### Identification To spot *Achtheinus oblongus*, you must look past the intimidating silhouette of a shark and focus on the skin. These specialized parasitic copepods are tiny—typically only 4 to 8 millimeters long. They possess a remarkably flattened, shield-like body (the cephalothorax) that is distinctly ovate or "oblong." Their coloration ranges from a translucent cream to a deep amber, often mimicking the hue of the shark’s dermal denticles to remain inconspicuous. Unlike the more common, rounded "sea lice" found on teleost fish, *A. oblongus* is elongated with visible segments and paired, string-like egg sacs trailing from the female’s posterior.
### Habitat & Range This is a creature of the open ocean, though its "habitat" is strictly biological. It is an ectoparasite found exclusively on elasmobranchs, most famously the Great White Shark (*Carcharodon carcharias*), but also Bronze Whalers and Dusky Sharks. Its range is as vast as its hosts’, spanning the temperate and subtropical waters of the Atlantic, Pacific, and Indian Oceans. You won't find them in the water column; they are tethered to the "living islands" of their hosts from the surface down to the mesopelagic depths.
### Behaviour & Diet The life of *A. oblongus* is one of extreme attachment. Once a free-swimming larva finds a host, it undergoes a transformation into a sedentary adult. Using specialized, hooked appendages called maxillipeds, it anchors itself firmly to the shark’s tough skin. You will most often observe them in "micro-refuges" where water turbulence is lower—specifically around the base of the dorsal fin or near the gill slits. They are not passive hitchhikers; they are active feeders. Using a specialized tube-like mouthpart (the siphonostome), they graze on the host’s mucus and epidermal tissues, occasionally piercing the skin to feed on blood.
### Fascinating Fact Because these copepods are so host-specific and have much faster generation times than sharks, they serve as "biological tags." By studying the DNA of the *Achtheinus oblongus* living on a shark, scientists can actually reconstruct the shark’s migration patterns and population history more accurately than by studying the shark’s own DNA!