The Luminescent Mating Displays of Photeros annecohenae: An Elaborate Light Show

     Come nighttime, bioluminescent ostracods live up to their name. Displays begin during the first truly dark hour of the night when no moonlight is present,  typically about 35 to 50 minutes post-sunset if there is no moon, otherwise after moonset. During this time, male Photeros annecohenae abandon their benthic lifestyle, swim up into the water column and produce an intricate series of vertically-progressing flashes to attract non-luminescent females of the same species (typically located in the grassbed habitat). These coded trains of pulses serve to attract the same species of females’ attention in hopes that at least one will approach the male, allowing for copulation and successful mating. These displays occur two to ten centimeters above the shallow grassbeds in which the organism resides and produce a species-specific “train” of flashing luminescence (Torres and Morin 2007). The protein luciferin and the enzyme luciferase are produced in the light organ of the ostracod and secreted from nozzles on the upper lip (Luminance in these organisms is always extracellular (Morin 1986).) This causes a chemical reaction with the surrounding water that produces flashes of bright blue light as the male rapidly swims in the upward direction (James G. Morin 2011).

     The displays typically consist of nine to 13 pulses, three bright, longer pulses followed by six to ten dimmer, quick pulses (Torres and Morin 2007). The length of each train of displays is typically 50 to 70 centimeters with each pulse lasting a second or less (Morin and Cohen 2010) and ranging from millimeters to centimeters apart. The display tapers after 45 to 50 minutes (Torres and Morin 2007). The train’s luminescence is typically brightest at the beginning, and, as the display progresses, pulses become dimmer, shorter and closer together. There are two phases of each display: an initial phase (stationary stage) and a terminal phase (helical stage) (Morin and Cohen 2010). The  stationary stage acts as the “alerting phase” by attracting the attention of potential female mates (as well as male competition). The first signal is usually what catches a female’s attention (Rivers and Morin 2009). This stage produces a specific pattern of bright, spaced out flashes and acts as a species identifier for females ready to mate. The following helical stage is a dimmer, quicker, more repetitive “trill” phase. It is also referred to as the orientation phase and creates a pattern of about a dozen evenly spaced pulses to be assessed and approached by a female (Morin and Cohen 2010). Males’ displays give females an idea of their quality as potential mates (Rivers and Morin 2009). These ostracods swim in a helical pattern that makes them appear to be swimming more slowly and, therefore, are more approachable to females. A receptive female approaches a displaying, swimming male of the same species, and the male grabs and holds her using his first pair of antennae.

      Other bioluminescent organisms can affect the success of bioluminescent ostracods' mating displays by distracting females from their own species' displays. These organisms include the luminescent syllid polychaete worm (See photo.) and bioluminescent dinoflagellates. These dinoflagellates also affect displays by creating background light when a lot of them are present in the water column (Gerrish Personal Communication).


     Photeros annecohenae reproduce by copulation, or sexual intercourse. There is variation between species in the morphology of a male’s eighth limb (copulatory limb), and, as mentioned on the Adaptation page, this acts as another way to keep mating within a species. Fertilization is internal and occurs as eggs enter the brood pouch. Two spermatophores containing sperm, produced by the male, are placed on the female’s genetalia; a male’s  spermatophores of a given species are shaped to fit perfectly into the female genetalia of that same species. It is believed that this limits the female’s ability to mate with other males for the time being.

     Because these females are unable to mate with other males, they retreat and incubate their embryos inside their bivalved shells. This removes them as potential mates, skewing the sex ratio of mating ostracods toward males (although the sex ratio in the population as a whole is equal (Cohen and Morin 1990)). This leads to competition between many males for fewer receptive female mates. Females are able to choose their mates, so males use their displays to "impress" females. Using bioluminescence for courtship has lead to strong sexual selection in Photeros annecohenae (Cohen and Morin 2010). This sexual selection has shaped the behavior , morphology and speciation of the ostracods (James G. Morin 2011). Evidence of sexual selection includes a high sex ratio toward males, males attracting females using luminescent signals, sexual dimorphism in adults and a higher parental investment by females than males. It is believed that strong sexual selection is ultimately what has lead to vast diversity of species and displays in ostracods of the Caribbean. Natural selection has lead to distinct divisions of displays and habitats. This is useful for females in identifying their own species' displays.
     Sexual selection is largely due to competition between males for female choice of mates. The higher number of receptive males has lead to competition between males. Competition between males has lead to different tactics in “scoring” female mates. Males may lead, entrain or sneak. Leading occurs when  males initiate the first mating display of the night. Entraining occurs when a competing male produces a display parallel to the “leader’s” (usually about half a meter away). This occurs frequently after the initial display and results in many displays visible at one time. This can create a spectacular “light show” (similar to the photo to the left) for diver spectators. Entraining may increase mating success by attracting more females to the area as well as increase competition between males once the females arrive (Rivers and Morin 2009). This is similar to what is seen in fireflies of Southeast Asia and North America. Lastly, sneaking involves a “photetically-silent” male that follows a luminescent male’s display (spirals above him) in hopes of intercepting a female swimming across his spiral path. Males have demonstrated an ability to switch between these mating tactics; however, once committed, a “sneaker” will not switch to another tactic. A sneaker is able to switch which ostracod he is ""sneaking on" but will never become an entrainer because he does not want to miss an opportunity to mate. The above photo illustrates what a typical ostracod display scene may look like. Notice evidence of entrainment and multiple species of bioluminescent ostracod.


     Remember that luminescent courtship displays occur only in the Cypridinid family of ostracods from reef systems in the Caribbean. Females produce luminance to deter predators but do not signal in response to males’ displays (unlike fireflies) (James G. Morin 2011).

     These flashing displays occur nightly (and year-round) over the ostracods' grassbed habitat. Photeros annecohenae’s displays all move upwards in a vertical direction, which is why slow water current is necessary for successful displays. In fact, it was found that a display occurring in a swift water current only produced three to five pulses, while a display in no water current produced 15 to 19 pulses.

     As mentioned on the Habitat page, darkness is an important ecological resource for Photeros annecohenae. Reproductive (and feeding) behaviors do not begin until the “dark threshold” is reached. This occurs when one third of the moon or less is exposed or two to three minutes before the beginning of nautical twilight on nights when no moon is present (Gerrish et al. 2009). The importance of darkness can also be proven by the fact that the number of displays increases on cloudy nights as long as the moon is well covered and it remains dark enough (Gerrish et al. 2009). Displays are most prevalent in the hour after sunset or moonset. Display numbers also peak after conditions that lead to a large number of receptive females. Females are constantly receptive after brood release and molting; however, they mate only when it is dark enough for mating displays, so the number of receptive females increases during the day or during long periods  of too much light when mating displays cannot occur. This is why the first hour of darkness on a night following a full moon will have the largest number of receptive females.

     As previously-stated, bioluminescent displays are species-specific and demonstrate a vast amount of diversity between species. The brightness, duration of, patterns, location and time at which they occur vary species to species. Characteristics of individual pulses and the display train as a whole vary as well. Most of the diversity among Photeros is a result of differences in displays due to sexual selection and differences of reproductive structures (Morin and Cohen 2010). There are three overall patterns of trains of bioluminescent ostracods, but each individual train is distinct to a given species. This allows for species recognition, which ultimately leads to mate recognition and mate choice by females. The below sequence also shows entrainment and species-specific displays. It illustrates how a typical series of displays would occur.


So, a male Photeros annecohenae wowed a female Photeros annecohenae with his glowing, sexy courtship display. Now what? To learn about the life cycle of the organism, click here!