The first and second emerged stages look like trochophore larvae, but already with traces of segmentation, especially in the second. On the first there appears to be an apical tuft near to the arrow head and on the second, it looks as though there is a prototroch with the cilia in focus at top and bottom of the image. To confirm this you would need to focus up and down to check that there is a band of cilia all the way round the larva. Also, it would be good to observe live larvae in which the beating of the cilia should be obvious. These are quite large larvae, so are likely to be lecithotrophic. There appear to be bundles of chaetae coming from each segment. Again, focusing up and down would give a clearer idea of the nature of these bundles.

A polychaete seem likely and there are some polychaetes that bore into seagrasses as adults. 

looks like a scale worm larva

It is an insect, definately!!! I see legs on first three thorax segments!!! But which one - that is a big question!!!! I may guess that it looks like I star larva of trichoptera, or coleoptera, or even lepidoptera... There is no posibility to say more, until you put high resolution pictures of the creature!!! Good luck!

I agree with a with some oligochaete larvae.

I would say it's a specimen of Oligochaeta, family Naididae, but double-check it if you can...determination of oligochaets is a bit tricky - you have to look in detail their anterior/posterior end, number and shape of chaetae (+ are they in bundles or single), genital organs, etc...so, a more detailed view is needed for more detailed determination...

Actually, I don't really have a clue, but if it is a larva, it doesn't look like a predatory one. So, I don't think it is Coleoptera or Trichoptera. It could be a Diptera larva, especially if no head or legs are visible. Otherwise, it might still be a Diptera or Lepidoptera larva. 

My best advise would be: try to grow it to the adult stage on leaves of eelgrass for an easier identification. 

The first and second emerged stages look like trochophore larvae, but already with traces of segmentation, especially in the second. On the first there appears to be an apical tuft near to the arrow head and on the second, it looks as though there is a prototroch with the cilia in focus at top and bottom of the image. To confirm this you would need to focus up and down to check that there is a band of cilia all the way round the larva. Also, it would be good to observe live larvae in which the beating of the cilia should be obvious. These are quite large larvae, so are likely to be lecithotrophic. There appear to be bundles of chaetae coming from each segment. Again, focusing up and down would give a clearer idea of the nature of these bundles.

A polychaete seem likely and there are some polychaetes that bore into seagrasses as adults. 

I think they are oligochaeta larvae, maybe Naididae or related family

Hi there,

Not totally sure, but the last pictures make me think that it could be an oligochaete: the creature is clearly segmented and I can see cholragogen tissue from segment IV on. The head shape seems ok for a Naidid oligochaete. On the other hand such a egg-accumulation seems weird for Oligochaetes, the segments look a bit too rounded and the chaetae seem too large: maeybe there is some organic matter attached on the chaetae? I would kill one of the creatures and try to take a look at it in glycerine or levulose syrup in between glass-slides and take a look at it using a compound microscope.

If it's a oligochaete, chaetae should look like on of those on the link below

Best wishes

Jan 

http://apps.biodiversityireland.ie/OligochaeteIdKey/introduction.php

I agree with Jan.

You can raise a doubt between an oligochaete or a polychaete in the initial instars, but ...... an insect? please..... definitely NO.

Please analyze setae under a microscope. I think is clearly an oligochaete.

Please forget about my previous post. I was misled by the fact that someone mentioned three pairs of legs. Good luck!

Concentrating on the chaetae, it is very difficult to be definitive but I am happy with the oligochaete/polychaete id. However, the chaetae look to be the beginning of winged capillary chaetae and possibly compound (again though, very difficult to say from photo) which would rule out oligochaetes (with simple, bifid or varied palmate chaetae). I would therefore be happier to go for polychaete larvae/juveniles. You don't mention the salinity regime of the lagoon. The presence of Zostera marina would suggest at the upper end towards 20 to 35 parts per thousand. The chaetae of the specimen remind me of a eunicidae polychaete but this taxa usually prefer fully saline water and the photo is too blurred, and speciemen likely too young, to be definitive.