Breaking it Down: The Taxonomic Classification of Photeros annecohenae

Domain: Eukarya
The word Eukarya contains the Latin roots "eu," meaning true, and "karya," meaning nut, or nucleus. Therefore, all members of this domain have cells that each contain a true nucleus. These cells also contain membrane-bound organelles.
Kingdom: Animalia
Photeros annecohenae are multicellular, heterotrophic (scavengers), lack a cell wall and, therefore, are motile during at least one stage of their lifecycle (Ostracods are motile as juveniles through adulthood.).

The above phylogenetic tree spans from the most broad of taxa, domain, to a more exclusive classification, animal phyla. It is based on comparative morphology and molecular data. The phylogeny of Photeros annecohenae can be traced through the tree by following the bolded lines and words.

Phylum: Arthropoda

Since Photeros annecohenae are Arthropods, they display bilateral symmetry, have segmented bodies, a chitinous exoskeleton, paired-jointed appendages and grow via molting. Arthropods also have a true, straight-through digestive system (from mouth to anus) to break down nutrients as well as an open circulatory system to transport them. Other Arthropods include spiders (including tarantulas), scorpions, centipedes and all insects (including bees, beetles, mosquitoes, mayflies and dragonflies).
Subphylum: Crustacea
Crustaceans' bodies are divided into two distinct regions (tagmata): the cephalothorax, which gives rise to the appendages, and the abdomen, which contains the reproductive tissue and the viscera. Members of this subphylum also possess two pairs of antennae and mandibles.
Other Crustaceans include lobsters, crayfish, crabs and barnacles.

Class: Ostracoda

Members of the class Ostracoda are separated from other Crustaceans by a laterally compressed body, undifferentiated head, seven or less limbs and a bivalved carapace with no growth lines.  

Subclass: Myodocopa

This group contains ostracods that are all marine species; the other subclass, Podocopa, may live in freshwater or marine environments. Myodocopa also brood their eggs, while Podocopa deposit theirs. The compound eyes of Myodocopa are more developed than those of the Podocopa in order to view and distinguish their own species' luminescent courtship displays. (Members of the Podocopa do not produce luminescent displays.)
Order: Myodocopida
The classification of Myodocopa is separated into two distinct orders, Myodocopida and Halocyprida. Members of the Myodocopida have a keel, while  some members of the Halocyprida have a spine-like structure. Also, there is no known bioluminescence in the Halocyprida like there is in the Myodocopida. The shape of the carapace also differs between the two groups.

Family: Cypridinidae
Cypridinidae is the only family that contains species of which males perform specific mating displays. This family contains 28 genera and more than 200 species (Morin 2001). 26 of these species are known to be luminescent and have been named, but it is estimated that there are 100 luminescent species of the family Cypridinidae out there. All members of this family produce upper lip secretions, but these secretions are not always used for bioluminescence. It is important to note that not all members of the Cypridinidae are bioluminescent. The family contains some species, such as Photeros annecohenae, that use luminescence in mating displays, some species that use luminescence solely for predator deterrence and some species that do not luminate whatsoever.

     This phylogenetic tree shows how Cypridinidae is separated into multiple  genera! It is based on morphological characteristics and limited DNA analysis. The green section of the tree indicates genera of ostracod in the family Cypridinidae that do not produce bioluminescence. The blue section indicates genera that use bioluminescence solely for predator deterrence and not in courtship behaviors. Lastly, the red section of the tree, including Photeros, separates the genera that use bioluminescence for both defense from predators as well as mating displays. The "C-Group" and "H-Group" of this section are largely undescribed; in fact, species, and even genera, classification of many bioluminescent ostracod groups are still under debate. There are many species and genera left to be named and discovered. Molecular data has just recently started to be used to change and support previous phylogenies that have been based on morphology and display behavior/bioluminescence (Gerrish Personal Communication).

Genus: Photeros (formerly Vargula)
The name of this genus refers to the organism’s ability to be luminescent (to deter predators as well as produce mating displays) (Gerrish Personal Communication). Aside from morphological differences in limb shape between genera, members of Photeros are designated by their pale color compared to other bioluminescent genera as well as the fact that they are transparent (have no pigment in their valves or limbs).
 Species: Photeros annecohenae (formerly Vargula annecohenae)
As mentioned above, the genus name Photeros refers to the organism's ability to be luminescent. The species is named for Dr. Anne C. Cohen. Dr. Cohen is a systematist/taxonomist who has carried out extensive research on Cypridinid ostracods.
The species Photeros annecohenae exhibits vertical mating displays in the upward direction, and, like other species of ostracod, may be distinguished by its specific habitat as well.

The word "bioluminescent," of "bioluminescent ostracod," obviously refers to the organim's ability to be luminescent. "Ostracod" stems from the Latin roots "ostrac," meaning shell, and "cod," meaning false imitation. This comes from the fact that an ostracod's "shell"/valves are a part of its carapace, not an independent structure like in most Arthropods.

     These ostracods have a dorsally-hinged outer bivalve carapace composed of chitin that entirely covers the extremities and functions to protect the organism. They possess a simple medial eye that detects light (extremely important since the organism is dependent on dark for mating and food acquisition) as well as  two large compound lateral eyes that can judge distance as well as light intensity. These compound eyes contain many ommatidia that increase the complexity of the eye. The evolution of these complex compound eyes is crucial for their reproductive strategy. Members of Photeros typically have seven pairs of appendages. The first pair, called the first antennae, act as sensory structures to help the ostracod sense the environment and allow males to grab and hold females during mating. The second pair of appendages, the second antennae, with setae, is primarily used for movement (swimming, crawling and burrowing) (Morin 1986) but is also used for deciphering food sources from other substances (Gerrish Personal Communication). These appendages are extended through a notch (called the incisur) to allow for swimming. The furca is a foot also used for swimming; it propels the ostracod along the sediment (Gerrish Personal Communication). The keel streamlines the organism and helps it stay upright (Gerrish Personal Communication) . A mandible and maxillae are present as well as an upper lip, on which three nozzles (See above photo.) are located that secrete chemicals creating luminescence. These chemicals are produced in a long, curved gland called the light organ. A pair of tusks is also present on the upper lip. Male and female genitalia demonstrate a "lock and key" morphology within a species, meaning the male genitalia of a Photeros annecohenae only fits comfortably with the genitalia of a female Photeros annecohenae. This is one reason mating between species does not occur. Photeros annecohenae are extremely small, reaching a maximum length of about two millimeters. As stated on the Home page, their valves contain no pigment and are, therefore, mostly translucent.

     Sexual dimorphism is evident primarily in the adult forms of Photeros annecohenae, especially in body size, eye size and differences in the first antennae. The first antennae of males have a grasping structure to hold females during mating. Males' first antennae also has smaller suckers than the females'. Adult males are smaller than adult females: Males average 1.62 millimeters in length, while females average 2.02 millimeters in length (Torres and Morin 2007)). The carapace of females is more rounded than that of males, and the lateral eyes of females are 20% smaller than males’ (Torres and Morin 2007). The genital lobe (eighth limb) of females is also a lot smaller than that of males and is unjointed. The presence of this sexual dimorphism indicates sexual selection (Morin and Cohen 2010).

Now that you know how Photeros annecohenae is classified and how it may be identified morphologically, check out where the species can be found in the world and what habitat it occupies!