Article: Simpson TS, Wernberg T, McDonald JI (2016) Distribution and Localised Effects of the Invasive Ascidian Didemnum perlucidum (Monniot 1983) in an Urban Estuary. PLoS ONE 11(5): e0154201. doi:10.1371/journal.pone.0154201
It’s seagrass monitoring season at work in Southwest Florida, so it’s time for me to post about seagrass again (Fig. 1)! I’ve been noticing some tunicates encrusting seagrass blades and as a scientist, decided I wanted to know what impact they may be having! Low and behold, I find a new article about how invasive tunicates affect seagrass in Australia! Now, I don’t think the tunicates I’ve been seeing are invasive, but they likely cause some of the same effects found in this study.
Our story takes place in the Swan River Estuary in Western Australia. Estuaries are semi-enclosed bodies of water where saltwater from the sea meets freshwater from at least one river. Estuaries are highly productive areas but also focal points for human impacts as they tend to be highly settled. They are also at risk of invasion since so many human activities occur around and within them.
Seagrass (our protagonist in this tale) is a major habitat within this estuary, and it is important for providing food and homes for many other organisms, stabilizing the substrate, filtering runoff, and sequestering carbon. Seagrass habitats worldwide are in decline due to a number of factors. The species of seagrass studied, Halophila ovalis (Fig. 2), is threatened by changes in temperature and sedimentation, increased nutrients from runoff (and jellyfish blooms!), and invasive species such as macroalgae.
So who is our antagonist? A colonial tunicate called Didemnum perlucidum (Fig. 3). Tunicates are chordates (the same broad category of organisms as humans) that begin life as tadpole-like larvae that then settle and dramatically change body form (resorbing their brain-like structure and other organs) to become filter feeders. Many species are invasives as they possess traits that allow them to succeed in establishing in new areas: rapid growth, high reproductive rates, and several reproductive strategies. Didemnids can even regenerate from fragments. D. perlucidum is native to the tropical Indo-Pacific and was first documented in Western Australia in 2010 on settlement panels and jetty pylons. These colonial tunicates are a type of bio-fouler. Fouling is the accumulation of material on a surface to the detriment of the function of that surface; bio-fouling is when the accumulating material is alive (like barnacles or tunicates!). These colonies were observed overgrowing other fouling organisms and have since spread along the coast to many artificial structures and now cover a broad latitudinal range. Therefore a number of agencies are calling “Foul”! Because of their potential negative impacts, they are on the Western Australian Prevention List for Introduced Marine Pests and the US National Exotic Marine and Estuarine Species Information System. From 2013 to 2015, it was found living on leaves of the seagrass H. ovalis, causing concern since tunicate fouling of seagrass has been implicated in seagrass decline in areas, including New England in the US.
The authors of this study, Simpson et al., set out with the goal of determining D. perlucidum’s extent and abundance within the estuary and what its impacts are to H. ovalis and associated plants and animals.
Act 1: The Data Collection
Scene 1: Document abundance and distribution
Snorkelers and SCUBA divers survey 12 sites during March, April, June, and August. At each site, they monitor three 60 m transects for percent cover of both seagrass and D. perlucidum. They take photographs of nearby artificial structures to determine presence/absence of the tunicate on those structures (Fig 3). They measure environmental conditions that may be correlated with the distribution of D. perlucidum. They establish permanent quadrats to assess growth and spread over time, and they photograph these quads every month for one year.
Scene 2: Measure effects on seagrass
The team collects biomass samples at five of the sites half with D. perlucidum encrusting and half without. They dry and weigh the seagrass biomass (after removal of the tunicate). They measure photosynthetic response and pigment levels for living leaves with and without tunicates covering them.
Scene 3: Test effects on associated organisms
They take sediment cores from three sites, half the cores with the colonial tunicate and half without. Mud snails (Battilaria australis) are sieved from the cores and counted (Fig. 4).
Act 2: Results Revealed!
Scene 1: Abundance and distribution
The researchers found D. perlucidum at 86% of the sites, and at 75% of those, it was growing on seagrass (primarily on H. ovalis but also on Zostera marina and some macroalgae) (Fig 5). They saw it on individual blades but most often spread across multiple and forming mats as large as 30 cm in diameter (Fig 3). The tunicates are more abundant near artificial structures such as mooring buoys and in fact, these structures may be a source of recruits for those growing on seagrass. Colonies on seagrass tended to be within 10s of meters of colonies on human structures.
The cover was variable, but colonies appear in March and are gone by August within the seagrass beds, with mean cover across sites of 3%, with a maximum mean cover at one site of 17% (Fig 5). The size of colonies averaged 106 cm2, ranging from 6.8 to 853 cm2, at the permanent quadrats. The size contracted by May and decreased each month through winter (N. Hemisphere readers: remember the southern hemisphere’s seasons are reversed!).
The two environmental variables explaining distribution and abundance were number of moorings within a site and salinity.
Scene 2: Effects on seagrass
Since the tunicates completely envelop seagrass blades, it’s not surprising they’d take a toll on the plants. Biomass of seagrass was lower when associated with D. perlucidum (Fig 6) and the plant tissue had often disintegrated and decayed within the colony. Surprisingly there was no difference in photosynthetic responses or pigments, but they only tested that with live blades. Seven percent of leaves within colonies were still healthy and bright green, 38 percent had some remaining green color, but 55% were brown (dead or dying). For comparison, 70 percent of leaves outside of tunicate colonies were green and alive (Fig 7). Since seagrass leaves remained attached to the plant, they were likely alive when D. perlucidum settled and covered them.
Scene 3: Effects on associated organisms
Fewer mud snails were living where D. perlucidum was. Cores contained 1.3 to 2.8 times more mud snails where colonies were absent (Fig 8).
The Take-Away from this Play
D. perlucidum is certainly a potential threat to this estuarine seagrass ecosystem and may have impacts on the ecosystem services seagrass communities supply. They have the potential to contribute to 2-3.0 hectare loss of seagrass per year (keep in mind that the grassy area inside a track is approximately 1 hectare). These invasives are likely spreading from human infrastructure and then recruiting to grass blades when conditions are right. Once in the seagrass, they may spread if covered blades break off and get moved to other areas by currents. Studies of their temperature and salinity tolerances are needed to predict their future range. All told, fouling is a huge problem for seagrass since it blocks the light blades need, causing them to die and decompose within the tunicate colony. Seagrasses will have to rely on seed and vegetative fragments to regrow under these conditions. In addition, the impact to mud snails is likely not one to ignore. These snails impact sediment movement, release nitrogen, filter water, and product shells that act as substrate and shells for hermit crabs after the snails die. There may be large ecosystem wide effects from the interactions between seagrass, tunicates, and snails, which will require further study. This invasion and the spread of tunicates to seagrass beds is an important consideration for the sustainable management of these estuarine habitats. Finally, on a personal note, I might have to take a closer look at the coverage of tunicates in seagrass beds where I work!