**4. Bioluminescence in animals: distribution and ecological significance**

As it is expected, the complexity of bioluminescence certainly upgrades as we proceed upwards in the tree of life. There are no plants (terrestrial or aquatic) that exhibit bioluminescence. Fungal bioluminescence is rare, and has been discussed in the previous sections. Coming to bioluminescence in animals, there is a strong agreement that the evolution of bioluminescence first occurred in the ocean, as the oceanic ecosystem offers many favorable conditions like optical homogeneity, stability of environment, large areas that are almost or completely perpetually dark and a large diversity of organisms that can engage in a variety of ecological interactions [4]. This, and the fact that both luminous as well as non-luminous prey in the ocean are rich in luciferins ensures that the emergence of bioluminescence in the

ocean must have been a comparatively easy process [4, 33, 34]. The phenomenon of bioluminescence is so significant in the oceanic ecosystem, that it serves as the predominant source of illumination in many parts of the ocean [35]. Furthermore, courtships involving bioluminescence have been reported to show higher species accumulation rates than those without bioluminescence [36]. The presence of many independent coelenterazine-mediated bioluminescent systems, nine different phyla to be exact [10], indicates dietary linkage, as coelenterazine is procured by most species mainly through their diet [16]. Bioluminescence is encountered most commonly in the topmost 1 kilometer layer of the ocean, and is doubtlessly the most efficient mode of communication in the oceanic ecosystem [35]. The ability to glow is strongly habitat dependent because of various selection forces described earlier, and it is observed that there is a marked difference in the occurrence of this trait as we go deeper in the ocean [35].

Bioluminescence is also common in the terrestrial ecosystems, though it is nowhere as abundant as in the ocean. Various worms and arthropods are known to exhibit complex behaviors related to this phenomenon. It is clear that bioluminescence has a powerful impact on behavioral and ecosystem dynamics [4].

In this section, bioluminescence has been followed as a trait through various animal phyla, both terrestrial and aquatic, and its ecological significance is simultaneously discussed.

#### **4.1 Bioluminescence in ctenophores**

Comb jellies are the phylogenetically the most basic examples of bioluminescence in animals. Many species like *Mnemiopsis* [20, 37] use calcium activated coelenterazine as their bioluminescent substrate [4]. Some species, for example *Beroe forskalii* are known to produce myriad, cascading wave-patterns of intrinsic glow on their bodies, and some even emit a haze of glowing particles to startle the predator as a defensive measure, coupled with an escape response [38]. A majority of pelagic species are likely to exhibit bioluminescence [35]. The photo-proteins involved in bioluminescence in various genera like *Mnemiopsis* and *Beroe* have been studied, and are known to depend on calcium ions for their activity [39, 40].

Many comb jellies like *Pleurobrachia* and some species of the genus *Beroe* also show a startling display of rather colorful lights, in various wavelengths found in the visible spectrum. This was mistakenly believed as bioluminescence in the past. However, the said lights were not actually "produced" in the organism itself, as was evident in some studies [41, 42]. This iridescence was rather found to be a result of refraction of ambient light through the moving combs as the organism swims around [43].

#### **4.2 Bioluminescence in cnidarians**

Cnidarians in both pelagic as well as benthic zones, including corals, anemones, hydroids and medusae are known to exhibit bioluminescence. All of them use the luciferin coelenterazine as the substrate for their biochemical pathways (hence the name "coelenterazine"). Most of the pelagic siphonophores encountered show bioluminescence [4, 35]. The most common examples of bioluminescent coelenterates is the shallow-living hydrozoan Crystal Jelly (*Aequorea victoria*), the sea pansy *Renilla* and also the bamboo corals from the pelagic zone [44]. Anatomically, light producing centers, or photocytes, may be clustered or widely scattered all over the body, located around the endodermal layer [20]. The bioluminescent system of *Renilla* has been studied extensively, and attempts have been

made to triangulate and engineer the genes from the source into various eukaryotic (plant) systems [45].

Cnidarians use bioluminescence for various defensive, agressive as well as warning purposes. Some jellyfish show glowing wave patterns on their umbrellas, and even emit clouds of glowing particles as a part of their escape response [4]. Siphonophores use bioluminescence to attract prey within reach of their cnidocytes. Some jellyfish are also known to show aposematic glow, which is indicative of distastefulness. Cnidarians can gain a lot from aposematic bioluminescence, as it would not only warn the predators of the unpalatability of the individual, but also protect them from any physical injuries [4]. However, many predator species like leatherback turtles use this to their advantage, and easily locate prey like jellyfish.

### **4.3 Bioluminescence in annelids**

Bioluminescence in annelids has independently emerged in several lineages [46], resulting in a rich taxonomic diversity [36] spanning across 45 different genera in 13 lineages of clitellates and polychaetes [7]. They are found in diverse terrestrial and aquatic habitats all across the globe.

Clitellates are the only terrestrial annelids known, including potworms and earthworms from families Lumbridae [47] and Megascolidae [48]. Most of them emit brief flashes, and secrete a slimy coelomic fluid packed with bioluminescent granules [47, 49] under mechanical, chemical [50] or electrical stimulation. The same trend is seen in benthic species from the family Chaetopteridae [46, 51]. This is basically a form of aposematism or advertisement of distastefulness or toxicity [52], due to which predator species avoid such individuals from a distance [7].

In the marine ecosystems, polychaetes are the predominant annelid species in both pelagic as well as benthic zones [53]. Unlike their terrestrial counterparts, marine annelids show an interesting diversity of adaptations of bioluminescence, which they use for a variety of functions. The swarming behaviors of *Chaetopterus* and *Odontosyllis* spp. [51] and their flashing patterns [54] have been studied in detail. The bioluminescent "bombs" of the deep-sea genus *Swima* are detonated upon the slightest disturbance, facilitating an almost ninja-like distraction while the animal swims to safety [55]. Several members of the family Tomopteridae are known to produce golden yellow light, which is quite rare in aquatic ecosystems [56]. Scale worms (family Polynoidae) emanate flashes when disturbed, and even break off one or more bioluminescent scales or even whole parts of the body [57] as decoys or sacrificial lures for the predator while they flee [46]. Some species even shoot sticky glowing mucus at the predators to hamper their mobility, distracting them while making them even more conspicuous [58]. Arrow worms (Chaetognatha) are also known to adapt similar defensive measures. Light production also wards of symbiotic bacteria that overcrowd the tubules of some annelids [59]. Bioluminescence is also used as a mode of intraspecific communication in annelids [7]. Some members of the families Syllidae and Cirratulidae exhibit bioluminescence as a part of their mating behaviors. Elaborate bioluminescent courtship displays of the genus *Odontosyllis* are even known to align with lunar cycles [52, 60].

### **4.4 Bioluminescence in molluscs**

Bioluminescence in molluscs is represented by many unusual taxa, for example the bivalve *Pholas*, the biochemical machinery of which has been extensively studied. Also, the sea-firefly *Cypridina* is a specimen of significance, as its bioluminescent system was among the first to be studied and analysed in detail [3, 61]. The only bioluminescent organism from freshwater ecosystem, the snail *Latia* 

*neritoides*, is also a mollusc [62]. Also, the terrestrial snail *Dyakia striata* is another bioluminescent organism that has been studied in great detail [63, 64]. Also, the snail *Hinea brasiliana* uses flashes of blue light as an aposematic signal to ward off predators [65].

Cephalopods are the prominent representatives of bioluminescent molluscs, and some of these may have been the source behind the fables of the mythical Kraken. Among squids alone, there are about 70 bioluminescent genera, both symbiotic and intrinsic [66]. Most luminescent cephalopods use coelenterazine as substrate for bioluminescence [67]. Squids are almost flamboyant in their exhibition of bioluminescence. *Euprymna* is known to be symbiotic with the bioluminescent bacteria *Vibrio fischerii* to form exclusive light organs [10] which it uses for counter illumination [68]. The vampire squid *Vampyroteuthis* has light organs all over its body, and it even shoots glowing particles from the tips of its tentacles. The squid *Taningia danae* has light organs on the tips of its arms, which it uses for intraspecific communication as well as to lure, stun and baffle prey [69]. Even some octopods are known to use bioluminescence to lure prey into their glowing suckers [4]. Cephalopods are also known to autotomize entire glowing arms as decoys if threatened. Some species of octopus also use bioluminescence in courtship displays.

An interesting fact about sperm whales is that they hunt squid by triggering the burglar alarm mechanism around themselves to attract unsuspecting squids.

#### **4.5 Bioluminescence in insects**

Insects are the most predominant terrestrial organisms that exhibit the phenomenon of bioluminescence. A majority of the bioluminescent insects are beetles (Coleoptera), click beetles (Elateridae), glowworms & railroad worms (Phengodidae), and fireflies (Lampyridae) [70]. The biochemical mechanism of luminescence is similar in all of these [71], even though each of them emit a diverse palette of wavelengths [20]. Other insects like lantern flies (Homoptera), springtails (Collembola), etc. also show bioluminescence.

Among springtails, only two families exhibit bioluminescence upon mechanical stimulation. Bioluminescence occurs only during sexual phases, and is crucial for sperm transfer. Lantern flies, for example *Fulgora lanternaria*, emit bright white light when both the sexes fly together [72]. Glowworms and Fungus gnats from the order Diptera show bioluminescence only in the larval stages, where they use their glow to attract prey and snare them in webs [73]. The larvae of *Arachnocampa luminosa* are a prime example of such behavior [74]. Female glowworm pupae also glow to attract males [72].

Click beetles show bioluminescence in all stages of life [75]. In the larval stage, bioluminescence serves as a tool to attract prey, as well as for defense. The pupae also glow when illuminated, and adults use bioluminescence for various functions like defense, mating communication and even general illumination [72]. In glowworms, on the other hand, bioluminescence is only secondary to pheromone-mediated communication. Males are rarely bioluminescent, only in the sexual stages for seductive purposes, whereas larvae and females are very luminescent. The railroad worm *Phrixothrix* is highly aposematic, as its body is lined with bright green glowing patches, while it has red headlights, which is very rare among all animals [70].

Fireflies are among the most studied bioluminescent systems, especially the north American *Photinus pyralis* [76]*.* All life stages in fireflies are luminescent, and firefly larvae are known to use their glow for defensive purposes [73, 77]. Illumination patterns of fireflies may differ even for different individuals of the same species, and are highly encodable [72, 77]. Fireflies have specialized organs

### *The Ecology of Bioluminescence DOI: http://dx.doi.org/10.5772/intechopen.96636*

called lanterns in their abdominal segments, which can be controlled by the nervous system [20]. Since bioluminescence in fireflies forms the basis of various complex interspecific as well as intraspecific interactions, visual sensitivity according to the environment, time of activity and other parameters has evolved in parallel [78]. The signaling systems in firefly species are highly encodable, species specific, and crucially timed for maximum efficiency. Synchronous flashes are seen in various species, sometimes in swarms spanning 30 meters [72], producing spectacular displays like the ones at Chaophraya river, Bangkok. The biological significance of such displays are still not understood [73]. Due to the uniqueness of the signaling mechanism, some species have evolved to mimic other species specific signals. For example, female fireflies of the genus *Photuris* mimic the female signal of *Photinus macdermotti* to attract and prey upon their males [72]. Fireflies are also highly distasteful to predators, which is exhibited by their aposematic signals, a necessary counter measure to compensate for their high conspicuousness. Today, fireflies are adversely affected by the growing numbers of artificial lighting systems, which hamper their signaling and even cause direct mortality in some cases [79].
