A Coi Nudibranch (Goniobranchus coi) slowly traverses a rock on a Balinese reef. Nestled amidst its anal gills—located around the animal's anus, in accordance with the typical rosette gill structure of most Cryptobranch Dorid species—we can observe two small, sausage-like orange objects.
These peculiar structures happen to be the egg sacs of a female copepod. These sacs firmly adhere to the body of the female copepod, which, as small parasitic crustaceans, sustains themselves by feeding on the skin tissue or bodily fluids, including blood, of their host. The shape, form, structure and configuration of their mouthparts vary depending on their food source, leading to two distinct types: tissue-feeders, which feed on skin an flesh, and haemolymph feeders, which draw nourishment from the host's body fluids.
Detecting copepods can be quite challenging, as most of their bodies remain ensconced within the host. The sausage-shaped, often vividly coloured egg sacs represent the sole exposed portions of the female copepod and are discernible to the naked eye. Besides nudibranchs, numerous aquatic species contend with copepod infestations, including other mollusks, fish, sharks, rays, marine mammals, and a plethora of invertebrates such as corals, various crustaceans, and even sponges and tunicates. Some freshwater fish also play host to copepods. The transmission of copepods from one individual to another occurs during their free-swimming larval phase. Once the eggs hatch, the emerging larvae are released into the water column, where they seek out a suitable host and embed themselves in an opportune location.
Copepods, or Seed Shrimps as they are sometimes called, rank among the most abundant creatures on the planet, thriving in a wide range of aquatic environments. The majority of copepods lead a planktonic existence, drifting along, subject to the whims of ocean currents, and subsisting on phytoplankton—microscopic oceanic plants that harness solar energy through photosynthesis to produce sustenance. In turn, copepods become the prey of other animals further up the food chain. Many marine species heavily rely on these diminutive crustaceans for their daily dietary needs. It's worth noting that only specific copepod species, like the one currently residing within the branchi of this nudibranch, adopt a parasitic lifestyle. It is believed that only about 30 percent of all copepod species are parasitic.
Ironically, even these parasitic copepods often serve as hosts themselves for another parasite: marine dinoflagellates, which happen to be gut parasites.
In wildlife photography, the optimal approach typically involves capturing the subject head-on, the same applies with underwater videography. It’s far more aesthetically pleasing for the viewer when the animal faces the camera. The impact is even greater when the animal turns, advances, or moves toward the camera. Filming an animal departing from the scene, although a shot I occasionally use to conclude a clip or scene, is less rewarding, both for the audience and the videographer recording the scene. This principle holds true for shooting an animal from behind or from an overhead perspective unless the specific focus is the rear or upper part of the animal itself such as highlighting the copepod’s egg sacs in-between this nudibranch’s gills.
This short underwater videoclip has been filmed in Bali, Indonesia 🇮🇩
For another in-depth description about a nudibranch infected with parasites please go to our vlog post 92 or click on this link: https://www.beyondscuba.com/post/tryon-s-risbecia-nudibranchs-with-a-pair-of-imperator-shrimps
and for another in-depth description about a moray eel plagued with copepods go to post 130 or click this link: https://www.beyondscuba.com/post/plagued-with-sea-lice
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