Morphological responses of mitochondria-rich cells to hypersaline environment in the Australian mudskipper, Periophthalmus minutus.

A population of the Australian mudskipper, Periophthalmus minutus, was found to inhabit mudflat that remained uncovered by tide for more than 20 days in some neap tides. During these prolonged emersion periods, P. minutus retreated into burrows containing little water, with a highest recorded salinity of 84 ± 7.4 psu (practical salinity unit). To explore the mechanical basis for this salinity tolerance in P. minutus, we determined the densities of mitochondria-rich cells (MRCs) in the inner and outer opercula and the pectoral fin skin, in comparison with P. takita, [corrected] from an adjacent lower intertidal habitat, and studied morphological responses of MRCs to exposure to freshwater (FW), and 100% (34-35 psu) and 200% seawater (SW). Periophthalmus minutus showed a higher density of MRCs in the inner operculum (3365 ± 821 cells mm(-2)) than in the pectoral fin skin (1428 ± 161) or the outer operculum (1100 ± 986), all of which were higher than the MRC densities in p. takita. [corrected]. No mortality occurred in 100% or 200% SW, but half of the fish died within four days in FW. Neither 200% SW nor FW exposure affected MRC density. Transfer to 200% SW doubled MRC size after 9-14 days with no change in the proportion of MRCs with apical pits or plasma sodium concentration. In contrast, transfer to FW resulted in a rapid closing of pits and a significant reduction in plasma sodium concentration. These results suggest that P. minutus has evolved morphological and physiological mechanisms to withstand hypersaline conditions that they may encounter in their habitat.

Ontogenetic phase shifts in metabolism: links to development and anti-predator adaptation.

The allometric relationships between resting metabolism (VO(2)) and body mass (M), VO(2) = a(i)M(b), are considered a fundamental law of nature. A distinction though needs to be made between the ontogeny (within a species) and phylogeny (among species) of metabolism. However, the nature and significance of the intraspecific allometry (ontogeny of metabolism) have not been established in fishes. In this study, we present experimental evidence that a puffer fish ranging 0.0008-3 g in wet body mass has four distinct allometric phases in which three stepwise increases in scaling constants (a(i), i = 1-4), i.e. ontogenetic phase shifts in metabolism, occur with growth during its early life stages at around 0.002, 0.01 and 0.1 g, keeping each scaling exponent constant in each phase (b = 0.795). Three stepwise increases in a(i) accompanied behavioural and morphological changes and three peaks of severe cannibalism, in which the majority of predation occurred on smaller fish that had a lower value of a(i). Though fishes are generally highly fecund, producing a large number of small eggs, their survivability is very low. These results suggest that individuals with the ability to rapidly grow and step up 'a(i)' develop more anti-predator adaptation as a result of the decreased predatory risk.

Molecular cloning and expression analysis of a GnRH-like dodecapeptide in the swordtip squid, Loligo edulis.

In vertebrates, gonadotropin-releasing hormone (GnRH), which is synthesized in the brain, is a key peptide involved in gonadal maturation regulated by the brain-pituitary-gonadal axis. GnRH isoforms and their primary structures have recently been determined in two species of non-chordates, the octopus (Octopus vulgaris) and sea hare (Aplysia californica), which are mollusks. Octopus and sea hare GnRHs are dodecapeptides that contain the structural core of chordate GnRH; however, chordate GnRHs, including tunicate GnRH, are decapeptides. in this study, we examined a GnRH-like peptide in the swordtip squid, Loligo edulis, to provide information on the structural evolution of GnRH in non-chordates. We isolated the full-length cDNA of a GnRH-like molecule from the central nervous system (CNS) of the squid. The open reading frame of this cDNA encodes a protein of 90 amino acids, which consists of a putative signal peptide, a GnRH dodecapeptide, a processing site, and a GnRH-associated peptide. This architecture is generally conserved in chordates. Compared to octopus GnRH, Squid GnRH is identical in the deduced amino acid sequence of the peptide, and 80.5% similar in base sequence. in a phylogenetic analysis, prepro-GnRHs of octopus, sea hare, and squid were segregated from all chordate prepro-GnRHs, in a group designated GnRHS. The squid prepro-GnRH mRNA was expressed mainly in the CNS. This study is the first report of GnRH cDNA cloning in squid and the third in non-chordates.

Bidirectional sex change induced by sex steroid implantation in the hermaphrodite fish, Pseudolabrus sieboldi.

Sex steroids have been suggested to be involved in gonadal sex change in hermaphrodite fish. Aromatase, the enzyme that catalyzes the conversion of androgens into estrogens, is a principal enzyme regulating gonadal sex. However, the detailed functions of each steroid hormone remain to be evaluated. Recent studies have demonstrated that estradiol-17ß (E2) is synthesized via estrone (E1) in some hermaphrodite species. On the other hand, 11-ketotestosterone (11KT) is produced in the testis via testosterone (T). In this study, we hypothesized that E1 and T are also involved in the sex change as precursors for E2 and 11KT, respectively. We implanted females of the wrasse, Pseudolabrus sieboldi, with T and 11KT, and males with E1 and E2, by use of sustained-release capsules. In females, testicular tissues and body color change were observed after androgen administration, in which 11KT was more effective than T. In contrast, after estrogen administration, the gonads of males contained oocytes. In females, the administration of T and 11KT resulted in reduced serum E2 levels. Conversely, serum 11KT levels decreased in the E1- and E2-treated males. Thus, we successfully induced bidirectional sex change in the gonad by estrogen and androgen administration in vivo. Moreover, this study raises the possibility that E1 and T are involved in the sex change as precursors for E2 and 11KT, respectively.

Mudskippers brood their eggs in air but submerge them for hatching.

Intertidal mudflats are highly productive ecosystems that impose severe environmental challenges on their occupants due to tidal oscillations and extreme shifts in habitat conditions. Reproduction on mudflats requires protection of developing eggs from thermal and salinity extremes, O(2) shortage, dislodgement by currents, siltation and predation. Mudskippers are air-breathing, amphibious fishes, and one of few vertebrates that reside on mudflats. They lay their eggs in mud burrows containing extremely hypoxic water, raising the question of how the eggs survive. We found that the Japanese mudskipper Periophthalmus modestus deposits its eggs on the walls of an air-filled chamber within its burrow. To ensure adequate O(2) for egg development, the burrow-guarding male mudskipper deposits mouthfuls of fresh air into the egg chamber during each low tide, a behaviour that can be upregulated by egg-chamber hypoxia. When egg development is complete the male, on a nocturnal rising tide, removes the egg-chamber air and releases it outside the burrow. This floods the egg chamber and induces egg hatching. Thus, P. modestus has developed a reproductive strategy that allows it to nurture eggs in this severe habitat rather than migrating away from the mudflat. This requires that mudskipper eggs be specialized to develop in air and that the air-breathing capacity of the egg-guarding male be integrated in a complex behavioural repertoire that includes egg guarding, ferrying air to and from the egg chamber, and sensing O(2) levels therein, all in concert with the tidal cycle.