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Turtle Leeches (Ozobranchus) are a genus of leeches (Hirudinea) that feed exclusively on the blood of turtles. Only two species--Ozobranchus margoi and Ozobranchus branchiatus--are found in the Atlantic coast of the United States and the Gulf of Mexico. Because of difficulties in studying sea turtles, little is known about other species of the Ozobranchus genus.

Physiology
Because species of Ozobranchus can be very small (down to a few millimeters in length), it is often difficult to distinguish species, and Ozobranchus margoi and branchiatus are the most anatomically documented. According to researchers from the University of Wales and the Gulf Coast Research Lab in Mississippi, they look very similar externally, with their main difference being the number of pairs of finger-shaped gills; Ozobranchus margoi has 5 pairs while Ozobranchus branchiatus has 7. They both have a light or slightly pink color, and dark spots sometimes form from ingested blood visible through the epidermis. Ozobranchus branchiatus range generally from 3.5-10 mm while Ozobranchus margoi generally range from 4-22 mm. The body is composed primarily of two unequal annuli. Both have large suckers on each end of their bodies. The sucker on the anterior end is not distinguishable from the neck. The anterior end also has two light-sensitive eye spots. The finger-like gills tend to get smaller as they progress towards the posterior end.

Reproduction
Like other leeches, Ozobranchus are hermaphrodites with separate male and female reproductive systems. As hermaphrodites, they have both testes and ovaries. Unlike some hermaphrodites, however, leeches cannot self-fertilize. They have four pairs of testes connected to various ducts and to an external gonopore with an eversible penis controlled by a muscular bulb. The gonopore also functions as a receptacle for sperm with a connection to the ovaries. Like other leeches, Ozobranchus lay eggs, and Ozobranchus branchiatus are known to lay eggs directly on their host and cement them there.

Life cycle
Because of the difficulties in studying turtles, relatively little is known about the life cycle of Ozobranchus. Particularly, it is not fully known if these leeches are capable of surviving independently of a turtle host for any extended period of time. It is also unknown if they can feed off of organisms other than turtles, although the possibility was indicated by the discovery of a single Ozobranchus margoi specimen on a longbeaked dolphin. Ozobranchus branchiatus are known to complete their entire life cycle attached to their host turtles, which is an unusual trait shared only by a few other species of leeches. This is accomplished by laying eggs on the turtle and attaching them with a cementing substance, where they will hatch and attach to the host turtle. Further, Ozobranchus margoi have been observed to lay eggs directly on heavily infested turtles in the event of an epizootic.

Blood Extraction
Ozobranchus branchiatus are known only to attack green turtles (Chelonia mydas) while Ozobranchus margoi target several species but are found mostly on loggerhead turtles (Caretta caretta). They have also been found on several other species of turtles and, in one case, on the longbeaked Dolphin. The leeches attach themselves on the mouth, neck, cloaca, and the undersides of the flippers of turtles. Once there, they use the same technique for blood extraction as other sanguivorous leeches: opening of a small wound, use of an anticoagulant to prevent blood clotting, and ingestion of the blood.

Epizootics
Occasionally, as a scientific journal reported in 1974, overwhelming infestations of leeches called “epizootics” break out. Frank J Schwartz of the University of North Carolina’s Institute of Marine Sciences described it thusly:

Vector Organism
Apart from the direct damage that they deal to turtles, Ozobranchus margoi are suspected by scientists to transfer diseases to turtles. Fibropapillomatosis, a viral disease of marine turtles known to be associated with a turtle herpesvirus, is suspected to be transferred between turtles by a vector organism: specifically, Ozobranchus margoi. In a study conducted by several members of Cornell University's Department of Microbiology and Immunology along with several United States Government employees, very high quantities of the DNA for this virus were found in Ozobranchus margoi specimens, implicating it as a possible vector for transmission of the disease.

Genetic “barcoding”
Because of difficulties in use of common taxonomic techniques to identify leeches and the fact that distinguishing anatomical characteristics apply only to mature specimens, recording the genetic pattern of leeches has utility in their identification. For this reason, a team of scientists from Wright State University, the University of Central Florida, and the Inwater Research Group captured several Ozobranchus margoi and branchiatus for examination. Once captured, the specimens were all carefully examined and morphologically identified based on the number and shape of their gills. Then DNA was extracted from areas of the leeches far from the digestive tract so as not to mix in any of the host turtle’s DNA, which could alter the results. The DNA was then analyzed and the findings were added to GenBank, which did not already have any data on Ozobranchus margoi.