Euplokamis is a genus of ctenophores, or comb jellies, belonging to the monotypic family Euplokamididae. It shares the common name sea gooseberry with species of the genus Pleurobrachia. Despite living for hundreds of millions of years in marine environments, there is minimal research regarding Euplokamis, because they are uncommon. Research on the evolution of the basic body structures of diploblastic metazoans revealed that there are four major phyla, including the Ctenophores. Although the morphology of Euplokamis often resembles the medusa stage of Cnidarians, their eight rows of combs are one distinguishing feature that led to the official classification of Ctenophores. After being originally described by Chun (1879), the family Euplokamididae was expanded by Mills (1987) due to the discovery of a new species, Euplokamis dunlapae. Further research indicated that Euplokamis should be identified from Mertensiidae due to the rows of combs and some compression. They may also be distinguished from the genus Pleurobrachia due to their more elongated shape. Additionally, various adaptations of Euplokamis have been observed such as the use of tentacles for movement/feeding, a complex nervous system, and bioluminescent capabilities. Other characteristics including a defined mesoderm, lack of stinging cells, developmental differences, and symmetry supported the reclassification of these organisms.
Distribution & Habitat
Euplokamis have been found in fossil records dating back to the Cambrian period, part of the Paleozoic Era, and it is estimated that some species may have evolved before this period. Originally, Cnidarians and Ctenophores were classified under the same phyla, Coelenterata. Ctenophore bodies are made up of a gelatin substance, similar to Cnidarians, but the multiple rows of combs present in fossil records are unique to ctenophores. Records of Euplokamis sp. indicate they are distributed widely around the world, but are most often found in warm coastal waters. Euplokamis prefer marine, or saltwater, environments and are classified as free swimmers, due to their ability to move through the water column. They have been identified in the Mediterranean Sea, the North Pacific, the Gulf of Maine, and off the coast of Sweden. However, since this genus was originally grouped in the family Pleurobrachiidae, there is limited information regarding their actual distribution and habitat.
Anatomy & Morphology
Ctenophores are divided into two classes based on either the presence(Tentaculat)) or the lack (Nuda) of tentacles. Within each class, there are multiple orders to further distinguish their structures and characteristics. The class Tentaculata contains the following orders: Cydippida, Lobata, and Cestida. The genus Euplokamis is part of the class tentaculate, which indicates that tentacles are present. They are also part of the order Cydippida, distinguished by their tentacles and their round body shape. Euplokamis tentacles are long, with side branches, and have a sheath allowing them to be retracted inside of the body. The tentacle side branches are known as tentilla, which in the case of Euplokamis are held tightly in coils except during the act of prey capture. Further, the widely spaced tentillia droplets allow for organisms to be classified to the genus level and are one of the only examples of striated muscle found in ctenophores.
Additionally, these organisms have bi-radial symmetry with a mouth on their front end and a statocyst, or sense organ, at the other end. The sides of their stomachs are lined with distinct bulbs, shaped like tadpoles, and unlike other well-known jellyfish, Euplokamis do not have any nematocytes, known as stinging cells. The mouth is connected to the digestive tract via the pharynx. The digestive system, or gastrovascular cavity, is made up of intricate canals that allow for both digestion and circulation to occur. They also lack an anus but are able to excrete some waste through pores on the adoral end. Typically, Euplokamis are small, only growing up to approximately 20 millimeters (mm) in length.
Another distinct feature of this genus is the eight rows of combs present. While they are known as comb plates, they are actually made up of large cilia, which are hair-like structures. These plates are unique because they consist of some of the largest known cilia found on any organism. Additionally, the combs primarily function in movement, allowing some species to move forward and backward. Since their bodies are made of the mesoglea—a translucent, gelatin-like substance—the 8 comb rows can be easily identified. These combs function in movement, due to their large ciliary structures.