We speak with Dr Rod bray about his research on parasitology, and the fascinating life cycles of these elusive species.
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Dr rod bray
Scientific associate at the Natural History Museum, London.
Hey rod! so, what are the main groups of parasites we find in the deep sea?
Well, the ones I'm looking at are digeneans; the “trematodes”, a flatworm group very common in the marine environment. Within the group of flatworms there are three major parasitic groups: alongside the digeneans, there’s the tapeworms “cestodes” and the ectoparasitic flukes, the “monogeneans”. Neither monogeneans or tapeworms are that common in deep sea teleosts. (Teleosts = fish). Cestodes are very common in elasmobranchs (sharks and rays) in the deep sea and everywhere else, and monogeneans seem to die out towards the deeper waters. These have a direct life cycle so you have to have contact between hosts for it to pass on - whether there's not enough close contact between host organisms to sustain the life cycle, might be the reason why there are few of them.
there isn't really a swimming stage then, they've got to actually bump up against each other?
No, not a free swimming stage. Because digeneans have a much more complicated life cycle. They usually have three hosts and go into a mollusc (often a snail or bivalve) as the first intermediate host. In that host they reproduce parthenogenetically (offspring is produced with only one parent; i.e. using an unfertilised egg). Organisms at that stage are called parthenitae. Then they produce enormous numbers of offspring called cercaria, which come out of the first intermediate host, and in various ways, get taken up or swallowed or burrow into the skin of the second intermediate host. Which could be almost anything: crustaceans, annelids, echinoderms, small fishes and things like that. And then the final host gets the parasite by feeding on the intermediate host, and that's where the adult worm matures.
A typical life cycle of a digenean.
They’re virtually all hermaphrodites and they produce eggs, having usually been fertilised. We think sometimes by self-fertilisation, but usually as far as we know, they tend to cross breed between individuals.
So the digenean life cycle is complicated, but because of the enormous numbers of cercaria produced and the enormous amount of eggs produced, they do seem to be able to sustain a deep sea life cycle much better than some of the other groups.
how much do we know about deep-sea parasites as a whole?
From my view, most of what I understand about deep sea parasites comes from digeneans. And, it does emphasise the fact that we know so very little. A lot of what we presume about what's going on down there, is related to what we know happens in shallower waters to the same sort of groups of parasites. So we presume the same sort of thing is going on in the deep but in most cases, we we have no direct evidence of that.
some of your work found a link between cold-adapted species and being able to become a deep sea species, is that seen a lot?
Yeah that definitely appears to be the case. Certainly the deep sea fauna does seem to be very closely related to the Arctic and the Antarctic fauna. There's been quite a bit of work done on the Antarctic stuff, and the parasites are very similar. A bit of work has been done on the molecular phylogeny and we're pretty happy that they are pretty closely related.
so after one of these Mass anoxic events, as the host evolves and recolonises the deep sea, I guess it brings it’s parasites along with it?
Coryphaenoides armatus
Yeah that's right, it does look as if that is the case. We don't know how much of the life cycle is going on in the abyss, whether most of the stuff that we're finding has been taken down there by fish as they as they get deeper. Coryphaenoides armatus is the really common fish that we've looked at and that has quite a varied depth profile. So it probably picks up parasites and most of the life cycle of the parasites is in the slightly shallower waters.
yeah of course, they've got shallower stages as juveniles so there's plenty of chances to pick up some hitchhikers
The other thing that we find is that the bathypelagic fishes (mid-water) have a much smaller parasite burden. When you are demersal (live on the bottom), you've got a surface where everything falls down to. Whereas, if you're in the pelagic regions (mid-water), particularly in the deeper waters, there seems to be a much poorer parasite burden there.
do the parasites have to become less specialised in the deep?
Yeah I think that's true. The few that have been picked up in bathypelagic fishes, as we know, are very generalised; very non-specific parasites. Whereas, the ones in the benthic fishes are often pretty host-specific. The further way you get away from the slope, the smaller number of parasites you pick up until you get into the epipelagic areas (max 200m depth) where there's a lot of photic stuff going on and then you do pick up parasites there. There's a sort of big zone in in the middle of the ocean where there doesn't seem to be a lot of parasite transmission going on, everything's a bit too spread out.
there's a lot of scavengers in the deep sea. do you think Scavenging is a good Vector?
Yeah, I think probably scavenging is how most of these things are picked up.
Another thing is, we’re not at all certain how long the parasites actually live, but certainly some of them seem to grow to quite a big size for a parasite. It's not a straightforward life cycle down there, and I think it is to do with the fact that their hosts are so very long-lived, that they manage to sustain parasite life cycle down there the complexity of the of the life cycle.
Image Courtesy of Alan Jamison
the multi-host aspect is probably a bit of a blessing to these long-lived fish because they've got such a tight energy budget, that if the parasites could reproduce within the final host, you could easily overwhelm and kill it
Yeah, none of these things actually reproduce in the final host like that so yes, you're right, it's a balance. Obviously if they started reproducing so much that they killed off the host, then it wouldn't be an efficient way of running a parasite life cycle.
However, sometimes at the intermediate host stage, the parasite does want the death of the host. For example, at the intermediate stage, some parasites get into arrow worms (Chaetognatha) and they change the colour of the worm so they become more prominent. This means small fish go for them more readily than the other uninfected worms.
It's common thing at the intermediate stages to try and get their host eaten by making it more prominent or less mobile so it's consumed more easily.
what would you say is the most Grisly parasite among the deep sea?
Well, it's not very easy to say because we're usually looking at the final host, and these are usually in a pretty good balance with the host and the parasite. There's no point in killing the final host. They are probably grazing on the surface of the gut but they're actually causing very low levels of pathogenicity (damage), so they're not really very grisly in that regard.
It’s much more of a balance in the deep sea, everything is so close to the edge of survival, any major influence would kill them off pretty quickly.
“It’s common thing at the intermediate stages to try and get their host eaten by making it more prominent, or less mobile...”
I think one of the things that surprised me the most is: it is parasitism it is a one-sided relationship, but it's it's much more in-harmony than I anticipated
Well a lot of parasites don’t seem to have much pathogenicity, and you've got to be in balance to survive as a parasite. So from the parasites point of view, in lots of cases it's against your intentions to kill off the host. You want to keep it alive as long as possible and by doing that you you have very little pathogenic effect. Of course, you do you have some effect because you're taking nutrients away from the host. But usually, they are small relative to the host size, so they're having pretty small effect on the actual host survival.
Parasites are a problem when you get into certain conditions e.g. where there's a large number of parasites in a fish farm or in human society where you get a lot of transmission of viruses and things like that. But in a normal, wild situation, a lot of the parasites have very little effect on the final host.
so it's perfectly normal to be living in balance with these parasites?
Yes, parasitism is a very normal arrangement and quite often, it's a indicator of a good well-balanced ecosystem if there's a good parasite fauna in there. Some of the free-living parts of life cycle, like the cercariae (which come out of the snail and find the second intermediate host), these are living in the environment and if the environment is polluted, you're going to lose those parasites.
These sort of things have been used as indicators of pollution, and indicators of movements for a while. Talking about shallow water stuff that we know more about: for some of the life cycles, we know that the intermediate host doesn't occur in a certain area, so we know if that parasite is in that certain area, that host has come from somewhere else. It's a rough indicator of movement and stock conditions and so it has been used fairly successfully as an indicator in in shallow water conditions.
There'll be one or two cases where they've tried to do it with deeper water, but we often don't know enough about the life cycles to make very strong findings from that.
When you're working on parasites you get used to thinking of them as fascinating organisms who deserve to live. Whereas, generally people probably think of them as problems and something to get rid of. Which, they are in some cases, but in lots of cases they're just part of a natural system.
Gyroctyle (a type of tapeworm) - Image Courtesy of Rod Bray
is there any competition on the host - are there any parasites that that actively fight over their host?
Well there are, yes! There's some fascinating research done in the mollusc where there's parthenogenetic reproduction. In some molluscs where there's two or three parasite species in the same mollusc, there is predation of one parasite on another.
There are groups of these individual organisms that are developed by parthogenesis who are like soldier ants; soldier parthemeti that actually consume the other species.
Wow! could that be potentially seen as advantageous?
Maybe, but often it just means that one parasite is more dominant in that host. Also, the mollusc hosts are often castrated by their parasites, so it’s not overall advantageous for the host.
Of course, if you've got a group of parasites in a gut of a host, they'll be segregated. Some in the front end, some in the rectum. Also, it's been found that if you've got two parasites in a host, they'll segregate a bit but if you've got several parasites, they'll segregate more tightly to stop competition.
C. armatus with ectoparasites. Image courtesy of Alan Jamieson
oh that's fascinating! are there any parasites that eat other parasites within the host?
You do find occasionally parasites with lumps out of them as if somebody's chewed on them, but whether that's just a bit of cannibalism or whether it's actually a competition between different things, I don't know.
any final thoughts?
Well, it's a fascinating area that needs a tremendous a lot more work done, and I would recommend to anybody to look at parasites; they're a fascinating group and there's an enormous amount of work left to be done on them!