Illuminating the ocean_submerge magazine.pdf

FEATURE | ILLUMINATING THE OCEANIlluminatingthe oceanBy Aida VerdesBioluminescent creatures are found throughoutmarine habitats, from the ocean’s surface to the deepocean floor. Enlighten yourself about this alluringability that the majority of sea creatures possess.22 | SUBMERGE Let the dive begin

ILLUMINATING THE OCEAN | FEATUREA round stingray illuminated.Image by David Gruber and John Sparks.As a PhD student with research interests in thebioluminescence and fluorescence properties of polychaetes(segmented worms), my research aims to clarify the biochemicalprinciples of bioluminescence as well as to understand thedifferent functions and behavioural implications of lightproduction. As part of my research work, I recently headed toBermuda to witness the illuminating mating ritual of fireworms.It is 12 August 2014 and the sun has just set over the calmblue waters of Bermuda. There was a full moon two nightsbefore and here I am in the water, ready for the show to begin.Around an hour later, the first female fireworm appears, glowingmagically with an intense green light and swimming in circles onthe ocean’s surface. The mating ritual has its desired effect andjust a few seconds later, a male fireworm swims to the surface,emitting short flashes of light. In just about 15 minutes, dozensof fireworms leave the ocean bottom and swim to the surface todisplay their bioluminescent mating dances, turning the watersof Ferry Reach Point into a spectacular light show.Bioluminescence is the emission of light produced bya chemical reaction. The Bermuda fireworm is a marinepolychaete worm that uses bioluminescence to attract matesfor reproduction. Other members of this family also usebioluminescence in their mating rituals, which are periodic andsynchronised with the moon cycle. Females can glow for up toeight seconds and can repeat the process 33 times over a periodof 12 minutes or until they attract a male. Males and femalesgather at the surface, swimming in narrow circles and secretingtheir gametes for external fertilisation. Odontosyllis worms arejust a few millimetres long, but their glow is so bright that it iseasily visible from 50m away.These shallow-water polychaete worms are not theonly marine bioluminescent organisms out there. In fact,bioluminescence is widespread across the tree of life, rangingfrom bacteria to vertebrates, and the great majority of lightproducingorganisms are marine dwellers. More than 80% of theapproximately 700 known bioluminescent genera are marineorganisms, and within the marine environment, over 90% ofthe biota can produce their own light. The main reason whyorganisms produce light is not completely understood, butthis feature is closely related with a variety of vital functionssuch as reproduction, feeding, defence and/or communication.In general, longer bioluminescent glows are thought to havean attracting function and short bioluminescent flashes areassociated with repellent signals.Many species of squid use bioluminescent flashes to confuseor startle their predators, which cause them to hesitate andallows the squid enough time to quickly escape. The vampiresquid and other animals, including copepods, shrimps andctenophores, exhibit variations of this defensive behaviour inwhich they exude or secrete a bioluminescent cloud that actslike a smoke screen, thereby making it difficult for the predatorto locate the escaping prey. Another common defensiveAugust/September 2015 SUBMERGE | 23

FEATURE | ILLUMINATING THE OCEANBelow left:An Acanthurid Iarva radiatesa magnificent green light.Image by David Gruber andJohn Sparks.Below right:A flathead emits red light.Image by David Gruber andJohn Sparks.Bottom left and right:Examples of bioluminescentand fluorescent worms.Images by Aida Verdes.mechanism is the use of a sacrificial tag. Organisms like the deepseasquid or the polynoid worm autotomise luminous body partswhen they encounter a predator. The lost body parts continue tomove and can glow for hours, drawing away the attention of thepredator, while the rest of the animal escapes in the dark.Many marine animals such as crustaceans, cephalopods andfishes also use a defensive technique called counter-illumination.This is a form of camouflage in which the animal uses ventrallight-producing organs, called photophores, to match the lightcoming from the surface and to disguise their shadow to avoidbeing detected by predators looking upwards.Bioluminescence is also commonly used in predatorybehaviours to lure or search for prey. The most famousbioluminescent predator is probably the anglerfish, which hasa light organ where bioluminescent bacteria are cultured. Thefish can control the amount of light that is emitted by alteringthe conditions in the light organ. Dragonfishes also haveluminous barbels which they use to attract prey, although inthis case, bioluminescence is not produced by bacteria, but bythe dragonfish itself. A particular type of dragonfish, called theloosejaw dragonfish, has special photophores that are able toemit red light, which represents a great advantage since mostmarine organisms can only see blue light. When loosejaws lookfor their prey, mostly copepods, they can see but cannot be seen.The cookiecutter shark seems to use bioluminescence tocombine both counter-illumination and prey-attraction in itspredatory behaviour. This small shark feeds by taking bites out ofthe flesh of larger fishes, squids and cetaceans. It is hypothesisedthat the cookiecutter shark uses counter-illumination to emulatethe silhouette of these larger species’ prey and attract them closeenough to be able to bite them.Whether for defence, predation or communication,bioluminescence is widespread and very common in the marineenvironment. From the shallow depths of Bermuda to thedeepest depths of the ocean floor, the underwater world is filledwith organisms that can light up your dive. So, turn the lights off,watch carefully and enjoy the show. SBioluminescence is widespreadand very common in themarine environment.24 | SUBMERGE Let the dive begin