Diel vertical migration (DVM) by zooplankton is a universal feature in all the World’s oceans, as well as being common in freshwater environments. The normal pattern involves movement from shallow depths at night to greater depths during the day. For many herbivorous and omnivorous mesozooplankton that feed predominantly near the surface on phytoplankton and microzooplankton, minimising the risk of predation from fish seems to be the ultimate factor behind DVM. These migrants appear to use deep water as a dark daytime refuge where their probability of being detected and eaten is lower than if they remained near the surface. Associated with these vertical movements of mesozooplankton, predators at higher trophic levels, including invertebrates, fish, marine mammals, birds and reptiles, may modify their behaviour to optimise the exploitation of their vertically migrating prey. Recent advances in biotelemetry promise to allow the interaction between migrating zooplankton and diving air-breathing vertebrates to be explored in far more detail than hitherto.

http://www.swansea.ac.uk/bs/turtle/reprints/hays_Hydrobiologia2003.pdf

A review of the adaptive significance and ecosystem consequences of zooplankton diel vertical migrations

Bioluminescence spans all oceanic dimensions and has evolved many times— from bacteria to fish—to powerfully influence behavioral and ecosystem dynamics. New methods and technology have brought great advances in understanding of the molecular basis of bioluminescence, its physiological control, and its significance in marine communities. Novel tools derived from understanding the chemistry of natural light-producing molecules have led to countless valuable applications, culminating recently in a related Nobel Prize. Marine organisms utilize bioluminescence for vital functions ranging from defense to reproduction. To understand these interactions and the distributions of luminous organisms, new instruments and platforms allow observations on individual to oceanographic scales. This review explores recent advances, including the chemical and molecular, phylogenetic and functional, community and oceanographic aspects of bioluminescence.

/pdf/bioluminescence-in-the-sea-43cqr67xhm.pdf

Bioluminescence in the Sea

The study of vertical migrations in aquatic organisms has a long and colourful history, much of it to do with the effects of changing sampling technology on our understanding of the phenomenon. However, the overwhelming majority of such studies carried out today still depend on detecting differences in vertical distribution profiles during some course of time, or acoustic echoes of migrating bands of organisms. These can not distinguish migratory activity of individual organisms, but can only assess net results of mass transfers of populations, which may integrate many individual migrations. This is an important distinction, for without knowing the actual movements of individuals it seems unlikely that we will be able to understand their causes, nor the effects of vertical migrations on the environment or on the migrators themselves. This review examines evidence for individual vertical movements gathered from 'tracers', mainly gut contents, and reviews the evidence for the hypothesis that such movements are in fact driven by hunger and satiation. The more recently appreciated vertical migrations of phytoplankters and their similarities in form and driving forces to those of zooplankton and nekton are also discussed. Finally, the role of vertical migrators in vertical fluxes of materials is discussed, along with the consequences of satiation-driven descent for such estimates.

Eat and run? The hunger/satiation hypothesis in vertical migration: history, evidence and consequences

Summary 1. Diel vertical migration (DVM) is a widespread phenomenon among marine and freshwater organisms and many studies with various taxa have sought to understand its adaptive significance. Among crustacean zooplankton and juveniles of some fish species DVM is accepted widely as an antipredator behaviour, but little is known about its adaptive value for relatively large-bodied, adult predatory fish such as sharks. Moreover, the majority of studies have focused on pelagic forms, which raises the question of whether DVM occurs in bottom-living predators. 2. To investigate DVM in benthic predatory fish in the marine environment and to determine why it might occur we tracked movements of adult male dogfish ( Scyliorhinus canicula ) by short- and long-term acoustic and archival telemetry. Movement studies were complemented with measurements of prey abundance and availability and thermal habitat within home ranges. A thermal choice experiment and energy budget modelling was used to investigate trade-offs between foraging and thermal habitat selection. 3. Male dogfish undertook normal DVM (nocturnal ascent) within relatively small home ranges ( ∼ 100 × 100 m) comprising along-bottom movements up submarine slopes from deeper, colder waters occupied during the day into warmer, shallow prey-rich areas above the thermocline at night. Few daytime vertical movements occurred. Levels of activity were higher during the night above the thermocline compared to below it during the day indicating they foraged in warm water and rested in colder depths. 4. A thermal choice experiment using environmentally realistic temperatures supported the field observation that dogfish positively avoided warmer water even when it was associated with greater food availability. Males in laboratory aquaria moved into warm water from a cooler refuge only to obtain food, and after food consumption they preferred to rest and digest in cooler water. 5. Modelling of energy budgets under different realistic thermal-choice scenarios indicated dogfish adopting a ‘hunt warm − rest cool’ strategy could lower daily energy costs by just over 4%. Our results provide the first clear evidence that are consistent with the hypothesis that a benthic marine-fish predator utilizes DVM as an energy conservation strategy that increases bioenergetic efficiency.

Hunt warm, rest cool: bioenergetic strategy underlying diel vertical migration of a benthic shark

Habitat selection processes in highly migratory animals such as sharks and whales are important to understand because they influence patterns of distribution, availability and therefore catch rates. However, spatial strategies remain poorly understood over seasonal scales in most species, including, most notably, the plankton-feeding basking shark Cetorhinus maximus. It was proposed nearly 50 yr ago that this globally distributed species migrates from coastal summer- feeding areas of the northeast Atlantic to hibernate during winter in deep water on the bottom of con- tinental-shelf slopes. This view has perpetuated in the literature even though the 'hibernation theory' has not been tested directly. We have now tracked basking sharks for the first time over seasonal scales (1.7 to 6.5 mo) using 'pop-up' satellite archival transmitters. We show that they do not hiber- nate during winter but instead undertake extensive horizontal (up to 3400 km) and vertical (>750 m depth) movements to utilise productive continental-shelf and shelf-edge habitats during summer, autumn and winter. They travel long distances (390 to 460 km) to locate temporally discrete produc- tivity 'hotspots' at shelf-break fronts, but at no time were prolonged movements into open-ocean regions away from shelf waters observed. Basking sharks have a very broad vertical diving range and can dive beyond the known range of planktivorous whales. Our study suggests this species can exploit shelf and slope-associated zooplankton communities in mesopelagic (200 to 1000 m) as well as epipelagic habitat (0 to 200 m).

/pdf/seasonal-movements-and-behaviour-of-basking-sharks-from-5e0g6uxtas.pdf

Seasonal movements and behaviour of basking sharks from archival tagging: no evidence of winter hibernation

A 4.9 m TL megamouth shark, only the sixth specimen known to science, was tracked continuously for 50.5 h, during which it exhibited distinct vertical migrations at the dawn and dusk transitions. The male shark was captured on 21 October 1990 in a drift gill net off Dana Point, California, restrained overnight in a harbor, and released at sea the next afternoon. Horizontally, the shark moved slowly southward, covering 62 km on a relatively straight path with no significant diel changes. For the major part of the tracking, its rate of movement was 1.15 km h−1, as determined from positions at 15 min intervals. Considering a probable head current of 10–25 cm sec−1, its estimated through-the-water swimming speed was more likely 1.5–2.1 km h−1 (X¯ = 1.8, representing 0.1 body lengths sec−1). Vertically, the shark stayed shallow at night (12–25 m depth range, X¯ = 17) and deep during the days (120–166 m, X¯ = 149) but still well above the bottom at 700–850 m. The four twilight depth-change events were very distinct and always spanned the times of sunset or sunrise. The ascent and descent profiles are a reasonble match to isolumes on the order of 0.4 lux for an extinction coefficient (0.07) calculated from water transparency measurements. Furthermore, the steepest parts of the shark's profiles correspond closely to the times of maximum rate-of-change of illumination. These findings suggest that, except during nights, the shark's chosen depth was to a large degree determined by light level.

https://faculty.washington.edu/sisneros/Nelson%20et%20al.%201997.pdf

An acoustic tracking of a megamouth shark, Megachasma pelagios: a crepuscular vertical migrator

Three of four new jawfishes described herein have sexually dimorphic jaws and dichromatic maxillary markings: O. smithvanizi, with a simple nasal cirrus, buccal pigmentation and other traits similar to members of the O. macrognathus group, is known only from Isla del Cano, a continental shelf island off southern Costa Rica; O. fossoris, with a multifid nasal cirrus, a broad dorsal membranous subopercular flap and a black spot on tips of first dorsal-fin spines lives in the Gulf of California and is a sister-species of O. galapagensis; the maxilla of O. walkeri terminates in a flexible scimitar-shaped lamina in adults of both sexes, but is longer in males, the species lacks nasal cirri and is also restricted to the Gulf of California. Opistognathus brochus is a small species with dark speckling on head and body, and several dark blotches along the dorsal fin and two bars on the tail; it is found in moderately deep water on the Costa Rican coast and Gulf of California. Opistognathus mexicanus is placed in the synonymy of O. punctatus. Opistognathus galapagensis, O. rhomaleus and O. fenmutis are recorded for the first time from Costa Rica and a description of the latter is given and an identification key and summary table are provided for all known eastern Pacific species of Opistognathus

Robert J. Lavenberg

Papers

An acoustic tracking of a megamouth shark, Megachasma pelagios: a crepuscular vertical migrator

Four new species of eastern tropical Pacific jawfishes ( Opistognathus : Opistognathidae)