The Invertebrates

The invertebrates are a rather diverse group to attempt to address but due to the nature of bioluminescence and the problems faced by organisms in the deep-sea the systems and characteristics of the various taxa are surprisingly uniform.

Few crustacean groups are known to include luminous species; namely the decapods, ostracods, copepods, amphipods, euphausiaceans and mysids (Herring 1985). All accounts of crustacean bioluminescent are of self-illuminating systems.

Figure 5.1 Bioluminescent Ostracod (Ocean Lab, University of Aberdeen)

Ostracods (Figure 5.1) have been recognised for their bioluminescence since the 1850s (Okado & Kato 1949, Herring 1985). These were mainly coastal, shallow-water species and were well studied (Herring 1985). Like dinoflagellates they emit pulses and flashes when disturbed as a defence mechanism. Large swarms have also been seen to synchronize flashes during mating displays. The family Halocypridae has luminous representatives that do occur in the mesopelagic and bathypelagic zones though, unlike the shallow water Cypridininae, glands are focused along carapace margins (Oakley 2005).

Due to the problems faced in simply identifying copepod species there are many conflicting reports about how many species and Genus exhibit bioluminescence. Many copepods are bioluminescent with the ability to deliver high intensity flashes as distractions and deterrents to predation (Herring 1985).

Figure 5.2. Gnathophausia, a deep-sea bioluminescent decapod (tolweb.org)

Decapods are far better understood and express bioluminescence in a variety of different ways with the majority of known shrimp and prawn species displaying the ability in the deep oceans. Gnauthophausia (Figure 5.2) is a well studied pelagic species with many recorded displays of bioluminescence. An extreme response to disturbance is the secretion of a bioluminescent cloud from the anterior end of the animal (Herring 1985). Other decapod species such as Acanthephyra purpurea exploit similar survival and evasion techniques. As well as secretions, adult Sergestes lucens have as many as 184 photophores located ventrally and while males have two pairs of photophores between the bases of the fifth pereiopods, females have only one (Herring 2007). These are employed as a counter-illumination defence.

Figure 5.3. Stauroteuthis syrtensis, a deep-sea bioluminescent octopod (biology.duke.edu/johnsenlab)

Some 63 of the 100 squid and cuttlefish genera have bioluminescent members while only 2 out of 43 octopod genera have substantiated observations of bioluminescence (Johnsen et al. 1999). Bioluminescence is common but highly complex among the Cephalopoda¬†with “lenses, reflectors, irises, interference filters, pigment screens, and shutters” being utilized within photophore organs (Herring 1988, Johnsen et al. 1999). Stauroteuthis syrtensis (Figure 5.3) is one of the only two octopod species to exhibit bioluminescence and this is reported to only be in females. Caught specimens tested for bioluminescence with gentle stimulation were found to have 40 sucker-like photophores lining the oral side of the animal’s eight tentacles, clearly visible in Figure 5.3 (Johnsen et al. 1999). The reason for the evolution of the bioluminescent organs in place of suckers is unknown. Johnsen et al. (1999) has speculated as to why, stating the obvious applications of defence and reproductive pressures but no clear expression has been recorded. After relocating to colonise the pelagic deep-sea from shallow seas the ability to adhere to substrata became obsolete, while, like with the vast majority of bioluminescent species, the need to deter predation became paramount (Johnsen 1999). The bioluminescent response to disturbance of the animal’s photophores suggests it is at least a defence mechanism but the sheer number and the ability of the animal to use them in bright short bursts, as strobes or for periods of up to 5 minutes suggests they serve a secondary purpose (Johnsen et al. 1999). Herring (2000) refers to work on Vargula back in 1986 which clearly states recorded observations of bioluminescent courtship displays. Johnsen et al. (1999) failed to cite these claims but mention unsubstantiated evidence they identify members of the same species for mating. Personally, I believe in the absence of ambient light bioluminescence seems an ideal solution to providing a sexual display.

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