It may not be obvious to the casual observer, but most fish caught in the wild carry parasites of one form or another. In fact, most fish parasites are internal, but even then, because of their small size, they are only noticed if they form an unsightly cyst or lump in the flesh.
On the other hand, some parasites occur on the outside of the body and are very large as far as parasites go. In these cases, they naturally come under the gaze of the observant angler.
On black, blue and striped marlin, two species of external parasites are found on the skin. The first is a primitive flatworm - though a veritable giant in the world of flatworms. It is oval-shaped, about a third-of-an-inch in diameter and lives in tight clusters. In appearance, it is similar to what hot wax would look like if dropped on a surface. Usually, each worm has a set of both female and male reproductive organs (hermaphroditic).
The next is a crustacean, a species of copepod that is a purple/blue critter about an eighth-of-an-inch in size. It looks like a small horseshoe crab as it skitters across the surface of the marlin and is called Branchiura or "fish lice," They are the only crustacean fish-parasite known to infect humans. All fish lice should be treated with caution.
Studies have shown that both types of parasite eat blood and body slime. The less mobile flatworm respires through its skin and exudes a toxin of ammonia, which harms the crustaceans and keeps the copepods away from their patch. Biological warfare on a fish sized planet.
There is, however, another type of more specialized external parasite on marlin that can cause debilitating injury and possibly death. This is the long "worm" often seen growing from the backs of marlin. In fact, these parasites are not worms but highly modified copepods with wicked, anchor-like jaws. Once they've taken hold, they are destined to remain in the one spot sucking blood throughout their lives.
Their long, slender bodies flow with the current as they protrude from the marlin, and their barnacle-looking head brings up thoughts of "Alien."
In the normal life of a marlin, only a few of these parasites will find and infect a single fish, but occasionally an unfortunate marlin may play host to dozens, or even hundreds, of these gruesome creatures.
Parasites need their host - that's obvious. Without them, they wouldn't have anywhere to live. But parasites, by definition, are harmful to their hosts. So, the problem for any parasite is not to push a given host to the point of death.
There is not enough information about the life cycles of either the copepods or the flat worm to be sure how they find and attach themselves to a marlin. However, it's likely both parasites are tiny larvae when they find their host. Both are fairly specific about what species of fish they can live on, being only found on marlin.
Presumably, chemical cues alert the larvae to the proximity of an appropriate host as it swims by, but in the ways of nature, millions upon millions must die without ever hitching the ride of their lives