Future Directions in Bioluminescence Research

---

UNCLASSIFIED WORKGROUP SESSIONS

Physical and Biological Driving Mechanisms
Discussion Leader: Peter J. Herring

Bioluminescence in the ocean can be produced by a very great number of different species and groups, ranging from unicellular dinoflagellates to large fish and squid. At any upper ocean location many of these organisms will be present; it is almost certain that some measurable (and visible) bioluminescence will be produced anywhere in the global oceans by mechanical or other stimuli. At many locations the densities of bioluminescent organisms will be low and their integrated light output negligible in operational terms. The strategic significance of bioluminescence arises in circumstances where the densities of bioluminescent organisms are high enough to generate an intense light signal.

We therefore consider the mechanisms whereby bioluminescent organisms may multiply by rapid population growth and/or existing populations may be aggregated by other factors. Dinoflagellates and copepod crustaceans are the most frequent causes of intense bioluminescence and we concentrate on these organisms. Ostracod crustaceans and euphausiid shrimp may occasionally be important in particular areas.

1. Physical mechanisms

A. Population growth

Dinoflagellates reproduce rapidly, with generation times of only a few days, compared to copepods whose generation times are more usually weeks or months. Dinoflagellate population growth will be boosted by the development of a thermocline and a stable upper mixed layer located above the critical depth. In these circumstances the provision of sufficient nutrients and a high light intensity is likely to initiate a growth phase, modified in coastal regions by the effects of run-off on nutrients and salinity levels. There is already a very large database on the growth parameters of dinoflagellate populations, driven primarily by the economic effects of red tides. It is also noteworthy that several dinoflagellate species implicated in red tide events are bioluminescent (e.g. Lingulodinium polyedrum and Alexandrium tamarense)

B. Aggregations

Many of the most intense bioluminescent areas arise from the aggregation of dinoflagellate populations. Aggregations may occur in the vertical plane (as thin layers) or in the horizontal plane (as patchiness). The physiology of some heterotrophic dinoflagellates leads to an increase in buoyancy in stressed populations, so that in calm conditions the individuals accumulate as a scum on the surface (at densities of ~10¹⁰ m^-3). This is a vertical layering at the air/water interface, homologous to thin layers of these and other organisms which may accumulate at density interfaces at different levels beneath the surface. Surface layers of this type are usually further aggregated by the action of Langmuir circulations and other, larger-scale, frontal systems. Because the vertical mixing at fronts may also enhance nutrient supply these features are in any event often associated with increased levels of bioluminescence.

Passive accumulation of material such as marine snow at subsurface density interfaces may render these layers more attractive to foraging copepods and result in their secondary aggregation in high densities, with the potential for enhanced bioluminescent signals.

2. Biological mechanisms

Biological driving mechanisms result from the behavioural and physiological characteristics of different species, and include circadian rhythms, predator-prey relationships, diel vertical migration and swarming. Many dinoflagellates (especially the autotrophic, photosynthetic species) have a light-driven circadian rhythm of bioluminescence. Mechanical stimulation produces flash responses by night, but not by day. Measurements of stimulable bioluminescence made in daylight will greatly underestimate the population’s potential bioluminescence at night.

The distribution of dinoflagellates is primarily determined by the physical mixing processes but that of zooplankton such as copepods is more behaviourally controlled. The grazing of zooplankton on dinoflagellate populations will greatly affect both their rate of population increase and their absolute densities. For some species there will be a density at which dinoflagellate metabolites (e.g. toxins) will inhibit grazing and provide a protective feedback loop. Levels of bioluminescence may act similarly in dense populations, as a consequence of the burglar-alarm effect of flashing produced by grazing organisms.

Diel vertical migration (vertical movements with a diel rhythm) does occur in calm conditions over ~10-15m in some species of dinoflagellate, but it is generally of more importance to zooplankton species. Many animals migrate into near-surface waters at night from greater daytime depths (probably to feed). Many of these are luminous animals such as copepod and ostracod crustaceans, euphausiid shrimp and lanternfishes. Their migratory patterns will determine the bioluminescence potential of the near-surface waters, and the extent of the migration will in turn be affected by factors such as water clarity and moonlight intensity. Shoals of these large animals may themselves produce intense bioluminescent signals as a result of their disturbance of other organisms (e.g. dinoflagellates) and have the potential for generating "false positives" in operational terms (the same effect can be used to guide fishermen to shoals at night). Migrations with a lunar periodicity are a phenomenon of some shallow water luminous polychaete worms ("fireworms"), whose mating swarms may produce brief local patches of high bioluminescence.

The daytime distribution in the upper 200m of closely related species (e.g. luminous copepod crustaceans) are vertically stratified, with the result that population maxima of different species are separated by 10s of m or more. Vertical variation in bioluminescence will result, exacerbated by finer-scale accumulations of particular species as thin layers on density interfaces.

Behavioural aggregations (swarms) are not well-documented in the open ocean, and those of small animals (ostracods, copepods) are likely to be easily dispersed by local turbulence in coastal regions. Nevertheless these swarms may produce large-scale bioluminescent phenomena in particular regions (e.g. NW Indian Ocean where bioluminescent ostracods form huge local swarms). It is not known what cues initiate these swarms, but there may be a lunar element involved. Other large-scale phenomena such as "milky seas" may involve luminous bacteria. Some empathetic stimulation of one organism by the luminescence of another could result in propagated bioluminescence. Artificial lights certainly induce a bioluminescent response in many different animals (ostracods, copepods, euphausiid shrimp, lanternfish etc). Calculations suggest that the light of one Pyrosoma colony can potentially stimulate another colony at a range of several 10s of m, but there are no environmental observations that this intraspecific stimulation occurs other than in some shallow tropical ostracods.

There are many bioluminescent species which live on or in the bottom, sometimes emerging into the plankton at night (e.g. some ostracods) and it should be recognised that operations very close to (or on) the sea floor may encounter different bioluminescent risks (e.g. sea pansies, brittlestars etc), regardless of whether there are bioluminescent organisms in the overlying water.

3. Model systems

Dinoflagellates are usually the dominant source of any stimulable bioluminescent signal in the ocean. A few genera account for most of the observations, and a focus on Noctiluca, Pyrocystis, Protoperidinium, Gonyaulax and Lingulodinium would probably provide proxies for all others. The red tide research programmes are hugely valuable as a data source.

Copepods are the dominant bioluminescent zooplankton in most circumstances, and in the upper ocean they are almost invariably species of the genera Metridia or Pleuromamma. Research focused on the luminescence of any one species from these genera could be extrapolated to the others with little loss of fidelity. There is already a considerable database on their bioluminescence and (separately) on their biology, recognising their importance in the trophic relationships in many regions.

   Next summary >>

  [Download printable PDF version of this page]