The spread of Didemnum sp. has been prolific in recent years and is causing environmental and economic impacts. There are many questions regarding how this invasive tunicate proliferates. Will fragments of Didemnum sp. reattach and grow through reproductive budding after the larval period?
Didemnum sp. is a recent invasive species of tunicate in the waters of the east coast of the United States. Since 1988 it has spread from Damariscotta, Maine to as far north as the US border with Canada and as far south as Niantic, Connecticut in Long Island Sound. The Didemnum can overgrow other species but no species appears to grow over it. It will cover other tunicates and shellfish which causes concern for the health of shellfish beds. Didemnum covering the shellfishing beds of George’s Bank and the near-shore.
Didemnum spreads by larval (sexual) and asexual (budding) reproduction. Larval reproduction is limited to up to an area of about a quarter mile from its parent but occurs only when the water stays constantly above six degrees Celsius. According to Gretchen Lambert, Friday Harbor Lab, University of Washington-Seattle, larvae have been observed in New England Didemnum dissections in late June to early October. The larval reproduction stage is limited to these months, however all the limitations on larval production have not been discovered.
Didemnum can increase in colony size through reproductive budding. This asexual growth adds to the area of the colony, and allows for reattachment of fragments. Colonies fragment themselves as a means of spreading and increasing the population. Sandwich tide pool is exposed at low tide and covered by about nine feet of water during high tide. This site is near the east entrance to the Cape Cod Canal and is exposed to Cape Cod Bay. It is feasible to induce fragmentation there.
Asexual growth and fragmentation may occur year-round. A possible explanation for the spreading of this invasive species may be found in its ability to tolerate fragmentation throughout the year. The experiment involved artificial fragmentation of Didemnum in the Sandwich tidal pool into containers in November 2005. The containers were observed for four weeks. Out of nine containers, five reattached in a span of two weeks. The trial was allowed to run another two weeks until the fragments exhibited growth. The most significant proof that budding growth occurred in the attached specimens is the irregular shapes of the specimen: they all started square and grew irregularly.
Didemnum has been known to cover organic substrate such as clams, mussels, and oysters. If a drag net artificially fragmented a shell fishing bed covered in Didemnum, new colonies could be distributed all over. Dragging for shellfish occurs year-round in the areas that have seen the explosion of Didemnum. Thus fragmentation explains the spread ofDidemnum in the short time span and its abundance in offshore beds such as those of Georges Bank and in the nearshore like at Wellfleet. The mystery of the speed at which this tunicate has spread can be attributed to its ability to fragment and establish new colonies year-round.