It looks to me like a marine Sphaeromatidae, like Cymodoce, Dynamene or similar genus. Were was it collected?

Hi Ton, found them in seagrass nursery tank. They had been grazing on the seagrass. Location Sabah, Malaysia

Hi Tzuen,

as Ton already said it is a sphaeromatid isopod. I'm not familar with the Indo-Pacific region, for South Africa I use the very good key book of Brian Kensley (1978): Guide to the marine isopods of southern Africa. Maybe it helps a little bit to get in touch with this very diverse family with more than 100 genera.

Cheers

Michael

Thanks Micheal for the reference, really appreciate it.

Hi Tzuen

The habit of rolling in a ball is typical of sphaeromatids and the general appearance, though the telson posterior margin does not resemble that of the members of the genus Sphaeroma, so consider other genera.   The paper by Bruce and Wong attached may be useful as it covers Singaporean isopods.

Best wishes

Simon

Hi Tzuen

The photograph is litle blurred but this looks like an adult male of the genus Cymodoce. That would be a good place to start.  The good news is that an adult male is exactly what you want when it comes to identification. The bad news is that there are many, many species of Cymodoce worldwide.

See the list at WoRMS ('marinespecies' link below).

You may get females or juveniles in the same location but they do not look like the males. You can see the difference between males and females of Cymodoce for a British species in the 'Aphotomarine' link below (once at that page, just hover over the list on the left and the image changes).

Keith

http://www.marinespecies.org/aphia.php?p=taxdetails&id=118426

http://www.aphotomarine.com/isopoda_cymodoce_truncata.html

@Simon. Ya, their body relatively longer than genus Sphaeroma, will consider other genus. Thanks for the paper. Really appreciate it.

@Keith,I got at least 50 specimen collected  from seagrass nursery tank, with different size, and colour. Most with hairy forked tail appendages. Will keep this species under consideration. Thanks and grateful for your help.

It looks like male of Cymodoce truncata from Sphaeromatidae. ??