Isopods Cause Reproductive Death in Shrimp
The isopod in question is the Argeiopsis inhacae, a member of the parasitic family of isopods - Bopyridae. They don't start off as the lovely parasite "friend" of shrimp. The larvae begins life as a free swimmer until it finds a copepod to attach itself too, then metamorphoses into another larval stage and looks to buddy up with the nearest shrimp it can find.
Calado et al. found that when pairing unparasitized males and females together, females laid perfectly fine clutches of eggs. However, when unparasitized males were paired with females containing the isopod, there were never any egg clutches laid. This is in spite of similar courtship behavior and no differences in moult patterns. Furthermore, parasitized adult female shrimp did not develop a key feature denoting fertile production, a bright green spot on the back that marks the presence of large yolky oocytes. It appears that this bopyrid isopod causes "reproductive death" in females Stenopus hisidus. Unfortunately, they never tested whether parasitized males can make viable offspring. It is still not known whether parasitism is sex-biased or appears as such because of the author's limited sampling.
This short study is interesting because it is the first experimental study to nail down reproductive cessation due to the isopod parasite. What use is it to stop reproduction? One reason may be to divert the host's resources away from reproduction, an energy expensive process. The isopod would ensure its survival and its continuance to leech off the shrimp.
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Calado, R., Bartilotti, C., Goy, J.W., Dinis, M.T. (2008). Parasitic castration of the stenopodid shrimp Stenopus hispidus (Decapoda: Stenopodidae) induced by the bopyrid isopod Argeiopsis inhacae (Isopoda: Bopyridae). Journal of the Marine Biological Association of the UK, 88(02) DOI: 10.1017/S0025315408000684
I admit it...I have a lol problem.
ReplyDeleteBopyrid isopods are no fun. Up here in the Pacific Northwest we have a recently discovered bopyrid isopod parasite (Orthione griffensis)that infects one of the thalassinid (i.e., burrowing) shrimp species, Upogebia pugettensis which can occur in densities of up 200+ shrimp per sq. meter on the large intertidal flats of of Oregon and Washington. A colleague of mine at Oregon State, John Chapman, has been studying this bopyrid for several years. He has found a similar effect - impact on reproduction (let me add a little aside here and state that technically bopyrid isopods do not castrate if by that you mean directly impact reproductive organs. They are attached in the gill chamber sucking blood so their real impact is reducing energy reserves of the host such that they cannot invest energy in reproduction...maybe a picky point but potentially important evolutionarily if in a field population some of the parasitized adults can still squeeze out some eggs. To the use of castration has been because it is a dramatic and immediately brings up...um...let's say unpleasant images). The interesting (or nightmarish) aspect of O. griffensis is that is thought to have been introduced via ballast water from Japan to the W. Coast and subsequently adapted to two intermediate hosts and the adult host in the California current system and W. coast estuaries. Some of the early info on this parasite can be found here: http://oregonstate.edu/dept/ncs/newsarch/2005/Aug05/isopod.htm
ReplyDeleteThe first paper on this isopod parasite is coming out shortly but early observations are that some of the Upogebia populations have crashed (see the figure in the above press release for a Willapa Bay, WA population) with the exception of a few in central Oregon. Even more interestingly, these surviving populations in OR have had three successive huge recruitment years...so there are obviously some of the females that are still able to produce eggs and larvae. So, are the non-infected females (<20%) producing all these potential recruits? Are infected females still producing enough eggs to maintain the population? Can an introduced parasite push natural populations on a trajectory towards extinction? Too many cool questions for the invertebrate geek in me...but we need to figure it out quickly!
But if they don't let the shrimp reproduce, aren't they going to run out of hosts?
ReplyDeleteAydin-
ReplyDeleteYes, that is one of the arguments that has been discussed regarding non-native species...that since in this case the parasite did not co-evolve with their host species it is possible that the parasite could cause a localized extinction in the host population.
Tony, thanks for your comment! I still like the word castration, "energy thief" just isn't as a good a headline grabber lol
ReplyDeleteSince this is an invasive isopod, is affecting any other shrimp? Have you looked at other potential hosts? Commercial shrimp fisheries? I wondering if there are reservoir hosts for the isopod.
Could the shrimp population be maintained from pools up current where the isopod is not present? i.e. fresh shrimp larvae being swept south to get infected.
Kevin-
ReplyDeleteInterestingly there is another thalassinid shrimp species, Neotrypaea californiensis (Pacific ghost shrimp) that co-occur with Upogebia and are even more abundant (higher densities and cover larger intertidal area). We have not found any of the invasive isopod parasites on the ghost shrimp which has a native bopyrid isopod parasite (Ione cornuta), albet at a very low prevalence (<3% infected). The only commercial shrimp in the systems with the invasive isopod are the Pacific bay shrimp (Crangon franciscorum and there are no reports of the isopod infecting this species to date.
As far as maintaining the Oregon Upogebia populations, the upstream populations (in WA state) have crashed so it is more likely that recruits have come from the local populations. We are starting to test this hypothesis through a combination of early life history studies (larval rearing, tracking offshore larval patterns, recruitment back to tideflats) and population genetics (of adults and larvae) so stay tuned.
Funny, I just found this post while I was trying to make sure I spelled Ione correctly.
ReplyDelete"Since this is an invasive isopod, is affecting any other shrimp? Have you looked at other potential hosts? Commercial shrimp fisheries? I wondering if there are reservoir hosts for the isopod."
A lab mate of mine is examining isopod prevelence on ghost shrimp all along the west coast, in bait shops and in the field, to try and get at some of those questions... Although with the native isopod (native to oregon NOT southern CA, hence the worry)
I get to help collect and disembowel the little critters! Small price to pay to travel up the coast (but I can't imagine 3 years of it).