Parasitoid - Parasitoidal and Parasitic Vertebrates

Parasitoidal and Parasitic Vertebrates

Perhaps because they are less specialised and their relationships with their hosts are less intimate than is the case with many invertebrates, it often is more difficult to distinguish parasitism from parasitoidy in vertebrates. In fact many of their relationships of such types do not immediately suggest parasitism to most people at all. However, the very concept is so open to interpretation that it emerges frequently in vertebrate biology.

Kleptoparasitism for example is ubiquitous, and is a major constraint on reproduction or even survival among vertebrate predators, especially in times of famine. Male lions in a pride for example, largely leave hunting for non-threatening prey to females. However, prides that specialise in very large prey such as giraffes, elephants, or buffalo, may behave differently.

Other predators such as cheetah, leopard, and even lions sometimes may be chased from their kills by hyaenas. Hyaena may sometimes follow such predators so routinely in the hope of confiscating their kills, that the hunters spend more effort on avoiding hyaena than on hunting.

Ethologists could multiply examples of kleptoparasitism many-fold; it may be intraspecific or interspecific; it ranges from the smallest foragers and predators to the largest, and may combine with predation, where the robber is happy to eat both hunter and prey. Curiously though, interspecific robbers often show at least some constraint as though they were robbing conspecifics, and do not necessarily attack the host as directly as they would have done had there not been a "robbery" situation. Interpretation and speculation about the nature of such behaviour is beyond the scope of this article however.

It is not easy to classify such relationships, because many of them involve degrees of payment in terms of protection and other benefits; for example the male lions who preempt the females' kills do at least offer protection from hyaenas and rival males.

Kleptoparasitism occurs in many other forms among vertebrates (see here for example), but for it to lead to the death of the host is not so common, and this would seem to disqualify it from the category of parasitoidy. Still, when the hosts are hard pressed in hard circumstances, the resulting injury and famine could cause reduced reproduction and even death.

Lampreys present both parasitic and parasitoidal examples. Most species are not parasitic, but among the North American species for example, there are several species ectoparasitic on freshwater fishes. They rasp away the skin of the host and suck the blood, but most do only superficial damage. In contrast, the most notorious species is the sea lamprey, Petromyzon marinus. Its rasping wounds can extend deep into the host's flesh, and the muscle damage and loss of blood commonly weaken the host severely, affecting its reproduction unfavourably. Often the harm is severe enough to kill the host.

Hagfish, are distant relatives of lampreys. They are largely carrion feeders and predators of large worms and similar small creatures, but various species also attack weakened fishes much as some lampreys do, and accordingly rank as opportunistic parasitoids under at least some conditions.

The sabre-toothed blenny presents a curiously difficult example of parasitism to classify. It parasitises the relationship between some cleaner fish and their client fishes, more than it parasitises either party to the relationship; it attacks the client fish, approaching it in the guise of cleaner wrasse and snatches a mouthful of scales or other convenient tissue. Clients often react violently, and thereafter trust neither wrasse nor the wrasse-mimicking blenny. In its violence and the pernicious effect on a valuable relationship, it suggests parasitoidy as well as parasitism.

Another form of parasitism that can approach parasitoidy occurs in the Perissodini, Cichlids from Lake Tanganyika. Seven species in the genus Perissodus are specialised in eating scales from other fish. Their teeth are variously suited to being able to grab bits of skin with the scales attached, and such bits of skin and scale formed major components of the stomach contents. At least some of the species also have adaptations in their behavior to enable them to approach potential hosts They also have an adaptation of the jaw that enables them to lash out sideways in passing a victim; the jaw is asymmetrical, and there is continuous selection for the asymmetry that currently is less frequent in the population, because host fishes are more alert to defend themselves on the side on which they have been attacked in the past.

Such a lifestyle is reminiscent of sharks of the genus Isistius, which is known as the Cookiecutter shark because of the circular wounds it leaves in the skins of whales and large fish that it has bitten in passing. Isistius species have been referred to as partly ectoparasitic, but they sometimes overwhelm their hosts and kill them, which by definition amounts to parasitoidy.

Candiru and related fishes in the Family Trichomycteridae, subfamilies Vandelliinae and Stegophilinae, present unusual examples of vertebrate parasitism, and occasionally parasitoidy. Most popular accounts are obsessed with the idea of candiru entering the human urethra and other orifices, but they are very varied in their habits. Some burrow partway into the skin of larger fish, apparently largely for purposes of protection and transport rather than food. Several at least are haematophagous, commonly entering the gill cavities of larger fishes and feeding on blood drawn from the gill filaments. At least when large fishes are tethered by fishermen where large numbers of the parasites occur, the hosts may die. Possibly this effect is analogous to the effect of soft ticks on hosts that do not avoid the sand patches where they assemble.

Among birds the best-known forms of parasitism are brood parasitism by various species of cuckoos, honey-guides, cowbirds, and several more. They qualify as parasitoids because many of them will cause the starvation of the host's chicks by competing with them for food, and many others either will remove host eggs when laying eggs in host nests, (sometimes eating the eggs removed), or the chick will eject or kill the eggs or chicks of the host when they hatch. Some hatchlings actually have hooked beaks adapted to attacking the host chicks and eggs, hooks that vanish before fledging.