IN PRAISE OF PAR­A­SITES

Asian Diver (English) - - Contents -

By Richard Smith

Blood-suck­ing, flesh-fix­at­ing marine life forms crop up in the wildest of places, and they might be strangely creepy, but they are also sur­pris­ingly com­mon

For some they are the stuff of nightmares, but th­ese in­cred­i­ble crea­tures are strange and fas­ci­nat­ing as­pects of life on the reef

For most peo­ple, par­a­sites are the low­est of the low in the an­i­mal king­dom: They’re creepy-crawlies that spread dis­ease and ex­ploit other crea­tures.

This is prob­a­bly why call­ing some­one a par­a­site is so down­right deroga­tory. In fact, the term “par­a­site” was first used in an­cient Greece to mean a hanger-on and was later adopted as a bi­o­log­i­cal term. A par­a­site lives in or on another liv­ing or­gan­ism, but usu­ally with­out killing it, and gets its food from or at the ex­pense of its host. Hu­mans are the vic­tims of a huge va­ri­ety of par­a­sites, from liver flukes to in­testi­nal worms, dengue and malaria, the lat­ter ob­vi­ously be­ing one of the world’s big­gest killers.

Even small fishes can sup­port rather large par­a­sites. Once they reach a cer­tain size, par­a­sites be­come im­mune to the work of clean­ers

It’s not only hu­mans that suf­fer at the hands of par­a­sites. Sci­en­tists be­lieve that there may be at least one species of par­a­site for ev­ery non-par­a­sitic species on the planet. Even par­a­sites get par­a­sites but be­cause they're so ma­ligned there has been rel­a­tively lit­tle re­search into their di­ver­sity. Par­a­sites can live on the out­side or in­side of their hosts and they tend to be very spe­cific about the host species they in­fect. Ob­vi­ously, as divers we’re more likely to see the ex­ter­nal ones clamped to the out­side of a hap­less fish, but if you look closely you might even be able to see one liv­ing in­side the trans­par­ent shell of a crus­tacean. There’s a real sim­plic­ity and fi­nesse to the life his­to­ries of par­a­sites that per­fectly il­lus­trates Dar­win’s the­ory of nat­u­ral se­lec­tion.

THE CAR WASH

Divers might be more fa­mil­iar with the fishes that work tire­lessly to rid other marine crea­tures of their pesky par­a­sites than the par­a­sites them­selves. A diver favourite is the cleaner wrasse, which serves over 2,000 clients per day, re­mov­ing on av­er­age 1,200 par­a­sites. They gen­er­ally work in pairs, main­tain­ing a ter­ri­tory on the reef. Their

work is so valu­able that medium- to lar­ge­sized fishes visit them ev­ery five min­utes, and some­times the clean­ers will even refuse a clean to an in­di­vid­ual that they think has been ex­ploit­ing their ser­vices. As well as cleaner wrasses, there are around 130 other fishes that are known to clean at some stage in their life, usu­ally when ju­ve­nile. Of course there are many shrimps that clean too.

By Dr Richard Smith

ABOVE This goby has a large fe­male cope­pod. The much smaller males are al­most im­pos­si­ble to see with the naked eye

LEFT Most likely par­a­sitised by some kind of worm, many in­ter­nal par­a­sites shut down all non-es­sen­tial sys­tems in their host to max­imise their own nu­tri­tional de­mands. Some par­a­sites even ster­ilise their host

STRANGE SCI­ENCE

PAR­A­SITES

HELP REG­U­LATE POP­U­LA­TIONS OF THEIR HOST SPECIES. A HIGH DI­VER­SITY OF PAR­A­SITE SPECIES MAY BE AN INDI­CA­TOR OF A HEALTHY ECOSYS­TEM!

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