The Stowaway

This is the final installment of The Velella Chronicles (at least for now!).

While I was watching the gonozooids feeding, something amazing happened.  A small, brownish, worm-like object became visible among the gonozooids.  It's at the top center of this image:

 

I struggled to understand what was happening at first...was this a part of Velella that I hadn't seen before?  But then it emerged fully and I could see that it was a completely separate animal.  Ah ha!  A stowaway!

 

The mystery animal was generally long and slender, but it was sometimes thicker and rounder in the middle.  And the anterior end (on the left) would sometimes contract dramatically:

 

One gonozooid began opening its mouth extremely wide as the stowaway approached.  You can follow the progression of the mouth opening by watching it in the upper right corner of these pictures (start with the first picture of the post above).

The stowaway started crawling along the rim of the open mouth:

 

Here it is fully on the rim, moving to the left: 

 

At one point it started "reaching around" and exploring the opening to the mouth:

 

And then I couldn't believe it, but the stowaway crawled into the mouth of the gonozooid!  The gonozooid didn't seem in a hurry, but it slowly closed in on the stowaway so that all I could see was the posterior end of the stowaway sticking out of a small opening (see below).  And then the stowaway disappeared completely inside of the gonozooid!

 

Wow.  What just happened?  Was the stowaway some sort of parasite?  Did the gonozooid eat the stowaway?  Or would the stowaway have some way of protecting itself against being digested by the gonozooid?

I haven't heard anyone talking about parasites of Velella velella.  And I wasn't really sure where to start learning about them.  Doing a quick search on the Internet didn't reveal much.  But I did find one reference to parasitic flatworms (trematodes and cestodes) in comb jellies, jellyfish, hydrozoans, and siphonophores.

One trematode genus was mentioned that seemed to have potential — Opechona spp.

I don't know if this stowaway is a species of Opechona, but it *might* be a trematode larva.  So it's worthwhile to talk about a basic trematode life cycle, with Opechona as an example.  This gets complicated, but here's a diagram to help visualize the different stages (with an explanation below).  The inner circle shows the life stages of the parasitic flatworm, while the outer circle shows the transitions between its three hosts:

 

Modified from Marcogliese, D.J. 1995. The role of zooplankton in the transmission of helminth parasites to fish. Reviews in Fish Biology and Fisheries 5: 336-371; Koie, M.M.  1975.  On the morphology and life-history of Opechona bacillaris (Molin, 1859) Looss, 1907 (Trematoda, Lepocreadiidae). Ophelia 13: 63-86; and  Siphonophores and Velellids by Kirkpatrick and Pugh (1984)

The stage we saw on Velella was possibly a free-swimming cercaria larva.  Once inside Velella it will lose its tail and encyst in a stage called a metacercaria.  If a fish eats Velella, then the metacercaria will develop into an adult fluke inside the fish (the definitive host).

The adult fluke can reproduce sexually with other flukes.  Fertilized embryos develop into miracidia larvae which leave the host with feces.  Once in the water, the miracidium larva locates and bores into the first intermediate host (e.g., a snail).  

Several additional larval stages occur within the snail (e.g., redia) before free-swimming cercariae leave the snail in search of the second intermediate host (e.g., Velella).

Whew!  Again, we're not sure about the identity of this stowaway, but it's fascinating to think about possible parasites on board Velella velella.