Starfruit

Damasonium alisma

Description/identification

Starfruit is a medium sized aquatic plant, named for the distinctive star shaped fruit it bears in late summer. Its appearance will vary depending on conditions, but typically it has broad leaves on long stalks. If the plant is submerged the leaves often float on the surface of the pond, and these may be somewhat narrower than the broad aerial leaves. If the plant is not submerged, the leaves are more robust. The flowers are white with three petals like other water plantains, however Starfruit can easily be recognised by it’s ripe fruit which have 6 pods arranged in a star shape.

Habitat

Starfruit is a specialist of small ponds and pools that either dry-out every year, or have a broad drawdown zone.  In the UK, its recent ponds have been in lowland heathland, common land or rough pasture where there is regular cattle grazing. In the past there are records from a wider range of habitats such as muddy road-side ditches. It favours ponds with bare exposed margins, where livestock grazing and poaching, and seasonally fluctuating water levels control competition from terrestrial and aquatic vegetation.

Starfruit is now confined to a small number of isolated ponds. However, it would have once likely inhabited many ponds, pools or ditch ruts across sites, existing as a metapopulation, with repeated recolonizations and local extinctions. In modern landscapes these muddy wetland complexes have been almost completely eliminated.

Life Cycle

Starfruit typically grows as an annual, but can sometimes over winter. Its seeds may need a period of desiccation: drying out when water levels are low in summer, but germinating underwater between autumn and spring when the water levels are high. Livestock may assist germination through poaching – bringing seeds to the surface and trampling potential competitors. The seedlings start to grow as submerged plants but can later develop into emergent or floating plants that generally flower between May/June and August. An individual plant can produce as many as 150 flowers (Birkinshaw, 1990).

In late summer the plant produces distinct fruits with 6 two-seeded pods arranged in a star shape. When the water levels begin to rise in late summer/early autumn and the fruits become submerged. The first seed is released, the second is retained until germination, when it floats to the water surface. This may be a mechanism for dispersal during high water periods (Birkenshaw, 1990). Seed which do not immediately germinate in the spring or summer enter a dormancy phase. Some seeds have been shown to remain viable for over 50 years in the sediment. This is a mechanism which allows them to survive the uncertain conditions of temporary ponds, where there may not be suitable conditions every year.

Distribution and threats

Starfruit is listed as Critically Endangered on the Red Data list for Great Britain and is protected by law under section 41/42 of 2006 Natural Environment and Rural Communities (NERC) Act. Starfruit has declined catastrophically in the last century, and in the last decade there have been several years where no plants have been seen at all. Pond management to try and recover the species has so far proved unsuccessful in the short to medium term. Attempts at translocation so far appear to be partially successful.

Multiple factors have led to the decline of Starfruit, including habitats loss, habitat degradation, competition from the invasive plant Crassula helmsii and nutrient pollution.

Habitat loss

Change in land use has led to the loss of many commons and rough pastures that contained suitable habitat for Starfruit. Many grazed heathlands and pasture have developed into woodland, amenity grassland or, especially around London, been lost to urbanisation.

Habitat degradation

Starfruit likes bare, open sunny habitats. Loss of grazing at many sites has allowed ponds to become shaded and overgrown and led to the loss of the bare substrate required by Starfruit. Lack of grazing may have also negatively impacted seed germination through the reduction in poaching and soil turnover.

Pollution

Nutrient pollution is a widespread problem across the country and in all freshwater habitats. Excess nutrients come from multiple sources including agricultural run-off, household waste water, sewerage and road runoff. Many species including Starfruit require clean, low nutrient water.

Current status

Starfruit now appears at only one of its former sites, and even there irregularly. It also occurs at two translocation sites. Past attempts to recover the species by clearing back marginal scrub and emergent vegetation at ponds on Headley Heath and Gerrards Cross have not proved successful. Dredging of Daisy Pond on Naphill in the 1980s produced a short-term flush of up to 300 plants, but further germination stopped within a couple of years. Seed bank analysis undertaken for the People Ponds and Water Project in 2017 showed that only a limited seed bank remained in pond sediments from Naphill and Headley Heath.

Starfruit seeds have been translocated to new ponds on at least three sites over the last 20 years. Unexpectedly, the two ponds where translocations have been most successful have both had clay substrates, rather than the sandy gravels which typify its former ponds in the UK.