and then larval idiosoma length and egg length (r²=0.579). Based on idiosoma length, Phytoseiulus persimilis had the largest larvae (non-feeding) among species and Euseius finlandicus had the smallest larvae (obligatory feeding). However, based on idiosoma length plus extended leg length, obligatory feeding larvae (on pollen or mites) of E. finlandicus and Euseius hibisci were largest and facultative feeding larvae (on mites) of Neoseiulus californicus and obligatory feeding larvae (on mites) of Galendromus occidentalis were the smallest. Among species with non- or facultative feeding larvae, Amblyseius andersoni and Neoseiulus barkeri had larger larvae and Typhlodromus pyri and Neoseiulus fallacis had smaller larvae when leg lengths were included in larval size. Setae lengths of larvae versus adult females (after adjustment for body sizes) showed high correlation for j6 (r²=0.942) and s4 (r²=0.854), but low correlation for larval Z4 versus adult female Z4 (r²=0.084) or Z5 (r²=0.063). Overall, larval morphological traits were most closely correlated to traits of other life stages, although for setae there were some exceptions. Differences in the functions of setae j6, s4 and Z4 in the larva versus adult female are discussed.     

Effect of delayed metamorphosis on larval competence, and post-larval survival and growth, in the abalone Haliotis iris Gmelin

Marine invertebrate species vary in their ability to delay metamorphosis, and in the degree to which delayed metamorphosis compromises juvenile performance. Abalone (Haliotis iris) larvae were deprived of metamorphosis cues and the effects of delayed metamorphosis on larval competence, and post-larval growth and survival were quantified. Larvae were exposed to a metamorphosis inducer (the coralline alga Phymatolithon repandum (Foslie) Wilks and Woelkerling) on Days 11, 18, 22, 26, 30 and 34 post-fertilisation (temperature 16-17 degrees C). Post-larvae were reared on diatoms (Nitzschia longissima Grunow) for 3-4 weeks post-metamorphosis. Delayed metamorphosis caused progressive negative effects on post-larval performance. Virtually all larvae initiated metamorphosis in response to P. repandum, regardless of larval age. The proportion of post-larvae that developed post-larval shell growth within 2 days of metamorphosis induction dropped only approximately 20% from Day 11 to Day 26 (P>0.05), but was significantly lower by Day 30 and Day 34 (P<0.001). Larvae that metamorphosed on Days 11, 18 and 22 showed high survival (>80%) and growth rates (means of 20-22 μm shell length per day). In contrast, larvae that metamorphosed on Day 26 and Day 30 had poor survival (30-40%) and lower (P<0.05) growth rates (15-16 μm/day). Of the larvae that metamorphosed on Day 34, only 7 (30%) survived their first week post-metamorphosis, and they grew only 2 μm/day on average. Only one of these post-larvae (4%) survived the second week. The visible yolk supply diminished over the life of the larvae and was near zero by Day 34. Nearly all larvae had died by Day 38. H. iris larvae remained competent to metamorphose for at least 3 weeks after they attained competence. Post-larval growth and survival were not reduced if metamorphosis occurred within 3 weeks of fertilisation. This extended period of larval competence implies that H. iris larvae can potentially disperse for up to several weeks before successful metamorphosis.     

Effects of delayed settlement on post-settlement growth and survival of scleractinian coral larvae

Demographic connectivity requires both the dispersal of individuals between sub-populations, and their subsequent contribution to population dynamics. For planktonic, non-feeding marine larvae, the capacity to delay settlement enables greater dispersal distances, but the energetic cost of delayed settlement has been shown to adversely impact post-settlement fitness in several taxa. Here, we assess whether delayed settlement influences mortality rates or growth rates for the first 6 weeks following settlement of the scleractinian coral, Acropora tenuis. Coral larvae that were settled at 2, 4, and 6 weeks after spawning, and then deployed in the field, showed negligible effects of delayed settlement on post-settlement survival and time to initial budding for colony formation. Between-cohort differences in budding rate appeared to be explained by temporal variation in the post-settlement acquisition of zooxanthellae. The potential for coral larvae to remain in the pelagic zone for increased periods of time with little to no effect on post-settlement survival and growth suggests that the capacity for delayed settlement is likely to have meaningful demographic consequences for broadcast-spawning reef-building corals, and that the predicted trade-off between delayed settlement and post-settlement fitness is less applicable to reef-building scleractinian corals than other taxa with non-feeding larvae.     

The surf zone: a semi-permeable barrier to onshore recruitment of invertebrate larvae?

The supply of larvae to the shore is important for population replenishment and intertidal community dynamics but its variability at most scales is not well understood. We tested the relationship between nearshore mussel larval abundance and intertidal settlement rates over several years at multiple spatiotemporal scales in Oregon and New Zealand. Abundance of competent larvae nearshore and intertidal recruitment rates were simultaneously quantified using collectors mounted at different depths on moorings 50-1100 m from shore, and at adjacent rocky intertidal sites. Total mussel larval abundance and oceanographic conditions were also measured in some locations. At all scales, abundance of nearshore mussel larvae was unrelated to intertidal recruitment rates. In the intertidal, patterns of mussel recruitment were persistent in space, with sites of consistently high or low recruitment. In contrast, nearshore competent larval abundance showed generally similar abundances among sites except for a high, and spatially-inconsistent, variability in Oregon during 1998 only. On moorings, recruitment tended to be greater on midwater collectors than shallower or deeper. Finally, on moorings larval abundance in traps and recruitment on collectors was unrelated. These results suggest that (1) among sites, the size of the nearshore larval pool is relatively uniform while onshore recruitment varies and is unrelated to larval abundance, (2) temporal variability in nearshore larval availability is not strongly expressed onshore, (3) vertical stratification of competent larvae nearshore is strong and may influence transport and recruitment, and (4) within-coast variability in onshore recruitment is strongly driven by processes occurring locally within the surf zone that need to be studied to understand coastal recruitment dynamics.     

Red foliage color reliably indicates low host quality and increased metabolic load for development of an herbivorous insect

Plant chemical defense and coevolved detoxification mechanisms in specialized herbivorous insects are fundamental in determining many insect–plant interactions. For example, Brassicale plants protect themselves from herbivory by producing glucosinolates, but these secondary metabolites are effectively detoxified by larvae of Pierid butterflies. Nevertheless, not all Brassicales are equally preferred by these specialist herbivores. Female Pieris butterflies avoid laying eggs on anthocyanin-rich red foliage, suggesting red color is a visual cue affecting oviposition behavior. In this study, we reared P. brassicae larvae on green and red cabbage leaves, to determine whether foliage color reliably indicates host plant quality. We did not find a difference in survival rates or maximal larval body mass in the two food treatments. However, larvae feeding on red cabbage leaves exhibited significantly lower growth rates and longer durations of larval development. Interestingly, this longer development was coupled with a higher consumption rate of dry food matter. The lower ratio of body mass gain to food consumption in larvae feeding on red cabbage leaves was coupled with significantly higher (ca. 10 %) larval metabolic rates. This suggests that development on red foliage may incur an increased metabolic load associated with detoxification of secondary plant metabolites. Energy and oxygen allocation to detoxification could come at the expense of growth and thus compromise larval fitness as a result of extended development. From an evolutionary perspective, red foliage color may serve as an honest defensive cue, as it reliably indicates the plant’s low quality as a substrate for larval development.     

Rapid settlement in broadcast spawning corals: implications for larval dispersal

Broadcast spawning of gametes with planktonic development of larvae is the most common reproductive mode in tropical corals, and is generally thought to optimize the dispersal potential of larvae. To this end, many previous studies of coral larval dispersal have focused on the maximum time larvae can remain competent to settle and consequently how far they might disperse. However, dispersal ability of broadcast-spawned coral larvae will be linked, at least in part, to the minimum time to settlement competency as well as the length of the planktonic period—although estimates of minimum time to competency remain largely anecdotal, with few rigorous studies of the pre-competent period. To determine the minimum time to larval settlement in two species of broadcast-spawning coral ( Platygyra daedalea and Goniastrea favulus), we monitored larval settlement rates in aquaria every 6 h from the time larvae commenced swimming (i.e. were ciliated, fully developed larvae) for a period of approximately 10 days. For P. daedalea, peak settlement occurred between 60 and 66 h following fertilization (2.5 and 2.75 days), which is markedly earlier than the 4- to 6-day time period commonly cited as the minimum time before broadcast-spawned coral larvae are competent to settle. Surprisingly, it was also clear from our experimental results that settlement in P. daedalea occurred as a distinct pulse during the 60- to 66-h period, rather than continuously throughout the study period. G. favulus larvae also appear to be able to settle quickly (from 54 h following fertilization). We argue, on the basis of these short competency times and apparently rapid settlement, that dispersal in broadcast-spawning coral larvae may not be as great as has previously been assumed.     

Loss of larval parasitism in parasitengonine mites

Larval Parasitengona are typically parasites, yet at least 29 species of water mites and one species of Trombidiidae forgo larval feeding and any association with a host. Species with non-feeding larvae are isolated cases within species groups or genera where the remaining species have parasitic larvae. Species without larval parasitism occur in at least 14 genera, eight families and four superfamilies of water mites; the loss of larval parasitism is presumably polyphyletic, having occurred at least 21 times. Lineages of water mites with non-feeding larvae frequently exist in parallel with almost identical populations or species that have parasitic larvae. Thus, there is tremendous potential for studies comparing the relative merits of the two life history strategies. Comparisons indicate that adults from lineages with non-parasitic larvae produce smaller numbers of larger eggs; the extra nutrition included in larger eggs permits the larvae to forgo feeding. Non-feeding larvae frequently have wider dorsal plates but reduced leg length, setal length and sclerotization when compared to parasitic larvae from sister lineages. The adults of lineages with non-feeding larvae are frequently smaller in comparison to adults of sister lineages with parasitic larvae. There is no apparent pattern in relation to habitat: lineages lacking larval parasitism occur in streams, temporary ponds and the littoral and planktonic regions of permanent lakes. © Rapid Science Ltd. 1998     

Seasonal variations in larval biomass and biochemical composition of brown shrimp, Crangon crangon (Decapoda, Caridea), at hatching

The “brown shrimp”, Crangon crangon (Linnaeus 1758), is a benthic key species in the North Sea ecosystem, supporting an intense commercial fishery. Its reproductive pattern is characterized by a continuous spawning season from mid-winter to early autumn. During this extended period, C. crangon shows significant seasonal variations in egg size and embryonic biomass, which may influence larval quality at hatching. In the present study, we quantified seasonal changes in dry weight (W) and chemical composition (CHN, protein and lipid) of newly hatched larvae of C. crangon. Our data revealed significant variations, with maximum biomass values at the beginning of the hatching season (February–March), a decrease throughout spring (April–May) and a minimum in summer (June–September). While all absolute values of biomass and biochemical constituents per larva showed highly significant differences between months (P < 0.001), CHN, protein and lipid concentrations (expressed as percentage values of dry weight) showed only marginally significant differences (P < 0.05). According to generalized additive models (GAM), key variables of embryonic development exerted significant effects on larval condition at hatching: The larval carbon content (C) was positively correlated with embryonic carbon content shortly after egg-laying (r ² = 0.60; P < 0.001) and negatively with the average incubation temperature during the period of embryonic development (r ² = 0.35; P < 0.001). Additionally, water temperature (r ² = 0.57; P < 0.001) and food availability (phytoplankton C; r ² = 0.39; P < 0.001) at the time of hatching were negatively correlated with larval C content at hatching. In conclusion, “winter larvae” hatching from larger “winter eggs” showed higher initial values of biomass compared to “summer larvae” originating from smaller “summer eggs”. This indicates carry-over effects persisting from the embryonic to the larval phase. Since “winter larvae” are more likely exposed to poor nutritional conditions, intraspecific variability in larval biomass at hatching is interpreted as part of an adaptive reproductive strategy compensating for strong seasonality in plankton production and transitory periods of larval food limitation.     

Adrenoceptor Compounds Prevent the Settlement of Marine Invertebrate Larvae: Balanus amphitrite (Cirripedia), Bugula neritina (Bryozoa) and Hydroides elegans (Polychaeta)

The effects of the neurotransmitter blockers idazoxan and phentolamine on the larval settlement of three marine invertebrate species belonging to three different phyla were investigated by using in vitro concentration-response bioassays. Since neurotransmitters are known to influence metamorphic transitions in invertebrate larvae, neurotransmitter blockers were tested to evaluate their sublethal effects on larvae. The α-adrenergic antagonists idazoxan and phentolamine inhibited settlement of Balanus amphitrite (Cirripedia), Bugula neritina (Bryozoa) larvae, and larvae of the polychaete Hydroides elegans (Polychaeta) in a concentration- and taxon-dependent manner. At concentrations of 10^?3 M of both agents, larvae of all three species became immobile and subsequently died within 24 h. While cumulative settlement rates were observed after 48 h for B. amphitrite and H. elegans, and after 5 h for B. neritina, > 90% of the larvae that settled did so within 24 h for the first two species and within 1 h for B. neritina. The tendency of the hydrophobic idazoxan and phentolamine to accumulate at solid surfaces most probably contributes to their successful inhibition of larval settlement. This ability makes them particularly attractive as candidates for the development of slow-release carriers in antifouling paints.     

Influence of lowered salinity and elevated cadmium on the survival and metamorphosis of trochophores in Capitella sp. I

Abstract. It is well known that the competent larvae of many marine invertebrate species can be stimulated to metamorphose by exposing them to elevated concentrations of certain ions, neuroactive substances, and pharmacological agents. In this study we report that larvae of the euryhaline polychaete Capitella sp. I are induced to metamorphose within 24–48 h by reducing salinity from 30%‰ to 12–15%‰. At 10%‰, however, fewer larvae metamorphosed and the normal metamorphic response to sediment was inhibited; competence was restored within 3 h of transferring the larvae back to full strength seawater (30%‰). Larvae also metamorphosed within 24 h in response to increased external cadmium concentrations of 1000–2000 μg 1^–1. Further understanding of how reduced salinity and elevated cadmium stimulate (or inhibit) metamorphosis may lead to a better understanding of signal perception and transduction in this and other species.     

Der respiratorische Stoffwechsel von Calliphoridenlarven in Beziehung zu Temperaturadaptation und Regulation

The oxygen consumption of five larval stages of the Calliphoridae Callitroga macellaria, Lucilia cuprina, and Calliphora vicina was analysed at different temperatures. There are differences in temperature adaptation in these three species as well as in the feeding and non-feeding larvae of Callitroga.Within fixed limits of tolerance the reaction to changes in temperature is more intensive in feeding than in non-feeding larvae. A high metabolic rate is related to high levels of sensitivity to temperature changes.At particular temperatures oxygen consumption generally decreases with age. The results partly confirm the well-established fact that at the same temperature the metabolic rate of ‘cold’-adapted stages and species is relatively high as compared to ‘warm’-adapted ones.During the course of experiments oxygen consumption was frequently inconstant as can be seen by comparing the oxygen consumption of the first and second parts of the experiments, or more exactly, by regression analysis of the oxygen consumption rates throughout these experiments. Increasing and decreasing oxygen consumption depends on temperature adaptation which indicates regulatory processes.A method to determine standard temperature using measurements of respiratory metabolism is discussed.     

Benthic assemblages and temperature effects on Paracentrotus lividus and Arbacia lixula larvae and settlement

Recruitment is a principal controlling factor in population dynamics of marine species. In marine invertebrates with a planktonic larval stage, such as echinoids, recruitment is assured by larval supply, settlement and juvenile survival. Larval supply and juvenile survival are affected by a wide range of factors, including temperature, presence of predators, quality and quantity of food. Echinoid larval settlement is mainly conditioned by the finding of a suitable substrate to metamorphose. The sea urchins Arbacia lixula and Paracentrotus lividus are considered key species of the Mediterranean infralittoral rocky shores. At high densities, the grazing activity of both species can produce and maintain barren grounds, a particular habitat condition characterized by extremely low cover values of erect algae with high presence of naked substrates and encrusting corallinales, poor in biodiversity and ecosystem functions. We tested the role of different settlement substrates on the metamorphosis competent larvae of the two species. Furthermore, from our larval rearing trails we were able to identify strong temperature effects on larval development of the two species. P. lividus and A. lixula larvae have been reared at 18 °C but for the second species it was necessary to use higher temperatures (22 °C) to perform settlement experiments, as in the 18 °C set all larvae died in the first week. Both species larvae have been fed Cricosphaera elongata. Metamorphosis of competent larvae has been induced using different substrates: naked stones, Lithophyllum incrustans, Stypocaulon scoparium, Corallina elongata, turf forming algae and Posidonia oceanica. For each species, two larval batches were used for settlement experiments; for each larval batch two replicates/substrates were set up. No differences in the rate of metamorphosis on any of the tested substrates were observed for P. lividus, while A. lixula showed to prefer naked stones and encrusting coralline algae Considering that A. lixula population growth may trigger barren extension on rocky shores, this may lead to a positive feedback between barren extension and A. lixula population density. Furthermore, our results suggest that the predicted rise in seawater temperature may favor A. lixula larval survival and inhibit P. lividus. Combining information on temperature tolerance with other sources of information for these species in the Mediterranean, it is possible to develop a conceptual model of the interaction between the two species and the alternative state of their habitats.     

Linking the evolution of habitat choice to ecosystem functioning: direct and indirect effects of pond-reproducing fire salamanders on aquatic-terrestrial subsidies

Shifts in life history traits and in the behaviour of species can potentially alter ecosystem functioning. The reproduction of the central European fire salamander (Salamandra salamandra), which usually deposits its larvae in first-order streams, in small pool and pond-like habitats, is an example of a recent local adaptation in this species. Here we aimed to quantify the direct and indirect effects of the predatory larvae on the aquatic food webs in the ponds and on the flux of matter between the ponds and adjacent terrestrial habitats. Our estimates are based on biomass data of the present pond fauna as well as on the analysis of stomach content data, growth rates and population dynamics of the salamander larvae in pond habitats. By their deposition of larvae in early spring, female fire salamanders import between 0.07 and 2.86 g dry mass m^?2 larval biomass into the ponds. Due to high mortality rates in the larval phase and the relatively small size at metamorphosis of the pond-adapted salamanders compared to stream-adapted ones, the biomass export of the metamorphosed salamanders clearly falls below the initial biomass import. Catastrophic events such as high water temperatures and low oxygen levels may even occasionally result in mass mortalities of salamander larvae and thus in a net 100 % import of the salamander biomass into the pond food webs. Indirect effects further accelerate this net import of matter into the aquatic habitat, e.g. the feeding of salamanders on aquatic insect larvae with the emergence of terrestrial adults—thus preventing export—and on terrestrial organisms that fall on the water surface (supporting import). This study demonstrates that the adaptation of salamanders to pond reproduction can alter food web linkages across ecosystem boundaries by enhancing the flux of materials and energy from terrestrial (i.e. forest) to the aquatic (i.e. pond) habitat.     

Residence time and drift patterns of larval June sucker Chasmistes liorus in the lower Provo River as determined by otolith microstructure

Estimates of age derived from daily ring counts from otoliths and capture rates of larval June sucker Chasmistes liorus were used to determine the relationship between discharge rates of the Provo River and residence time and patterns of larval drift. During 1997, larval drift occurred over a 22 day period when discharge rates were low (mean ±s.d. 3·2 ± 0·0 m³ s^?1). In 1998, larval drift occurred in two separate events over a 40 day period. Discharge was higher during the first larval drift period (19 days; 24·8 ± 1·3 m³ s^?1) and lower during the second larval drift period (17 days; 7·0 ± 0·9 m³ s^?1). In 1997, no larval fish were collected at the lowermost transect on the Provo River (nearest Utah Lake), and few larvae >21 days of age were found. During the first drift period of 1998, larval C. liorus were collected at all transects, and mean age of larvae collected between upstream and downstream transects increased by c. 7 days. During the second drift period of 1998, only a few were collected in the lowermost transects, and age did not increase with proximity to the lake. Patterns in catch and age distribution of larval C. liorus in the lower Provo River suggest that recruitment failure occurs during the larval drift period in years with insufficient discharge to transport larvae into the lake.     

Susceptibility of larval Crepidula fornicata to predation by suspension-feeding adults

The slipper shell snail Crepidula fornicata forms dense assemblages along much of the European coast, where it co-occurs with oysters. We examined the susceptibility of slipper shell larvae to predation by suspension-feeders, including adults of their own species. In particular, we compared filtration rates on phytoplankton with those on larvae, and determined the extent to which consumption of larvae varied with adult size, larval size, and with the presence of alternative food (phytoplankton). We also examined the ability of competent larvae to metamorphose successfully in the presence of feeding adults. For each experiment, adults were held in plastic jars with seawater or phytoplankton suspension and allowed to graze on larvae (101 larvae per jar) for 4-6 h at room temperature (21-23 °C); larvae were kept in circulation with gentle aeration. Adults of C. fornicata ingested substantial numbers of larvae over the complete range of sizes tested, about 450-850 μm shell length. Ingestion rates were reduced by 43-50% in the presence of phytoplankton, and were not correlated with adult shell length. The rates at which larvae were removed by adult slipper shells were generally lower than predicted from the rates at which the same adults ingested phytoplankton, suggesting either some ability of larvae to avoid capture or some difficulty of adults in consuming larvae entrained into their feeding currents. Slipper shell larvae were also readily consumed by adult oysters (Ostrea edulis and Crassostrea gigas), and indeed oysters consumed larvae at faster rates than predicted from their phytoplankton ingestion rates. Nevertheless, substantial numbers of competent larvae managed to metamorphose successfully during the test periods, either on the sides of the jars they were in or on the adults' shells, suggesting that recruitment probably continues in the field even when suspension-feeding adults are at high concentrations in the benthos.