The Mauthner cell in a fish with top-performance and yet flexibly tuned C-starts. I. Identification and comparative morphology.

Archerfish use two powerful C-starts: one to escape threats, the other to secure prey that they have downed with a shot of water. The two C-starts are kinematically equivalent and variable in both phases, and the predictive C-starts - used in hunting - are adjusted in terms of the angle of turning and the final linear speed to where and when their prey will hit the water surface. Presently, nothing is known about the neural circuits that drive the archerfish C-starts. As the starting point for a neuroethological analysis, we first explored the presence and morphology of a pair of Mauthner cells, which are key cells in the teleost fast-start system. We show that archerfish have a typical Mauthner cell in each medullary hemisphere and that these send by far the largest axons down the spinal cord. Stimulation of the spinal cord caused short-latency all-or-none field potentials that could be detected even at the surface of the medulla and that had the Mauthner cell as its only source. The archerfish's Mauthner cell is remarkably similar morphologically to that of equally sized goldfish, except that the archerfish's ventral dendrite is slightly longer and its lateral dendrite thinner. Our data provide the necessary starting point for the dissection of the archerfish fast-start system and of any role potentially played by its Mauthner cell in the two C-start manoeuvres. Moreover, they do not support the recently expressed view that Mauthner cells should be reduced in animals with highly variable fast-start manoeuvres.

The Mauthner cell in a fish with top-performance and yet flexibly tuned C-starts. II. Physiology.

The parallel occurrence in archerfish of fine-tuned and yet powerful predictive C-starts as well as of kinematically identical escape C-starts makes archerfish an interesting system to test hypotheses on the roles played by the Mauthner cells, a pair of giant reticulospinal neurons. In this study, we show that the archerfish Mauthner cell shares all hallmark physiological properties with that of goldfish. Visual and acoustic inputs are received by the ventral and lateral dendrite, respectively, and cause complex postsynaptic potentials (PSPs) even in surgically anaesthetised fish. PSP shape did not indicate major differences between the species, but simple light flashes caused larger PSPs in archerfish, often driving the cell to fire an action potential. Probing archerfish in the classical tests for feedback inhibition, established in the Mauthner-associated networks in goldfish, revealed no differences between the two species, including the indications for electrical and chemical synaptic components. Also, the established hallmark experiments on feed-forward inhibition showed no differences between the goldfish and archerfish Mauthner system. Extending these experiments to visual stimuli also failed to detect any differences between the two species and suggested that acoustical and visual input cause feed-forward inhibition, the magnitude, time course and duration of which match that of the respective PSPs in both archerfish and goldfish. Our findings question simple views on the role of the Mauthner cell and suggest that the archerfish Mauthner cell should be a good system to explore the function of these giant neurons in more sophisticated C-start behaviours.

Spatial and temporal variation of macro-, meso- and microplastic abundance on a remote coral island of the Maldives, Indian Ocean.

Plastic debris is ubiquitous in the marine environment and the world's shores represent a major sink. However, knowledge about plastic abundance in remote areas is scarce. Therefore, plastic abundance was investigated on a small island of the Maldives. Plastic debris (>1mm) was sampled once in natural long-term accumulation zones at the north shore and at the high tide drift line of the south shore on seven consecutive days to quantify daily plastic accumulation. Reliable identification of plastic debris was ensured by FTIR spectroscopy. Despite the remoteness of the island a considerable amount of plastic debris was present. At both sites a high variability in plastic abundance on a spatial and temporal scale was observed, which may be best explained by environmental factors. In addition, our results show that snapshot sampling may deliver biased results and indicate that future monitoring programs should consider spatial and temporal variation of plastic deposition.