Host (biology)

The life cycles of many parasites are not well understood, and the subjectively or economically more important organism may initially be designated incorrectly as primary.

For example trout and salmon are sometimes said to be "primary hosts" for salmonid whirling disease, even though the myxosporean parasite reproduces sexually inside the sludge worm.

One example is the buff ermine moth whose larvae feed on alder, mint, plantain, oak, rhubarb, currant, blackberry, dock, ragwort, nettle and honeysuckle.

Monophagous insects have developed specific adaptations to overcome those in their specialist hosts, giving them an advantage over polyphagous species.

There is a trade off between offspring quality and quantity; the specialist maximises the chances of its young thriving by paying great attention to the choice of host, while the generalist produces larger numbers of eggs in sub-optimal conditions.

The host provides a protected environment in a well-lit position for the algae, while benefiting itself from the nutrients produced by photosynthesis which supplement its diet.

[24] Lamellibrachia luymesi, a deep sea giant tubeworm, has an obligate mutualistic association with internal, sulfide-oxidizing, bacterial symbionts.

These grow over and eventually dissolve away the mollusc shell; the crab may not ever need to replace its abode again and is well-camouflaged by the overgrowth of sponge.

Such crops as beans, peas, chickpeas and alfalfa are able to fix nitrogen in this way,[28] and mixing clover with grasses increases the yield of pastures.

[4] The host appears to benefit from the interaction, but biologists have disputed whether this is a truly mutualistic relationship or something closer to parasitism by the cleaner.

[31][32] Remoras (also called suckerfish) can swim freely but have evolved suckers that enable them to adhere to smooth surfaces, gaining a free ride (phoresis), and they spend most of their lives clinging to a host animal such as a whale, turtle or shark.

[33] Many molluscs, barnacles and polychaete worms attach themselves to the carapace of the Atlantic horseshoe crab; for some this is a convenient arrangement, but for others it is an obligate form of commensalism and they live nowhere else.

[34] The medieval Persian physician Avicenna recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms.

[34] In Early Modern times, Francesco Redi recorded animal parasites, while the microscopist Antonie van Leeuwenhoek observed and illustrated the protozoan Giardia lamblia from "his own loose stools".