Rivers TJ and Morin JG. 2008. Complex sexual courtship displays by luminescent male marine ostracods. The Journal of Experimental Biology 211: 2252-2262.

Table 18.3 in our textbook by Jack Bradbury gives general predictions on the qualities of modality specific signals if they are to be used in a courtship context. Design rules for visual mechanisms predictions can be applied to the Rivers and Morin article, which is an in-depth discussion of the signal pattern and function of another bioluminescent animal: a Caribbean ostracod crustacean. A part of the general marine zooplankton, ostracods seem too small to attract mates via sound, and the targeting of olfactory mechanisms would be compromised in water. The species, Vargula annecohenae, instead produces ritualized luminescent “display trains” or swooping patterns, which are among the most complex light displays known in marine systems. The authors were interested in characterizing this display pattern to measure the between-male variation in and complexity of each part of the signal. As in the previous paper on Photinus greeni, males should produce similar signals to ensure the commonality of the function of the signal and to allow females to locate and choose males (in this case, in the water column).

As expected in table 18.3 for visual signals, Rivers and Morin state the V. annecohenae courtship signal is long in duration (up to one hour) and includes flashing patterns only produced by males, as in fireflies, accompanied by stereotypic movements. Strong differences, however, are seen in the mode of production and excretion of the luminescent molecules. The ostracod produces luminescent compounds and actually excretes them into the water using contracted muscle around the secreting light organ. Females don’t produce luminescence to respond to potential mates, although they do have the ability to do so when threatened by predators.

In order to film the luminescent displays of V. annecohenae without disruption, the researchers used infrared illumination, which subtly reflects off the bodies of the ostracods without interfering with the display. In this way the authors were able to characterize two clear phases of the “display train.” The first, a so-called ‘stationary phase,’ seems to be more of a luminescent attraction call, where the male moves very little and release four or five plumes of very bright luminescent molecules. The intensity of color slowly decreases as the male begins to increase swimming speed, leading to the second display phase, a ‘helical’ phase that the researchers found was much more conserved in pattern that the stationary. Here, the male ostracod moves vertically up to 60 centimeters and swims downward in the water column, maintaining chemical pulses at a consistent interpulse rate of about four seconds apart. After modeling this display graphically, the authors expect that the stationery phase functions to gather nearby conspecifics (including males, which may attempt to mate with assembled females without the presumably costly displays) and the helical phase demonstrates male body condition. I expect this hypothesis to be testable based on length and quality of signal, and resulting mating success of males.