Didemnum species
Figure 1. Didemnum sp.
Figure 2. Didemnum sp. close up
Distinguishing Features:
- Colonial tunicate
- Colour ranges from tan to orange
- Dark lines may run between groupings of zooids
- Large colonies form long slender lobes
- May have a ‘spotted’ appearance due to calcareous spicules embedded in the tunic
- To distinguish from local sponges, check for gelatinous texture (as opposed to soft and spongy texture)
Origin:
(Likely) Japan
Habitat and Growth:
Didemnum sp. is a subtidal tunicate that will grow on any hard substrate ranging from docks, to the shells of bivalves, to gravel seabeds. Mature colonies can form large lobes when growing from a suspended structure, or form large ‘blanket’ sheets when growing along the sea floor. On the east coast of North America, at Georges Bank, scientists have measured over 259 square kilometres of sea floor blanketed by Didemnum sp.
Reproduction:
The colony can regrow and reproduce from fragments. Larvae only remain in the water column for minutes to hours before settling on a hard substrate to grow into juvenile tunicate colonies.
Can be Confused With:
- Most sponges – Didemnum sp. has a gelatinous texture as opposed to a soft and spongy texture.
- Native tunicate – Cystodytes lobatus which also has calcareous spicules. Cystodytes lobatus is purple coloured and is thick and lobed.
Figure 3. Sponge
Figure 5. Cystodytes lobatus (Native tunicate)
Management:
Remove colonial tunicates manually and place in garbage receptacle or let dry out of the water. If you must pressure wash colonial tunicates off equipment, only do so on land (not on the farm) and make sure the out flow does not go into the sea, as these colonies can re-grow from small fragments. Completely dry culture gear before placing it back in the water or moving it between sites.