Tatsusuke Takeda

Mudskippers store air in their burrows

Mudskipper fishes can maintain their metabolism while they are confined in mudflat burrows filled with oxygen-depleted water, and their eggs, deposited in the burrows, can develop under severely hypoxic conditions. How they cope with such conditions has been unclear. We report here that a mudskipper species Periophthalmodon schlosseri (Fig. 1) accumulates air in its burrows. This behaviour seems to be an adaptation to provide oxygen for burrow-dwelling fish and for embryos developing in the burrows.

Gas exchange in the carp gills in normoxic and hypoxic conditions

Respiratory exchange measurements in a teleost fish, Cyprinus carpio, in normoxic and hypoxic conditions are presented. Oxygen pressure in the dorsal aortic blood in normoxic carp was much lower than that in most of teleosts and elasmobranchs in similar conditions. The most evident physiological adjustment to the hypoxic condition was the increased ventilation due to an increased frequency of operation of the branchial pump. Increased perfusion at the gills played a small role in the physiological adjustments.

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

SUMMARY 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, O2 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 O2 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 O2 levels therein, all in concert with the tidal cycle.

Ventilation, cardiac output and blood respiratory parameters in the carp, Cyprinus carpio, during hyperoxia.

The specific ventilatory flow rate (Vw), cardiac output (Vb) and blood respiratory parameters were determined in the carp (Cyprinus carpio) during hyperoxia. Vb changed little during moderate hyperoxia (240-330 Torr) but slightly increased during extreme hyperoxia (430-490 Torr) while Vw decreased. This means that the ventilation-perfusion ratio considerably decreased during hyperoxia. The CO2 tension (PCO2) of blood rose, causing a corresponding decrease in blood pH. The O2 tensions (PO2) of arterial and mixed venous blood increased but remained low (about 40 Torr and 15 Torr, respectively). Consequently, the hemoglobin in the arterial and mixed venous blood was not saturated with O2 (about 80 and 55%, respectively) even during extreme hyperoxia. This indicates that most of the O2 which is consumed by the fish remains transported in a combined form during hyperoxia. During hyperoxia, when the decreased Vw was artificially elevated to the normoxic level, the PO2 of arterial blood (PaO2) rose further and the PCO2 and pH of arterial blood became restored to the normoxic levels. This suggests that the CO2 retention and the depressed increase in PaO2 during hyperoxia are mainly due to the decrease in Vw in the carp.

Mudskipper Periophthalmodon schlosseri Can Repay Oxygen Debts in Air but Not in Water

We determined aerial and aquatic oxygen uptake rates (&Mdot;o(2)) of Periophthalmodon schlosseri at rest and after exhaustive exercise at 30 degrees C. Resting &Mdot;o(2) in air (3.16 +/- 0.10 [SE] &mgr;mol/g/hr) was significantly higher than that in air saturated water (2.41 +/- 0.06 &mgr;mol/g/hr). When the fish was placed in air after 2 min exhaustive exercise, &Mdot;o(2) immediately increased about 2.5 times, thereby repaying oxygen debt of 5.0 &mgr;mol/g. In contrast, &Mdot;o(2) failed to show any significant elevation from the resting level as long as the fish was confined in water after exercise. However, when the fish was subsequently emerged, &Mdot;o(2) did increase above the resting level, amounting to an excess oxygen uptake of 5.5 &mgr;mol/g. These results demonstrated that the gas transport system of P. schlosseri is better adapted to air breathing and the mode of adaptation limited the capability for water breathing. J. Exp. Zool. 284:265-270, 1999. Copyright 1999 Wiley-Liss, Inc.

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 GnRHlike 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 GnRH5. 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.

Direct evidence for aerial egg deposition in the burrows of the Malaysian mudskipper, Periophthalmodon schlosseri

The presence of mudskipper eggs in an air-filled chamber was confirmed by direct endoscopic observation of intact burrows of Periophthalmodon schlosseri in a mudflat in Penang, Malaysia. For all five burrows from which video images of egg chambers were successfully obtained, the presence of air was unequivocally demonstrated by the existence of an air–water interface inside the chambers. Of these burrows, eggs were found in two, but not in the others. Eggs were laid uniformly in a monolayer on the inner top surface of the chamber. The much brighter color of the surface mud of the egg chambers than the surrounding mud, irrespective of the presence or absence of the eggs, suggested that the surface mud had been oxidized by deposited air.

Scale effects of MS-222 on a marine teleost, porgy Pagrus major

Abstract The relationship between the effective concentration (CE) of the anaesthetic MS-222 and body mass was examined at 20°C in porgy ranging in size from 0.00022 g (just after hatching) to 320 g (595 days old). CE was defined as the concentration which required 3–5 min to induce deep anaesthesia in the fish. Values of CE (ppm) increased monophasically with increasing body mass (W in g), following the equation CE = 79 W0.0549 (N = 30, r = 0.936 between log CE and log W). The recovery ratio (RR in %), i.e., the number of fish as a percentage which recovered after being placed back in ordinary seawater, varied depending on the developmental stage. RR was lowest in fish of around 0.01 g (30 days old), corresponding to the transitional stage between the post-larval and juvenile stage, whereas it was highest (nearly 100%) in the pro-larval stage (about 0.00023 g in size and 0–6 days of age).

Ecology of the Australian mudskipper Periophthalmus minutus, an amphibious fish inhabiting a mudflat in the highest intertidal zone

A population of Periophthalmus minutus inhabiting a mudflat in the highest intertidal zone in Darwin was investigated for surface activity, feeding and reproduction in relation to environmental conditions in the dry (August) and wet (February) seasons. On days with tidal inundation, the fish were diurnally active on the exposed mudflat surface at low tide, but retreated into burrows during daytime inundation and at night. Temperatures above 40°C and heavy precipitation suppressed the daytime surface activity of the fish. During neap tides, the mudflat remained uncovered by the tide for nine days in both seasons. The fish confined themselves in burrows without ingested food throughout the nine-day period in August, but they remained active on the mudflat surface and kept foraging in February. The salinity of burrow water during the nine-day emersion was extremely high (72 ± 6 psu, mean ± s.d.) in August, but lower (46 ± 9), though still higher than the open seawater value (34), in February. The burrows were J-shaped in February, but were straight (with no upturn) in August. Fertilised eggs were collected from the upturned portionof the burrow, and hatched upon submersion. Juveniles occurred in water pools on the mudflat surface in March.

Effects of experimental ventilation and ambient Po2 on O2 uptake of the isolated cutaneous tissue in the carp, Cyprinus carpio☆

Abstract Effects of experimental ventilation and ambient Po 2 on cutaneous O 2 uptake in vitro were studied in the carp, Cyprinus carpio . Oxygen uptake rate of the isolated cutaneous tissue was determined by ventilating the epidermis side of the skin with normoxic water in flow-through respirometers. Oxygen uptake rate of the skin increased with ventilation rate across the skin between 2.5 and 40 ml/min and became 3.2 nmol/cm 2 /min at a flow rate of 40 ml/min, which corresponds to an apparent water velocity of 1.1 cm/sec. At a ventilation rate of 10 ml/min, oxygen uptake rate of the skin increased with the ambient Po 2 between 115 and 230 Torr and became constant (3.8 nmol/cm 2 /min) between 230 and 295 Torr. When both sides of the skin were ventilated with normoxic water, oxygen uptake rate of the skin increased and became 3.7 nmol/cm 2 /min at a flow rate of 20–40 ml/min. These results suggest that the oxygen requirement of the skin is 3.7–3.8 nmol/cm 2 /min at 21.3°C and that cutaneous O 2 uptake in vitro depends on experimental ventilation and ambient Po 2 , consistent with values measured in vitro in the carp (ref).