Characterization of a natural inducer of coral larval metamorphosis

External chemical signals used by scleractinian corals to recognize suitable substrata for larval settlement and metamorphosis were identified from crustose coralline red algae (CCA). A fragment of coral rubble with CCA induced larval metamorphosis of the scleractinian coral Pseudosiderastrea tayamai. A natural inducer and compounds that enhanced its effect in larval metamorphosis were isolated from the methanol extracts of coral rubble with CCA. A bromotyrosine derivative, 11-deoxyfistularin-3 (10^− 7 M) isolated from the CCA, induced the metamorphosis of P. tayamai larvae (27.5 ± 24.0%). In the presence of fucoxanthinol (10^− 9 M) and fucoxanthin (10^− 9 M), the percentage of metamorphosis induced by the bromotyrosine derivative was further enhanced to 87.8 ± 13.0 and 88.4 ± 17.8%, respectively. Both carotenoids are also found in the coral rubble with CCA. These results suggest that bromotyrosine derivative and carotenoids have a synergistic effect in the metamorphosis of P. tayamai larvae. The synergistic effect provides a higher selectivity for recruitment than a single-component natural inducer for recognizing suitable substrata for larval metamorphosis. Thus, the effect might offer a survival advantage for benthic marine invertebrates.     

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.     

Morphological and biochemical characterizations of the great scallop Pecten maximus metamorphosis

In order to characterize Pecten maximus metamorphosis within a hatchery environment, the relationships existing among the various larval rearing parameters, the biochemical composition of the larvae and metamorphosis have been determined. Metamorphosis levels are correlated with the percentages of double ring larvae, as well as with the larval lipid content. A multiple regression incorporating the percentage of double ring larvae and larval lipid content shows that these two combined parameters explain 50 % of the total metamorphosis variance, with an equal relative importance for each of them. In an attempt to identify other possible endogenous markers, the kinetics of biogenic amines were also examined throughout larval and post-larval development. A steady increase in serotonin and dopamine levels was recorded during larval development while a sudden decrease in both molecules was noted during metamorphosis. It is suggested that these two amines may be used as indicators of larval competence for P. maximus metamorphosis.     

Age determination and estimation of larval period in field caught abalone (Haliotis discus hannai Ino 1953) larvae and newly metamorphosed post-larvae by counts of radular teeth rows

We developed an age determination method for larval and newly metamorphosed post-larval abalone Haliotis discus hannai in a laboratory experiment and determined the age of field caught individuals. Laboratory experiments showed that competent veliger larvae (4 days after fertilization) had a radula and regularly added rows of radular teeth with age in the absence of metamorphosis. Under environmentally relevant temperatures (17-22 °C), the number of rows of radular teeth increased linearly with age, but slopes of the regression lines were different among temperatures. Rows of radular teeth were added more slowly at lower temperatures. The effect of temperature on the development rate of the radula was quantified by the regression and the temperature coefficient, Q₁₀. The radular development of newly metamorphosed post-larvae, which had not acquired a peristomal shell (adult shell), was comparable with that of veliger larvae, although older post-larvae had a larger number of rows of radula than those of the same age of veliger larvae. From these results, an age determination method of veliger larvae and newly metamorphosed post-larvae was established, using the number of rows of radular teeth. The age of veliger larvae and newly metamorphosed post-larvae was determined by the age determination method for samples collected in August to October of 2003 and 2004 for which the thermal history of the coastal water of Miyagi Prefecture Japan was available. Only 9.1% of veliger larvae (n = 8) captured in the field had formed a radula and these were estimated to be 4-6 days old. The remaining 90.9% of larvae (n = 80) that had not formed a radula were classified as younger than 4 days old. All newly metamorphosed post-larvae (n = 24) that had metamorphosed on substrata were estimated to be 4-6 days old. Results of the field study indicate that these abalone metamorphosed within a few days after the acquisition of competence (4 days after fertilization) at this site, which has suitable crustose algal habitat.     

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.     

Proteomic analysis during larval development and metamorphosis of the spionid polychaete Pseudopolydora vexillosa

Background  

While the larval-juvenile transition (metamorphosis) in the spionid polychaete Pseudopolydora vexillosa involves gradual morphological changes and does not require substantial development of juvenile organs, the opposite occurs in the barnacle Balanus amphitrite. We hypothesized that the proteome changes during metamorphosis in the spionids are less drastic than that in the barnacles. To test this, proteomes of pre-competent larvae, competent larvae (ready to metamorphose), and juveniles of P. vexillosa were compared using 2-dimensional gel electrophoresis (2-DE), and they were then compared to those of the barnacle.     

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.     

Laboratory culture of the larvae of Conus textile Linné (Gastropoda: Toxoglossa)

The larvae of the Indo-Pacific gastropod Conns textile Linné were reared in the laboratory from hatching through metamorphosis. Larvae fed a mixed phytoplankton culture of Isochrysis galbana and Phaeodactylum tricornutum grew at a rate of 0.06 mm/day and began metamorphosing 16 days after hatching. Unfed control cultures yielded no metamorphically competent larvae. Laboratory-reared larvae metamorphosed spontaneously on the walls of the fïberglass rearing tanks when their average shell length was 1.5 mm. Measurements made on field-collected Conns textile juveniles indicate that the larvae metamorphose at the same size in the laboratory as they do in nature.Rates of larval shell length increase and dry weight increase paralleled each other until metamorphosis. At this point, shell growth slowed while dry weight increased suddenly. It is suggested that this weight increase reflects calcification and strengthening of the fragile larval shell upon entering the benthic environment.     

Sediment-associated cues for larval settlement of the infaunal spionid polychaetes Polydora cornuta and Streblospio benedicti

Do patchy distribution patterns of infaunal polychaetes result from active site selection of larvae influenced by sediment-associated microbial cues? This hypothesis was tested with still-water laboratory settlement assays revealing the acceptance or rejection of polychaete larvae to qualitatively different sediments. Laboratory brood cultures of the spionid polychaetes Polydora cornuta and Streblospio benedicti yielded a sufficient number of larvae with planktotrophic development for bioassays. High settlement rates (75-95%) of test larvae were observed in response to natural sediment. Sterilization of natural sediment significantly decreased settlement of P. cornuta (25-55%) while combustion of sediment significantly decreased the settlement rate in both species (5-50%). Differences in settlement responses to sediments treated by sterilization or combustion most likely resulted from a variety of factors such as modified sediment fabric, grain size distribution and quantity of adsorbed organic matter. To experimentally address the potential role of microorganisms and microbial metabolites as mediators of larval settlement, ashed sediment was inoculated with viable microorganisms obtained from natural sediment. In both polychaete species, this treatment significantly increased larval settlement in comparison to the control of ashed sediment indicating that larval settlement was at least partially mediated by the presence of microorganisms associated with sediment.     

Pharmacological Induction of Larval Settlement and Metamorphosis in the Blue Mussel Mytilus edulis L.

The blue mussel Mytilus edulis L. is an important aquaculture and fouling species in northern seas. Although the general role of chemical cues for settlement of larvae of the blue mussel has been proposed, few studies have focused on induction of settlement and metamorphosis by pharmacological agents. In this study, the induction of larval settlement of the blue mussel by pharmacological compounds was investigated through a series of laboratory experiments with an aim of identifying artificial cues for laboratory bioassay systems in fouling and antifouling research. Gamma-aminobutiric acid (GABA), dihydroxyphenyl L-alanine (DOPA), isobutyl methylxanthine (IBMX) and acetylcholine chloride (ACH) at 10^{m 7}-10^{m 2} M as well as KCl at 10-40 mM K⁺ in excess of the level in normal seawater were tested for their inductive effect on larval settlement. In filtered seawater (FSW) <9% of the larvae settled after 48 h. Elevated K⁺ and GABA levels had no effect on larval settlement and metamorphosis. DOPA at 10^{m 5} M and IBMX at 10^{m 6}-10^{m 4} M induced 41-83% larval settlement and ACH at 10^{m 7}-10^{m 5} M induced < 40% larval settlement. While the highest settlement rates were observed after 48 h exposure to the chemicals, most of the larvae settled within 24 h. Compounds at concentrations of 10^{m 3}-10^{m 2} M were either toxic to larvae or retarded the growth of the post-larvae shell. Juveniles resulting from induction by lower concentrations of chemicals had a very high survival rate, completed metamorphosis and grew as well as the juveniles that metamorphosed spontaneously. IBMX at 10^{m 6}-10^{m 4} M and L-DOPA at 10^{m 5} M are effective agents for induction of settlement and metamorphosis for future studies using juvenile M. edulis.     

Low salinity stress experienced by larvae does not affect post-metamorphic growth or survival in three calyptraeid gastropods

Marine larvae that experience some sub-lethal stresses can show effects from those stresses after metamorphosis, even when they seem to recover from those stresses before metamorphosis. In this study we investigated the short and long-term effects of exposing the larvae of three calyptraeid gastropods (Crepidula fornicata, Crepidula onyx, and Crepipatella fecunda) to temporary reductions in salinity. Larvae of all three species showed slower larval growth rates, longer time to metamorphic competence, and substantial mortality after being stressed in seawater at salinities of 10, 15, and 20 for less than 48 h. Larval tolerance to low salinities varied widely within and among species, but longer stresses at lower salinities were generally more harmful to larvae. However, larvae in nearly all experiments that were able to metamorphose survived and grew normally as juveniles; there were no documented “latent effects.” For all three species, starving larvae in full-strength seawater was not as harmful as exposing larvae to low salinity stress, indicating that detrimental effects on larvae were caused by the salinity stress per se, rather than by an indirect effect of salinity stress on feeding. C. fornicata that were stressed with low salinity as juveniles were more tolerant of the stress than larvae: all stressed juveniles lived and showed reduced growth rates for no more than 3 days. Our data suggest that even though reduced salinity is clearly stressful to the larvae of these 3 gastropod species, metamorphosis seems to generally provide individuals with a fresh start.     

Alteration of biogenic structure and physical properties by tube-building chironomid larvae in cohesive sediments

From earlier research it has been suggested that chironomid larvae may significantly affect the physical properties of cohesive sediments, leading to chemical alteration in lake ecosystems. The present study was aimed to evaluate the effect of Tanytarsini larvae on the physical properties of cohesive sediments, both in controlled environments and in natural environments. For this the sediment surface was examined by low temperature scanning electron microscopy (LTSEM) and direct measurement of the shear strength. The larvae are able to modify the surface sediment significantly, e.g., by secretion of silk, which forms a matrix at the sediment surface in the vicinity of the larval tubes. The shear strength increased with greater sediment depth in all the experimental treatments, except where the densities were high. Considerably higher shear strength was observed in the natural sediment than in the experimental set up. A very significant density-dependent effect was observed within the top 1-cm by tube building Tanytarsini larvae. The relationship between larval densities and shear strength was not significant for 2 deeper sections (1–3 cm). The tube length was significantly greater in experimental containers with the low density (4960 ind. m^–2) than in containers with high larval densities (19840 and 39680 ind. m^–2).     

The influence of temperature on growth rate and length of larval life of the gastropod, Crepidula plana Say

The relationship between rate of larval development and the potential to prolong larval life was examined for larvae of the marine prosobranch gastropod Crepidula plana Say. Larvae were maintained in clean glass dishes at constant temperatures ranging from 12–29°C and fed either Isochrysis galbana Parke (ISO) or a Tahitian strain of Isochrysis species (T-ISO). Under all conditions, larvae grew at constant rates, as determined by measurements of shell length and tissue biomass. Most larvae eventually underwent spontaneous metamorphosis. Regardless of temperature, faster growth was associated with a shorter planktonic stage prior to spontaneous metamorphosis. Within an experiment, higher temperatures generally accelerated growth rates and reduced the number of days from hatching to spontaneous metamorphosis. However, growth rates were independent of temperature for larvae fed ISO at 25 and 29°C and for larvae fed T-ISO at 20 and 25°C. Where growth rates were unaffected by temperature, time to spontaneous metamorphosis was similarly unaffected. Maximum durations of larval life at a given temperature were shorter for larvae of Crepidula plana than for those of the congener C. fornicata (L.), although both species grew at comparable rates. Interpretations of the ecological significance of these interspecific differences in delay capabilities will require additional data on adult distributions and larval dispersal patterns in the field.     

The B Vitamins Nicotinamide (B3) and Riboflavin (B2) Stimulate Metamorphosis in Larvae of the Deposit-Feeding Polychaete Capitella teleta: Implications for a Sensory Ligand-Gated Ion Channel

Marine sediments can contain B vitamins, presumably incorporated from settled, decaying phytoplankton and microorganisms associated with decomposition. Because B vitamins may be advantageous for the energetically intensive processes of metamorphosis, post-metamorphic growth, and reproduction, we tested several B vitamins to determine if they would stimulate larvae of the deposit-feeding polychaete Capitella teleta to settle and metamorphose. Nicotinamide and riboflavin individually stimulated larvae of C. teleta to settle and metamorphose, generally within 1–2 hours at nicotinamide concentrations as low as 3 µM and riboflavin concentrations as low as 50 µM. More than 80% of the larvae metamorphosed within 30 minutes at a nicotinamide concentration of 7 µM. The pyridine channel agonist pyrazinecarboxamide also stimulated metamorphosis at very low concentrations. In contrast, neither lumichrome, thiamine HCl, pyridoxine HCl, nor vitamin B₁₂ stimulated larvae of C. teleta to metamorphose at concentrations as high as 500 µM. Larvae also did not metamorphose in response to either nicotinamide or pyrazinecarboxamide in calcium-free seawater or with the addition of 4-acetylpyridine, a competitive inhibitor of the pyridine receptor. Together, these results suggest that larvae of C. teleta are responding to nicotinamide and riboflavin via a chemosensory pyridine receptor similar to that previously reported to be present on crayfish chela and involved with food recognition. Our data are the first to implicate B vitamins as possible natural chemical settlement cues for marine invertebrate larvae.     

Larval settlement preferences and post-settlement survival of the threatened Caribbean corals Acropora palmata and A. cervicornis

The settlement specificity of two threatened Caribbean corals, Acropora palmata and A. cervicornis, was tested by measuring their rates of larval metamorphosis in response to crustose coralline algae (CCA) and other substrata. In the no-choice experiments, the coral larvae were placed in six treatments: filtered seawater (FSW), a fragment of biofilmed dead skeleton of A. palmata, or a fragment of one of four species of CCA (Hydrolithon boergesenii, Porolithon pachydermum, Paragoniolithon solubile, and Titanoderma prototypum). Within each CCA treatment, there were three different substrata on which to settle and metamorphose: (1) the CCA surface, (2) the rock under the CCA, or (3) the plastic dish. The 5-day-old larvae of both A. palmata and A. cervicornis had similar rates of total metamorphosis (all substrata combined) in every treatment (excluding FSW) even in the absence of CCA. However, there were differences in larval behavior among the CCA species since the larvae settled and metamorphosed on different substrata in the presence of different CCA species. In the no-choice experiments the larvae of both corals had higher rates of metamorphosis on the top surfaces of H. boergesenii and/or T. prototypum than on P. pachydermum. In the choice experiments, the coral larvae were offered two species of CCA in the same dish. When given a choice, both species of coral larvae had more settlement and metamorphosis on the surface of H. boergesenii or T. prototypum or clean rock than onto the surface of P. solubile. After 6 weeks in the field, transplanted A. palmata recruits had approximately 15% survival on both T. prototypum and H. boergesenii, but A. cervicornis recruits only survived on T. prototypum (13%). Some, but not all, CCA species facilitated the larval settlement and post-settlement survival of these two threatened corals, highlighting the importance of benthic community composition for successful coral recruitment.     

Larval and early juvenile development of Lithodes santolla (Molina, 1782) (Decapoda: Anomura: Lithodidae) reared at different temperatures in the laboratory

The southern king crab, Lithodes santolla Molina, is distributed in cold-temperate and subantarctic waters ranging from the southeastern Pacific island of Chiloé (Chile) and the deep Atlantic waters off Uruguay, south to the Beagle Channel (Tierra del Fuego, Argentina/Chile). Recent investigations have shown that its complete larval development from hatching to metamorphosis, comprising three zoeal stages and a megalopa, is fully lecithotrophic, i.e. independent of food. In the present study, larvae were individually reared in the laboratory at seven constant temperatures ranging from 1 to 18 °C, and rates of survival and development through successive larval and early juvenile stages were monitored throughout a period of 1 year. The highest temperature (18 °C) caused complete mortality within 1 week; only a single individual moulted under this condition, 2 days after hatching, to the second zoeal stage, while all other larvae died later in the zoea I stage. At the coldest condition (1 °C), 71% of the larvae reached the zoea III stage, but none of these moulted successfully to a megalopa. A temperature of 3 °C allowed for some survival to the megalopa stage (17-33% in larvae obtained from two different females), but only a single individual passed successfully, 129 days after hatching, through metamorphosis to the first juvenile crab instar. At all other experimental conditions (6, 9, 12 and 15 °C), survival through metamorphosis varied among temperatures and two hatches from 29% to 90% without showing a consistent trend. The time of nonfeeding development from hatching to metamorphosis lasted, on average, from 19 days at 15 °C to 65 days at 6 °C. The relationship between the time of development through individual larval or juvenile stages (D) and temperature (T) was described as a power function (D=aT^b, or log[D]=log[a]blog[T]). The same model was also used to describe the temperature dependence of cumulative periods of development from hatching to later larval or juvenile stages. One year after hatching, the 7th (6 °C) to 9th (15 °C) crab instar was reached. Under natural temperature conditions in the region of origin of our material (Beagle Channel, Argentina), L. santolla should reach metamorphosis in October-December, i.e. ca. 2 months after hatching (taking place in winter and early spring). Within 1 year from hatching, the crabs should grow approximately to juvenile instars VII-VIII. Our results indicate that the early life-history stages of L. santolla tolerate moderate cold stress as well as planktonic food-limitation in winter, implying that this species is well adapted to subantarctic environments with low temperatures and a short seasonal plankton production.     

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.     

Induction of metamorphosis with potassium ions requires development of competence and an anterior signalling centre in the ascidian Herdmania momus

 Increased K⁺ concentration in seawater induces metamorphosis in the ascidian Herdmania momus. Larvae cultivated at 24°C exhibit highest rates of metamorphosis when treated with 40 mM KCl-elevated seawater at 21°C. At 24°C, H. momus larvae develop competence to respond to KCl-seawater and initiate metamorphosis approximately 3 h after hatching. Larval trunks and tails separated from the anterior papillae region, but maintained in a common tunic at a distance of greater than 60 μm, do not undergo metamorphosis when treated with KCl-seawater; normal muscle degradation does not occur in separated tails while ampullae develop from papillae-containing anterior fragments. Normal programmed degradation of myofibrils occurs when posterior fragments are fused to papillae-containing anterior fragments. These data indicate that H. momus settlement and metamorphosis only occurs when larvae have attained competence, and suggest that an anterior signalling centre is stimulated to release a factor that induces metamorphosis. Received: 15 May 1996 / Accepted: 19 September 1996     

Development and Infectious Disease in Hosts with Complex Life Cycles

Metamorphosis is often characterized by profound changes in morphology and physiology that can affect the dynamics of species interactions. For example, the interaction between a pathogen and its host may differ depending on the life stage of the host or pathogen. One pathogen that infects hosts with complex life cycles is the emerging fungal pathogen of amphibians, Batrachochytrium dendrobatidis (Bd). We sought to determine how conditions at the larval stage can affect variation in development and patterns of Bd infection across amphibian life stages. We used outdoor experimental mesocosms to simulate natural pond habitats and manipulated the presence of Bd, the larval density, and the number of host species in larvae of two co-occurring amphibian species (Rana cascadae and Pseudacris regilla). We found that infection differed between species throughout development; P. regilla consistently had higher infection severity compared to R. cascadae. Additionally, while up to 100% of larvae were infected, only 18.2% of R. cascadae and 81.5% of P. regilla were infected after metamorphosis. This indicates that amphibians have the ability to recover from Bd infection as they undergo metamorphosis. Higher larval densities in P. regilla led to a shorter larval period, and individuals with a shorter larval period had lower infection severity. This led to a trend where P. regilla larvae reared at high densities tended to have lower infection prevalence after metamorphosis. We also found that exposure to Bd increased larval mortality and prolonged the larval period in P. regilla, indicating that P. regilla are susceptible to the negative effects of Bd as larvae. This study demonstrates that host density, species composition, and pathogen exposure may all interact to influence development and infection in hosts with complex life cycles.     

Short-term fluctuation in salinity promotes rapid larval development and metamorphosis in Dendraster excentricus

The effect of constant and fluctuating salinity on larval development and metamorphosis of the sand dollar Dendraster excentricus was investigated in the laboratory. Sand dollar larvae at different stages of development were kept either at 32‰ (controls), exposed to constant low salinity (22‰) throughout development, or exposed to fluctuating salinity (i.e. transferring larvae from 32‰ to 22‰ for 7 days then back to 32‰ for the rest of their development). Larvae exposed to constant low salinity were significantly smaller but developed all larval arms at a slower rate than larvae in all other treatments. Larvae exposed to fluctuating salinity recovered and developed significantly longer larval arms and bigger rudiments than larvae kept at constant low salinity. Larvae exposed to fluctuating salinity produced more juveniles than larvae at constant high salinity (32‰), while those at constant low salinity produced few or no juveniles. Four-arm larvae exposed to fluctuating salinity produced significantly more juveniles than six-arm larvae exposed to the same treatment. Transferring competent 8-arm larvae from 31‰ to 15‰ for 2 days then back to 31‰, induced metamorphosis with juvenile production being significantly higher than for those kept at a constant salinity of 20, 25 and 31‰. This study indicates that a short-term decrease in salinity might induce metamorphosis for this species.