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.     

Long-wavelength photosensitivity in coral planula larvae

Light influences the swimming behavior and settlement of the planktonic planula larvae of coral, but little is known regarding the photosensory biology of coral at this or any life-history stage. Here we used changes in the electrical activity of coral planula tissue upon light flashes to investigate the photosensitivity of the larvae. Recordings were made from five species: two whose larvae are brooded and contain algal symbionts (Porites astreoides and Agaricia agaricites), and three whose larvae are spawned and lack algal symbionts (Acropora cervicornis, Acropora palmata,and Montastrea faveolata). Photosensitivity originated from the coral larva rather than from, or in addition to, its algal symbionts as species with and without symbionts displayed similar tissue-level electrical responses to light. All species exhibited as much (or more) sensitivity to red stimuli as to blue/green stimuli, which is consistent with a role for long-wavelength visible light in the preference for substrata observed during settlement and in facilitating vertical positioning of larvae in the water column.     

Studies on the cryopreservation of Dictyocaulus viviparus (Nematoda) third-stage larvae

Various cooling (0.1-5,100 degrees C min-1) and warming (20-6,800 degrees C min-1) rates, stepped cooling schedules and four cryoprotective additives (dimethyl sulphoxide, methanol, ethanediol and glycerol) were investigated in cryopreservation studies with Dictyocaulus viviparus third-stage larvae. Exsheathment with sodium hypochlorite was essential to achieve significant survival. With uninterrupted cooling, highest survival (30% normally motile) was achieved with rates of 10-70 degrees C min-1. Survival was higher (50-75%) using 1 degree C min-1 to -10 degrees C followed by plunging into liquid nitrogen. The optimum warming rate was 6,800 degrees C min-1. The use of cryoprotectants led to marginally lower survival while varying the suspending media had no significant effect on survival. X-irradiated, exsheathed third-stage larvae cryopreserved by the optimum protocol yielded 38.3 +/- 4.2% survival. Two calves each infected with 45,000 (15,000 viable) exsheathed, unirradiated, cryopreserved third-stage larvae harboured 494 worms (1.1% infectivity) and 355 worms (0.8%) at necropsy. Numbers of first-stage larvae in the faeces reached 420/g and 105/g respectively 27 days after infection.     

Preliminary studies of sperm cryopreservation in the mushroom coral, Fungia scutaria

Coral species throughout the world are facing severe environmental pressures. Because of this, we began cryobiological studies on the sperm of the mushroom coral, Fungia scutaria. We determined that F. scutaria sperm had a mean length of 56 microm and head diameter of 2.5 microm, and a mean spontaneous ice nucleation temperature of -37.2 +/- 1.7 degrees C. When the sperm were exposed to the cryoprotectant glycerol for 5 or 20 min (at 10% v/v), no fertilized larvae were produced. However, when sperm were exposed for 20 min to propylene glycol (10% v/v), fertilizations were produced at the same rate as untreated control eggs and sperm (P > 0.05), but slightly less for dimethyl sulfoxide (10% v/v) (P < 0.05). Regardless, dimethyl sulfoxide caused less osmotic damage to the sperm membrane than did propylene glycol. Therefore, we used the dimethyl sulfoxide (10% v/v) to develop cryopreservation protocols that yielded good post-thaw morphology and motility (>95%) for coral sperm.     

Surface ultrastructure of the advanced third-stage larvae of Gnathostoma nipponicum

The surface ultrastructure of advanced third-stage larvae (AL3) of Gnathostoma nipponicum was studied using scanning electron microscopy. The larvae were recovered from the grass snake Rhabdophis tigrina in the Republic of Korea. Parasites had a globular head bulb with a pair of lips at the anterior end and 2 labial papillae and an amphid on each lip. The head bulb was characteristically armed with 3 transverse rows of hooklets, averaging 36, 38, and 43 in number, increasing posteriorly. A total of 213-232 minute unidentate cuticular spines were present along the entire length of the larvae, forming the transverse striations. Two pairs of cervical papillae were located between the 8th and 12th transverse striations, and a pair of body papillae was seen laterally on the posterior third of the body. A pair of caudal phasmids was recognized near the posterior extremity. The surface ultrastructure of AL3 of G. nipponicum is unique compared with that of other species.     

Water contamination reduces the tolerance of coral larvae to thermal stress

Coral reefs are highly susceptible to climate change, with elevated sea surface temperatures (SST) posing one of the main threats to coral survival. Successful recruitment of new colonies is important for the recovery of degraded reefs following mortality events. Coral larvae require relatively uncontaminated substratum on which to metamorphose into sessile polyps, and the increasing pollution of coastal waters therefore constitutes an additional threat to reef resilience. Here we develop and analyse a model of larval metamorphosis success for two common coral species to quantify the interactive effects of water pollution (copper contamination) and SST. We identify thresholds of temperature and pollution that prevent larval metamorphosis, and evaluate synergistic interactions between these stressors. Our analyses show that halving the concentration of Cu can protect corals from the negative effects of a 2-3°C increase in SST. These results demonstrate that effective mitigation of local impacts can reduce negative effects of global stressors.     

Auditory sensitivity in settlement-stage larvae of coral reef fishes

The larval phase of most species of coral reef fishes is spent away from the reef in the pelagic environment. At the time of settlement, these larvae need to locate a reef, and recent research indicates that sound emanating from reefs may act as a cue to guide them. Here, the auditory abilities of settlement-stage larvae of four species of coral reef fishes (families Pomacentridae, Lutjanidae and Serranidae) and similar-sized individuals of two pelagic species (Carangidae) were tested using an electrophysiological technique, auditory brainstem response (ABR). Five of the six species heard frequencies in the 100–2,000 Hz range, whilst one carangid species did not detect frequencies higher than 800 Hz. The audiograms of the six species were of similar shape, with best hearing at lower frequencies between 100 and 300 Hz. Strong within-species differences were found in hearing sensitivity both among the coral reef species and among the pelagic species. Larvae of the coral reef species had significantly more sensitive hearing than the larvae of the pelagic species. The results suggest that settlement-stage larval reef fishes may be able to detect reef sounds at distances of a few 100 m. If true hearing thresholds are lower than ABR estimates, as indicated in some comparisons of ABR and behavioural methods, the detection distances would be much larger.     

Effects of cryopreservation procedures on sperm membranes

Empirical approaches to semen cryopreservation have resulted in the production of young in a broad range of species. However, acceptable levels of fertility in most domestic animal species has not been achieved. In this review, an attempt has been made to describe the complexity of the sperm plasma membrane and the many steps in a cryopreservation procedure where membrane perturbations can occur. Improvement in sperm cryopreservation procedures will require a careful consideration of the complexity of the sperm plasma membrane, the interaction of its components and the influence of cooling, freezing and thawing on these interactions.     

Effects of pressure on swimming behavior in planula larvae of the coral Porites astreoides (Cnidaria, Scleractinia)

Mechanisms governing the behavior of coral planulae are not well understood, particularly those manifesting themselves between the time when the larvae are released and when they settle. Larvae from the hermatypic coral Porites astreoides Lamarck were exposed to different levels of hydrostatic pressure—103.4, 206.9, 310.3, 413.8, and 517.1 kPa (including ambient pressure). Data were collected at stops of the above pressures for 15 min each, respectively. This was done in both an increasing sequence and a decreasing one. When exposed to increases in pressure from 103.4 kPa, larvae swam upward (negative barotaxis) in a spiraling motion. Upon exposure to decreasing pressure from 517.1 kPa, larvae moved downward (positive barotaxis), but the magnitude of the vertical movement was much less than in the case of increasing pressure. This suggests that these larvae are more sensitive to increased pressure than decreasing pressure. High variance was also observed in the responses of these larvae at both the intra- and inter-colony levels. Thus, this behavioral trait is variable within the population. The trait may be genetically based, and thus may be susceptible to alteration by natural selection, although this remains to be demonstrated. This study is the first to document these behavioral mechanisms in coral larvae.     

Chloroplast ultrastructure of Hypericum perforatum plants regenerated in vitro after cryopreservation

The ultrastructure of leaf mesophyll cells of in vitro cultured Hypericum perforatum L. plants regenerated after cryopreservation was studied. Electron microscopy analysis revealed that the chloroplasts in plants pretreated with abscisic acid and regenerated after cryopreservation were round, with increased amount of starch, rather small volume of the thylakoid system, and destroyed envelope. Plants pretreated with 0.3 M mannitol and cooled at rates of 0.1 or 0.3 °C min^?1 possessed chloroplasts with high starch content that resulted in a reduction of a membrane system. However, the pretreatment with 0.3 M mannitol and cooling at a rate of 0.2 °C min^?1 was the best as chloroplast ultrastructure resembled the controls regenerated without cryopreservation.     

Laboratory studies of cryopreservation of sperm and trochophore larvae of the eastern oyster

The eastern oyster, Crassostrea virginica, is the most important cultured oyster species of the Atlantic and Gulf coasts of the United States. Cryopreservation of gametes and larvae of aquatic organisms has increased in importance in recent years. However, studies on the cryopreservation of sperm and larvae of mollusks have focused on the Pacific oyster, Crassostrea gigas. The present study was conducted to improve cryopreservation of sperm and trochophore larvae and to assess fertilizing ability and male-to-male variation of thawed sperm of the eastern oyster. Sperm were diluted in 12 cryoprotectant solutions composed of Hanks' balanced salt solution without calcium and 0, 5, 10, 15, 20, and 25% (v/v) propylene glycol with or without 0.25 M sucrose. Trochophore larvae were suspended in artificial seawater and 10 or 15% propylene glycol (v/v). Sperm or trochophore larvae were placed in 5-mL macrotubes and allowed to equilibrate for 15 min. The macrotubes were cooled in a controlled-rate freezer at a rate of 2.5 degrees C per min until reaching a final temperature of -30 degrees C and were plunged into liquid nitrogen. After storage for 2 weeks, the samples were thawed in a water bath at 70 degrees C for 15 s. Overall, for cryopreservation of sperm and larvae, best results were obtained using 10 or 15% propylene glycol. Thawed sperm presented significant male-to-male variation in fertilizing ability. Survival of thawed larvae decreased as the concentration of larvae per macrotube increased. The procedures developed in this study for sperm and larvae are suitable for production of seedstock in commercial oyster hatcheries.     

Effects of corallivory and coral colony density on coral growth and survival

A suite of processes drive variation in coral populations in space and time, yet our understanding of how variation in coral density affects coral performance is limited. Theory predicts that reductions in density can send coral populations into a predator pit, where concentrated corallivory maintains corals at low densities. In reality, how variation in coral density alters corallivory rates is poorly resolved. Here, we experimentally quantified the effects of corallivory and coral density on growth and survival of small colonies of the staghorn coral Acropora pulchra. Our findings suggest that coral density and corallivory have strong but independent effects on coral performance. In the presence of corallivores, corals suffered high but density-independent mortality. When corallivores were excluded, however, vertical extension rates of colonies increased with increasing densities. While we found no evidence for a predator pit, our results suggest that spatio-temporal variation in corallivore and coral densities can fundamentally alter population dynamics via strong effects on juvenile corals.

     

Morphology and ultrastructure of spiracles in phlebotomine sandfly larvae

The morphology and ultrastructure of the larval spiracle system of three phlebotomine sandfly species, Phlebotomus perniciosus, P. perfiliewi and P. papatasi, were examined by scanning (SEM) and transmission (TEM) electron microscopy and by confocal scanning laser microscopy (CSLM). During larval development, thoracic and abdominal spiracles show considerable modifications. In fourth instar larvae, the spiracles consist of a plate with a sclerotized central portion and a peripheral circle of papillae. The latter is distinctive in the larvae of P. papatasi, which are readily distinguished from the other species. Opening clefts across the papillae communicate with an internal chamber that encircles an electrondense plug. Many cylindrical projections cross the chamber, uniting the central plug with the larval body, forming an air filter. Spiracular development in successive larval instars has both a taxonomic and adaptive value.     

Parasites of recruiting coral reef fish larvae in New Caledonia

Recruiting coral reef fish larvae from 38 species and 19 families from New Caledonia were examined for parasites. We found 13 parasite species (Platyhelminthes: Monogenea, Cestoda and Trematoda) but no acanthocephalan, crustacean or nematode parasites. Over 23% of individual fish were infected. Didymozoid metacercariae were the most abundant parasites. We conclude that most of the parasites are pelagic species that become ‘lost’ once the fish larvae have recruited to the reef. Larval coral reef fish probably contribute little to the dispersal of the parasites of the adult fish so that parasite dispersal is more difficult than that of the fish themselves.