**4.8 Bioluminescence in echinoderms**

Four out of the five classes of echinoderms, namely Ophiuroidea (brittle stars), Asteroidea (starfishes), Holothuroidea (sea cucumbers) and Crinoidea (sea lilies) are bioluminescent [50]. Echinoderms mostly use coelenterazine dependent bioluminescent systems, although some of them also use a novel photoprotein [4]. Bioluminescence is more commonly exhibited by echinoderms inhabiting deep seas. Many new bioluminescent taxa are still being discovered, and 70 ophiuroid species have been recognized to exhibit bioluminescence till date [81, 82].

## **4.9 Bioluminescence in tunicates**

Many species of tunicates are known to exhibit bioluminescence, though planktonic tunicates are not as frequent exhibitors of the trait as planktonic larvacean Appendicularia. However, it cannot be ascertained accurately because some filter feeders (like *Pyrosoma*) may ingest and trap luminescent microbes and appear to be bioluminescent [50]. Species like *Balanoglossus* (Acorn worms) and *Ptychodera* of the class Enteropneusta are also known to be bioluminescent. Also, the sessile adult *Clavelina miniata* glows green when stimulated.

#### **4.10 Bioluminescence in fish**

Among vertebrates, fish are the only taxa that have the ability of bioluminescence. This trait is found in fish inhabiting all the depths of the ocean, but is most frequently encountered in specimens from the deepest recesses of the ocean [6]. Bioluminescence is found in about 1500 species of marine bony fish spanning 43 families in 11 different orders [4, 5, 83], out of which some like the anglerfish, flashlight-fish (*Photoblepharon*) and pony-fish (*Leiognathus*) harbor symbiotic bacteria in discrete, specialized light organs, while others produce glow intrinsically [84]. On the other hand, only a handful of shark species in three families of cartilaginous fish are known to exhibit bioluminescence [83]. Unlike bony fish species, cartilaginous fishes do not rely on symbionts for bioluminescence [85], but use an altogether different, unknown bioluminescent system [86]. Some other species like the midshipman fish *Porichthys* and various lantern-fish obtain their respective luciferins from dietary sources [13].

Fish use the ability of bioluminescence for a variety of applications like communication, evading predators, luring prey. The latter is highly expressed in various taxa inhabiting the deep seas. Various anatomical modifications (like the light organs in various bony fish and the esca of anglerfish) harbor symbiotic bacteria, which enable the fish to use the bacterial emission with ample control on the intensity as well as distribution of the emission [4]. Fish of the order Stomiiformes (like dragon-fish, etc.) have evolved most elaborately arranged photophores, including those emitting red light [4]. Cookie-cutter sharks are interesting examples of both counterillumination and mimicry, as they bait their prey with non-luminescent patches on their bodies that look like small fish.

Bioluminescence may also prove disadvantageous to some species in certain cases. For example, elephant seals follow bioluminescence to track down prey populations. Some studies have shown that seals prefer to hunt in locations where there are more bioluminescent individuals [4].

#### **5. Future prospects**

Even though we still need to understand the dynamics and biochemistries of many bioluminescent systems in nature, humans have already begun to put bioluminescence to various applications. Bioluminescent mechanisms have been used in the diagnosis of various pathological conditions in the form of Green Fluorescent Proteins (GFP) [20]. Furthermore, attempts are being made to incorporate bioluminescent systems into plants to supplement illumination [87–89]. However, these prospects are still in their developmental stages, and there are various challenges and issues that need to be tackled.

### **6. Conclusion**

The emergence of bioluminescence in nature has occurred independently on multiple occasions, which certainly means that it confers some significant evolutionary advantage(s) which we are yet to understand fully. This is bolstered by the fact that there are so many species that exhibit this trait, and show a plethora of behavioral, anatomical and ecological trends so as to survive and thrive in various habitats. With a better understanding of these systems and their interactions, we will certainly be able to use this phenomenon to our advantage. However, there are some challenges that keep us from fully exploring certain bioluminescent systems. For example, the deep sea bioluminescent systems are very hard to access, and thus *in-situ* observations are few and far between. With the advent of new tools and techniques, we shall be able to gain a better insight into the dynamics of these systems.
