The colonisation of Havergate Island, Suffolk by the Starlet Sea Anemone, Nematostella vectensis

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THE C O L O N I S A T I O N O F H A V E R G A T E ISLAND, S U F F O L K BY T H E S T A R L E T SEA A N E M O N E , NEMATOSTELLA VECTENSIS. W. WELSTEAD AND M.E. SHARDLOW This shorl paper reports the discovery of the starlet sea anemone, Nematostella vectensis, in a saline lagoon on the RSPB Havergate island reserve in Suffolk.

The macro-invertebrate fauna of the lagoon has been monitored each year from 1994 to 1998. The first appearance of N. vectensis was in 1997 when three individuals were found in ten core samples. In 1998 in the same lagoon there were 52 in ten cores. Sampling in adjacent lagoons produced four more individuals from one lagoon.

Status and Distribution of the Starlet sea anemone The starlet sea anemone, N. vectensis, is an edwardsid actinarian which takes its specific name from its discovery in a brackish pond at Bembridge on the Isle of Wight. It was first described by Stephenson (1935). The anemone is relatively small at less than 10 mm in length. It is translucent, except for small white spots, and contracts on fixation and is therefore easily overlooked. The 10 to 18 tentacles are almost as long as the column (Hayward et al„ 1990). All the specimens found in Britain are females. Reproduction is by transverse fission (see Plate 5).

N. vectensis is a brackish water specialist which is found in muddy sediment with its tentacles spread over the surface. It may also be attached to macrophytes. The south and east coasts of Britain are the only European sites where it has been recorded. It is also known from the Atlantic and Pacific coasts of North America. N. vectensis is listed as rare on the British Red Data list (Bratton, 1991), is protected under Schedule 5 of the Wildlife and Countryside Act 1981 and is included in both the United Kingdom and the Suffolk Biodiversity Action Plan (Anon, 1997). A fuller description of the life history and distribution in Britain is given in Sheader et al. (1997).

Background Havergate Island covers 108 ha and lies at the confluence of the Rivers Ore and Butley on the Suffolk Coast. The island, which has been owned by the RSPB since 1949, is part of the Havergate - Orfordness - Shingle Street National Nature Reserve (NNR) and is a Grade 1 Site of Special Scientific Interest.

Havergate Island is included in the Orfordness - Shingle Street proposed Special Area of Conservation.

Over half the area of the island is covered by six saline lagoons that are managed to provide habitat for breeding avocets, Recurvirostra avocetta, and Sandwich terns, Sterna sandvichensis, and for passage and wintering waders and wildfowl. Five of the lagoons are on the northern half of the island. Until 1994, these lagoons were interconnected by a series of sluices. Estuary water with a salinity of 35 parts per thousand (ppt) was admitted at the northerly lagoon and then allowed to flow through sluices to each of the other four lagoons.

In the 1980s the lagoons suffered from hypersalinity with consequent decrease in the fauna (Mason, 1986). In response to this problem and in

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recognition of the wider conservation importance of lagoon type habits, the RSPB modified the management of one lagoon. In 1994 Bclpcrs lagoon was isolated from the other lagoons. There was no further direct input of sea water other than by percolation through the shingle at spring high tides. Fresh water is now pumped, under licence from the Environment Agency, from a bore hole adjacent to the lagoon. The aim of this management is to maintain a lower salinity level than could be sustained through admission of sea water through a sluice.

The impact of this change in management practice on the salinity and on the macro-invertebrate fauna of Belpers lagoon (TM/416475) has been monitored annually since 1994.

Method Sampling of benthic invertebrates on Havergatc Island is carried out by taking 8 or 10 random cores to a depth of 10 cm. Core diameters of both 10 cm and 6-5 cm have been used.

Belpers lagoon was sampled on 18 August 1997 and again on 3 September 1998, on each occasion 10 samples were taken using a 6-5 cm diameter core.

The mud samples were broken up by swirling in a bucket of fresh water and were sieved through 2 mm, 1 mm and 0-5 mm sieves. The contents of each sieve were washed into a white tray from which all macro-invertebrates could be removed to petri dishes for examination under a binocular microscope (magnification x 10 and x 30).

Samples were sorted within 8 hours of taking, when the majority of invertebrates were still alive.

Results The first discovery of N. vectensis in Belpers lagoon was made in August 1997, during the routine monitoring of benthic macro-invertebrates on the site. In 1997 the density was 92 + 66 per m2. (p = 0 05). Further monitoring in September 1998 revealed 52 individuals in ten cores giving a density of 1600 + 420 per m : (p = 0 05). Four individuals were found in North lagoon in 1998.

Two of the three 1997 specimens of N. vectensis were found in one of the core samples. Both were still alive and were attached, one to each end, of a 10 mm long red chironomid larva. Such larvae are among the commonest prey items for N. vectensis (Sheader el al., 1997).

Identification The anemones were examined under both transmitted and incident light at a magnification of x 10. An auxiliary light source was used. For the two anemones attached to a chironomid larva, the head capsule of the larva could be seen clearly through the transparent body and inside the gut of one anemone. The other anemone was attached to the same larva by its tentacles.

The anemones had transparent elongated bodies slightly rounded at one end and with about 16 transparent tcntaclcs at the other end. Faint whitish longitudinal lines were visible on the body. The tentacles had white spots on them when the anemones were alive, although these disappeared after preservation. The body length was measured at 7 mm for one specimen and 6 mm for the other. The tentacles were almost as long as the body.

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Identification was confirmed in 1997 by Dr Martin Sheadcr who also determined that all the specimens were female. The specimens have been retained and are preserved in 70% industrial alcohol. There is considerable loss of distinguishing features in the preserved specimens.

Description of Belpers Lagoon Belpers is a shallow saline lagoon with an area of 8-6 ha. In the categorisation adopted by Smith and Laffoley (1992), it is now a percolation lagoon. Over most of its area, the water is between 5 cm and 20 cm deep. Around the perimeter, there is a 2 to 3 m deep borrow dyke from which clay has been dug to build the sea walls that protect the lagoon from inundation at high tide. The substrate is clay over coarse shingle. The lagoon is managed by controlling the salinity, by maintaining the sea walls and by raking the substrate to soften the mud. Raking is carried out a section at a time so that only a small part of the total area is disturbed in any one year. A detailed study of the impact of raking on the macro-invertebrate fauna was carried out in 1995 (Welstcad, 1996).

Before Belpers was sealed off from the other lagoons, salinity was typically about 35 ppt, but evaporation has in the past resulted in hypersalinity up to 65 ppt (Mason, 1986). Since Belpers became a percolation lagoon, salinity levels have been maintained mostly between 20 and 25 ppt. To avoid the risk that salinity will rise through evaporation, levels are now measured weekly during periods of peak risk and the input of fresh water is adjusted accordingly.

Associated species During late summer, the lagoon is vegetated over about 10-15% of its area.

The dominant alga is Chaetomorpha sp.. Enteromorpha sp. and Ulva sp. have also been recorded. In winter, the algal growth dies back and together with the cuttings from terrestrial vegetation, provides an essential input to the detritus food chain. Mason (1986) in a study of Doveys lagoon found an impoverished fauna dominated by three species: Nereis diversicolor, Corophium volutator and Chironomus salinarius. The faunal diversity of Belpers is now more varied. Although it has nowhere near the 50 species that have been recorded from the natural lagoons at Shingle Street (Barnes, 1980), there are more species than in earlier years and more of these are lagoon specialists. The species and higher taxa that were found in core and net samples taken from Belpers lagoon in 1997 and 1998 are listed in the table.

Taxa recorded from Belpers Lagoon 1997-98 Nematostella vectensis* Corophium volutator Idotea chelipes* Palaemonetes varians Ostracoda Nereis diversicolor Polydora ciliata.

Chironomid larvae Tubificids Macoma ballhica Cerastoderma glaucum* Pomatoschistus microps Gasterosteus aculealus * = specialist lagoon species

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Discussion N. vectensis has been found in natural brackish lagoons at Shingle Street which is some 4 km to the south of Havergate Island and in lagoons on Orfordness some 3 km to the north (Smith & Laffoley, 1992). There are also more recent records from lagoons at Benacre, Covehithe and Dunwich (English Nature, 1997). The addition of three new sites on Havergate Island together with the other three new sites in Suffolk represents a significant increase in the number of locations where this National Biodiversity Action Plan species has been found in Britain.

It would be easy to overlook such a small transparent animal, but as Belpers lagoon has been monitored extensively during the last ten years, it seems likely that this species was either absent or was only present at very low densities.

Movement of this species between sites is not fully understood. Barnes (1994) discounts movement on the feet of birds and believes that occasional overtopping of the sea wall is a more likely explanation. Individuals from Orfordness could be carried down Stoney Ditch into the River Ore where they would be very close to the sluice leading into the northern Havergate lagoons.

Until 1994, Belpers was also fed from this source. During low tide, estuary water can be seen upwelling from the shingle substrate of Belpers and this is therefore a further possible route into the lagoon.

Laboratory tests have shown that N. vectensis is more likely to flourish if the salinity is within the range 16-36 ppt (Sheader et al., 1997). The change in the status of Belpers to a percolation lagoon and tighter control of salinity levels have resulted in other lagoon specialists prospering. For example, C. glaucum has been found at densities of between 100 and 200 per m over the years 1995 to 1998. Prior to that date there was no record of that species in this lagoon (Mason, 1986; Robertson, 1993). Further monitoring is required to establish whether the large increase in the density of N. vectensis from 1997 to 1998 is sustained.

The intitial objective in changing the management of Belpers lagoon was to provide a more reliable source of prey species, particularly C. volutator and Nereis diversicolor, for breeding avocets. This objective has been achieved to the extent that in 1998 Bclpers lagoon had a higher density of C. volutator than that for all the other lagoons on the island. The present study has shown that such management is not at the expense of the non-avian species for which Havergate Island is a stronghold.

Under the management of the RSPB and with protection under United Kingdom and European law, the immediate future of this site is reasonably secure. However, the whole ecosystem is fragile. Apart from the risk of rising salinity levels through evaporation, the sea walls are at constant risk from erosion. In the long term, rising sea levels threaten all the coastal lagoons of Suffolk. The management of Belpers lagoon has demonstrated what could be done to create and maintain new habitat during managed retreat.

Conclusion The discovery of N. vectensis in Belpers lagoon is important for the conservation status of this lagoon. It is an important success for both the UK Biodiversity Action Plan and the RSPB reserve management policy. Further

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monitoring is required to confirm that this species is prospering and to follow the long term changes in the species composition of the lagoon. In the meantime, it is essential that the present management regime is continued. In particular the control of salinity and the maintenance of the sea wall arc regarded as priorities for the management of the lagoon.

The development of a specialist lagoon fauna at Havergatc Island has demonstrated that lagoon systems managed primarily for birds can also deliver conservation benefit for specialist species. At present, there is no conflict between the requirements of specialist species and avian fauna. The RSPB is confident that Havergate Island can be successfully managed for all aspects of its conservation importance.

Acknowledgments The authors are grateful for the assistance of Stephen Denny and John Partridge in the planning and execution of this monitoring programme.

References Anon (1997). The Suffolk Biodiversity Action Plan. Ipswich: Suffolk County Council.

Barnes, R. S. K. (1980). The shingle foreshore/lagoon system of Shingle Street, Suffolk: a preliminary survey. Trans. Suffolk Nat. Soc., 18: 168-181.

Barnes, R. S. K. (1994). The brackish water fauna of North-western Europe.

Cambridge: Cambridge University Press.

Bratton, J. H. (ed.) (1991). British Red Data Books: 3. Invertebrates other than insects- Peterborough: Joint Nature Conservation Committee.

English Nature (1997). Starlets revealed in Suffolk! Suffolk Nature. 4: Winter 1997.

Hayward, P. J. & Ryland, J. S. (eds.) (1990). The marine fauna of the British Isles and North-West Europe. Vols 1 and 2. Oxford: Oxford Science Publications.

Mason, C. F. (1986). Invertebrate populations and biomass over four years in a coastal, saline lagoon. Hydrobiologica. 133: 21-29.

Robertson, P. A. (1993). The management of artificial coastal lagoons in relation to invertebrates andAvocets. Ph.D thesis. University of East Anglia.

Sheader, M „ Suwailem, A. M. & Rowe, G. A. (1997). The anemone, Nematostella vectensis, in Britain: considerations for conservation management Aquatic Conservation: Marine and Freshwater Ecosystems,.

7: 13-25.

Smith, B. P. & Laffolcy, D. (1992). A directory of saline lagoon-like habitats in England. Peterborough: English Nature.

Stephenson, T. A. (1935). The British Sea Anemones. Vol. 2, London: Ray Society.

Welstead, W. (1996). Impact of management of salinity and raking of substrate on benthic macro-invertebrates in saline lagoons on Havergate Island, Suffolk. Unpublished Diploma report. Centre for Extramural Studies, Birkbeck College, University of London.

W. Welstead M. Shardlow Laurel Farm Royal Society for the Protection of Birds Capel St Andrew Stalham House Woodbridge 65 Thorpe Road Suffolk IP12 3NQ. Norwich NR1 1UD Trans. Suffolk Nat. Soc. 35 (1999)

Plate 5: Starlet Sea Anemone, Nematostella vectensis Stephenson, with egg masses. This Biodiversity Action Plan priority species has been found in several saline lagoons (p. 134) and on Havergate Island (p. 57) in 1998.