Symbioses
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Parasitism

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This part of symbioses deals with parasitism, while commensalism and mutualism are accessible via the icons. After viewing these 3 topics, you can test your knowlege by taking the symbiosis quizzes.

Parasitism is a relationship where one organism benefits at the expense of another, but usually without killing it. Parasites are usually smaller than their hosts. They may be ectoparasitic or endoparasitic. Scientists often are not sure of the benefits gained by ectoparasites, say, on the skin of a fish. They have a clean place to stay and perhaps eat food intended for the host or possibly loose skin, scales, or skin mucus of its host. This account deals only with ectoparasites, and not with any of the myriad disease-causing endoparasites such as nematodes, flatworms, protists, and the like

NOTE lit. "outside parasite": those that crawl on the outer skin, like fleas or lice on vertebrates

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of eagle ray swimming taken from a video

"Sharksuckers attach by suckers to their host. The suckers don't appear to infllict damage, so I guess the main cost to this eagle ray is frictional drag during swimming." - Turneffe Island, Belize 2000. Video courtesy Andy Stockbridge, Belize.

NOTE you may have to look carefully to see the sharksucker Echeneis naucrates manouvreing for position on the ventral surface of this spotted eagle ray Aetobatus narinari

 
 

photograph of a reef shark swimming
Sharksucking remoras are thought to parasitise larger fish by robbing them of food items and catching free rides, the latter creating hydrodynamic drag that must be overcome by the hosts. Black-tip reef sharks in the Bahamas are observed to twist and gyrate in apparent attempts to induce the sharksuckers to move to less "sensitive" locations on the body (whether physical or hydrodynamic is not known), rather than actually attempting to remove them. Ritter 2002 Envir Biol Fish 65: 111.

 

An apparently "sharksuckerless" reef
shark Carcharinus perezi 0.05X

 

 

photograph of a midnight parrotfish with sharksucker attachedIn other cases, sharksuckers have been observed to aid their hosts by eating ectoparasitic copepods. In these circumstances the relationship is better described as mutualism, not parasitism. Vaske 1995 Rev Bras Biol 55: 315.

 

 

 

 

Sharksucker Echeneis naucrates hitches
a ride on a parrotfish Scarus sp.

 
 
seashorse dive leader for Biology of Caribbean Coral Reefs website photograph of a blue parrotfish with sharksucker attached taken from a video

"This sharksucker is getting a free ride on a blue parrotfish, but at what cost to the parrotfish? Looks like a parasite to me." - Turneffe Island, Belize 2000. Video courtesy Andy Stockbridge, Belize.

NOTE Scarus coeruleus

 
 

photograph of 4-eye butterflyfish with cymothoid isopod parasite
Various species of reef fishes host parasitic cymothoid isopods on their head regions, although what the isopods are doing there is not always clear.

NOTE lit. "same foot", referring to the 7 pairs of undifferentiated walking legs on the body of the isopod.

 

Four-eye butterflyfishes Chaetodon capistratus are sometimes
infected with ectoparasitic cymothoid isopods Anilocra sp. 0.4X.
The parasites invariably attach under the eyes, but whether this
is to facilitate access to the butterflyfish's food, or to be on a
more stable surface (i.e., one that would make them more
difficult to be brushed off by the host), is not known

 

photograph of coney with cymothoid isopod attached under its eyeThere are several species of cymothoid isopods in the Caribbean region and most are specific to one or a few types of fishes. Moreover, they attach in about the same positions on a given fish species and often create scars where they attach. DeLoach 1999 Reef fish behavior. New World Publ., Florida.

 

This pinkish-coloured specimen of parasitic
cymothoid isopod is a different species of
Anilocra to the one shown on the butterflyfish
above. It is attached under the eye of a coney
Cephalopholis fulvus 0.33X

 

photograph of isopod parasite Cymotha exigua occupying the mouth of a red snapperMost cymothid parasites attach externally and are able to move about on the host's body. However, one unusual type of cymothoid Cymotha exigua, known as a "tongue-biter" for its habit of occupying the mouth cavity of certain fish species, tends to stay put. The parasite first attaches to the tongue by gripping with its legs. It then either severs critical blood vessels to the tongue, or the tongue just slowly degrades, and eventually falls off. The isopod then becomes the fish's new tongue, although how effective it is in that role is not known. The parasite is thought to feed its host's blood, or mucus, or perhaps bits of food passing by during the fish's own feeding. Photograph courtesy Matthew Gilligan, Savannah, Georgia.

A parasitic isopod Cymotha exigua attached to the tongue
stub of a red snapper Lutjanus campechanus. The female
of the species is larger and occupies the mouth, while
the smaller male may hang out amoung the gill slits of the
fish waiting for the female to become reproductive 1X

 

 
 

photograph of tubeworm growing in coral
Tubeworms embedded in corals are behaving like parasites even though they may not seem to overtly harming their hosts.

 

 

 

 

 

Partly withdrawn Christmas-tree worm Spirobranchus
giganteus
in a coral, possibly a brain coral 2X

 
 

photograph of heart urchin with commensal crab
Crabs and worms that crawl on the skin or spines of echinoderms are usually described as commensals but, in the absence of knowledge about the exact nature of the relationship, they might also be termed parasites.

 

 




A pea crab Dissodactylus primitivus living among
the spines of a heart urchin Meoma ventricosa
could be a commensal, a mutual, or a parasite 1.5X

 

 
  photo collage of a French angelfish and a brown tube-sponge
One would think that definitions of feeding relationships would be easy. However, where a herbivore or predator takes non-lethal bites from a plant or animal, respectively, and uses it as a continual food supply, the behaviour begins to overlap with the definition of a parasite. Since it would be nonsensical to categorise a parrotfish that takes bites of algae or scrapes coral as a parasite, it nonetheless underscores the difficulting of defining and distinguishing various symbioses
 
 

photograph of zoanthids on a vase sponge
Similarly, because their effects on their host sponge are generally thought to be negative, zoanthids that live on sponges are usually considered to be commensal. However, if the zoanthids harm the sponge by their presence, they are acting as parasites.

NOTE lit. "beside, by" + "food or feeding" Gr. Whoever described this zoanthid species as "parasiticus" may have originally thought that it was deriving nourishment from the sponge, but it does not appear to do so. Another thing to consider, in this particular case, is that zoanthids are mostly toxic, and any sponge-eating fish or turtle taking a bite containing zoanthids might get a nasty surprise. If this were to be the case, then one could argue that the zoanthid is actually a mutual...

 

 

 

The sponge-zoanthid Parazoanthus parasiticus
lives on several different species of sponges, but
favours the pink vase-sponge Niphates digitalis 0.5X

  photograph showing zoanthids on a boring sponge, with details of inhalent and exhalent openings on the spongeStudies in Barbados show that host-specificity is actually quite high amongst most species of zoanthids, including Parazoanthus parasiticus. Crocker & Reiswig 1981 Mar Biol 65: 231.
 

photograph showing close view of zoanthids on a boring sponge
A closer view of zoanthids on a boring sponge Cliona delitrix shows more detail of how they intersperse themselves amongst the inhalent openings of the sponge. Note the perforated, mesh-like covering on the inhalent openings of the sponge. These presumably function to restrict entry of particles that too large and/or inedible into the sponge.

 

 

 

 

Close view of the polyps of the sponge zoanthid Parazoanthus parasiticus
interspersed between the many inhalent openings of the sponge

 
  What do the bluehead wrasses make of this?:
 
cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge
cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge
cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge cartoon 1 in a series showing wrasses discussing a photo of zoanthids on a boring sponge
 
 
...the same issue, but from another perspective: cartoon 1 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue
cartoon 2 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue cartoon 3 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue
cartoon 4 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue cartoon 5 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue
cartoon 6 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue cartoon 7 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue
cartoon 8 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue cartoon9 in a series of 9 showing a zoanthid polyp and sponge discussing parasitism issue
 
 

The zoanthids, sponges, and wrasses in the above cartoons may find the following information helpful. Researchers in Key Largo, Florida and the Bahamas have compared pumping rates of giant barrel sponges Xestospongia muta with and without zoanthids Parazoanthus catenularis, and report that ones with zoanthids pump up to 75% less than ones without. The effect of the parasites in this species is mainly to compress and block the inhalent openings, as well as to restrict the surface-area of the osculum relative to overall volume. In other words, flow is restricted both going in and going out of the sponge. The effect could be mimicked by experimentally blocking the inhalent openings on sponges with plastic meshes with different porosities. So, in this regard at least, the the zoanthids are harming their hosts. In comparison, vase sponges Niphates digitalis hosting zoanthids P. parasiticus exhibit an average reduction of 35% in pumping rates. However, because much of this owed to disproportionate responses at a single location, the authors consider the data for this species to be less convincing than those for the barrel sponges. Lewis & Finelli 2015 Coral Reefs 34: 291; photograph of barrel sponge courtesy ::divepix::.

NOTE recent taxonomic changes have led to this barrel-sponge zoanthid species being renamed Bergia (Parazoanthus) catenularis and the vase-sponge zoanthid being renamed Umimayanthus (Parazoanthus) parasiticus. However, until the popularity of these changes can be assessed, the old names will continue to be used in the ODYSSEY

NOTE different species of zoanthids have different styles of habitation, from partially burrowing into their hosts in some, to sort of lying about on or in them in others. Now, unfortunately, the authors don't describe how the zoanthids are occupying their specific sponge hosts in the study areas reported on here. If, for example, the zoanthids P. catenularis are growing densely within the oscular cavity of the barrel sponge, it is easy to see how water flow would be disrupted (see photograph below Left). in comparison, by partially embedding themselves on the external sponge surface, the zoanthids P. parasiticus inhabiting vase sponges may not be so disruptive to water flow either in or out of the sponge (see photograph below Right)

 
photograph of zoanthids Parazoanthus catenularis occupying the spongocoel of a barrel sponge Zestospongia muta This barrel sponge Zestospongia muta is heavily encrusted with various organisms (and thus probably not very healthy), but note how the zoanthids Parazoanthus catenularis are mainly growing within, and thus presumably occluding water flow out of, the spongocoel, the main cavity of the sponge zoanthids Parazoanthus parasiticus inhabiting the external surface of a vase sponge Niphates digitalis In contrast, polyps of the zoanthid P. parasiticus are also dense, but are scattered more or less evenly between and around the inhalent openings of the vase sponge Niphates digitalis, thus possibly interfering relative less with water flow into and out of the sponge
 
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