Whalesharks with Parasites
Having just got back from the Insider Divers expedition to Tubbataha we are all still totally psyched about how many whalesharks we saw. On each on of the three atolls we encountered many whalesharks. Most of them were "social" as they turned around to swim back to the divers and sometimes spent long periods of time interacting with us.
Same as on our last trips we noticed that almost all of the whalesharks are covered with black spots. This year the sharks were covered with these patches even more than last time.
A lot of the area affected is around the mouth the fins and the back of the sharks.
The animals have different degrees of the patches such as this poor character dubbed "Lipstick" who has a dense coverage of the mysterious patches around the entire mouth area. (Lipstick also had just been freed of a rope around the neck, you can see that scar there. Really not a fortunate creature.)
This one is called mohawk for the line along his back.
So what is this gunk and why is it specific to Tubbataha? Upon a closer look we can see that the patches are actually clusters of individual animals.
Disgusting right? These are copepods which are a type of crustacean that are amongst the most abundant sea creatures in our oceans. A large section of these species have adapated to being parasitic - meaning they live on host animals.
This little sucker is called Pandarus rhincodonicus - and as the fancy name already says it lives exclusively on whalesharks (scientific name Rhincodon typus). They are specifically adapted to living on the trail edges of whalesharks, so mainly where there is a lot of water flow. Many whalesharks have them although where I have seen them I didnt notice them (Mexico, Galapagos, Indonesia, Maldives) I didnt notice them. Studies suggest though that they live on all populations of whalesharks around the world.
According to what I have read they mostly live of bacteria and algae on the sharks skin, although some scientists believe they are also parasitic and feeding on their hosts. If they just ate bacteria then this would be a "commensal" relationship which means the shark actually has no downside in this relationship. Some of the research suggests that the little things are hooked into only the very top layer of the shark's thick "skin" (its actually not skin, but that's for another session) and therefore not affecting the whaleshark.
It did however seem to us that the whalesharks were hanging around us primarily because of our bubbles. They seemed at times to be bathing in the bubbles, leading me to believe that the whalesharks are trying to get rid of the parasites by swimming through the bubbles and even sucking the bubbles in (there are other parasites living inside their gills rakers too...).
Another interesting thing is that the few older and larger adults that we encountered during our trip
a) did not stop to hang out with us
and b) did not have the parasites.
To me it seems logical that the whalesharks that interacted with us were mostly interested in the bubbles to help them scrub off the parasite.
(larger whalesharks where mostly clean of parasites)
Because here is an interesting question - as whalesharks travel around the globe - why do only the whalesharks in Tubbataha have that many copepods? Not even other sites in the Philippines have this phenomenon. This question baffles the scientists that I have asked. There just does not seem to be a good explanation for this phenomenon. Dr Simon Pierce, one of leading scientists for whalesharks and founder of the Marine Megafauna Foundation, literally told me "Honestly, no clue. It's just really odd."
One explanation could be that the whalesharks that "live" here have them, so they would get them when they come to this area. Or they pick them up somewhere in the area. Younger Whalesharks often stay in "nurseries" such as Tubbataha or Ari Atoll in Maldives. So they could potentially infect each other with such a parasite. Unfortunately studies show that none of the whalesharks seem to stay here long term.
My other, and wilder, theory is that the whalesharks are coming to the nurseries to get rid of the parasites. The whalesharks in these nurseries are the smallest that divers encounter anywhere. But scientists believe them to be 0.5-0.7m length when they are born. So they must live somewhere in the deep until they grow to be around 3m size, which is when we see them for the first time. Maybe they get the parasites in the deep and then stay in the nurseries until the parasites disappear for example thanks to cleaner fish, or scuba divers!
Anther idea that I had is that maybe the whalesharks like to frequent the warm water around the reefs that can be around 30 degrees - maybe the little crustaceans dont like the warm water and fall off.
Regardless of why the sharks have them, the reason they have the parasites might be a reason why they are in Tubbataha. The parl is one of the few places in the world where divers can reliably scuba dive with them (rather than snorkel). And they seem to be genuinely happy to see us, so it's a win-win all around.