Fairy Shrimp

This report has been prepared for and funded by The Verderers of the New Forest Higher Level Stewardship Agreement AG00300016. The HLS partners shall have the sole right to publish the report and results of the survey, with an appropriate acknowledgement of the work or material contributed by the Contractor.

This report should be cited as: Aquilina,2014. New Forest Fairy Shrimp (Chirocephalus diaphanus) Survey

Data contained within this report should be treated as confidential and access restricted. Please observe your data protection policies.

Fairy shrimp Chirocephalus diaphanus

Robert Aquilina MSc MIEEM is an independent consulting ecologist specialising in freshwater ecology. He is a Full member of the Institute of Ecology and Environmental Management and abides by their Code of Professional Conduct. For further details see

www.ieem.net

www.aquilina-environmental.co.uk

Summary

Surveys for Fairy Shrimp (Chirocephalus diaphanus) were carried out at known historical locations across the New Forest HLS agreement area using two methodologies. The purpose was to assess the locations for the presence of Fairy shrimp and offer management recommendations to ensure their continued survival.

The two methods used were collection of dry soil samples with ex situ hatching in aquaria which identifies the presence of eggs and a follow up visit in November/December 2013 to search for adults using visual observation and netting.

Fairy shrimp were hatched from 9 of the 28 locations sampled. They were observed from 6 of the 34 locations visited in Winter 2013/14 when some additional nearby sites were revealed to hold water.

Tadpole shrimp, Triops cancriformis, were successfully hatched from three locations, although none were found during the field visits.

Introduction

Fairy shrimp (Chirocephalus diaphanus) are rare freshwater crustaceans that have highly specialised environmental requirements.

The purpose of this project is to identify and survey the known locations within the New Forest HLS agreement area for the presence of Fairy shrimp and offer management recommendations for each site. The project consists of a number of phases which are outlined below.

1. Data gathering
2. Site surveys – dry
3. Ex situ hatching
4. Site surveys – wet
5. Reporting

Data was requested from HBIC who provided a spreadsheet and map of locations with records for Fairy shrimp from 1985 onwards. These sites were then visited by Robert Aquilina at the end of July and beginning of August when the ponds were dry. The weather conditions were sunny and warm and therefore suitable for the surveys. Soil samples were collected and processed according to the agreed method statement. The samples were dried and then wetted in aquaria and observed for a minimum of 6 weeks. Counts of hatched Fairy shrimp recorded.

A further visit to each pond location took place in November or December 2013 when the ponds where surveyed by visual observation and netting.

Methodologies are discussed below with results and management recommendations made.

Natural history

Fairy shrimp are widespread but highly localised across England, with a major population centre in the New Forest. They inhabit temporary ponds which dry over the summer. This requirement means that their distribution is limited to an area where evaporation exceeds precipitation between April and September. They appear insensitive to geology as they occur on acid sands and gravels, neutral loams and calcareous soils where they occur across the country.

They do appear to require a summer dry period which both removes predators from their ponds as well as satisfying a requirement for the eggs to dry before they can hatch (diapause). This hydroperiod allows Fairy shrimp to hatch from eggs, grow to adulthood and reproduce over a period of as short as six weeks. They lay eggs which then survive the dry period in the soil at the bottom of the pond. When wetted again by Autumn or Winter rains they hatch and the cycle begins again. The eggs possess differential wetting requirements to overcome the variability in water levels due to variations in weather, some will hatch on first wetting, others will require a period of wetting followed by drying and then further rewetting before hatching and yet others will require the cycle to be repeated a third time or possibly more. This ensures that they survive the vagaries of hydroperiod in years with little rain.

Fairy shrimp are particularly vulnerable to predation by a wide range of animals from other crustaceans such as Freshwater shrimp, Gammarus, and even Ostracods in their early stages, most predatory insects such as Diving beetle larvae, Dytiscidae, dragonfly larvae, fish, amphibians and waterfowl. Few of these occur in seasonal ponds because they have a development life cycle that requires water for longer than is usually present. Thus Fairy shrimp avoid predation by using habitats that are generally unsuitable for predators and completing their life cycle in a shorter period than most of their predators. They are suspension feeders and can complete their life cycle in as little as six weeks.

They also appear to require regular disturbance by livestock, ploughing or vehicles which create bare soil on which eggs are laid preferentially. Having said that, they can occur in quite well vegetated ponds although there is usually a period when this vegetation dies back to expose the substrate. This disturbance and preference for bare soil may also be associated with dispersal of eggs during the dry phase, where they are believed to be transported attached to soil by wind, grazing animals, vehicles and waterfowl (Friday (1996)).

Because of widespread habitat loss and these specific environmental requirements Fairy shrimp are listed as Red Data Book Vulnerable species and protected under Schedule 5, Section 9 of the Wildlife and Countryside Act 1981 (as amended), which makes it illegal to kill, take or sell specimens except under licence. All the work carried out here described below was under Natural England licence 20131076 Fairy shrimp (Chirocephalus diaphanus) and 20131348 Tadpole shrimp (Triops cancriformis).

Historical records

A data search for Fairy shrimp records within the bounds of the New Forest was performed by HBIC (Hampshire Biodiversity Information Centre). Naomi Ewald of Pond Conservation kindly provided all her records as well. The search yielded 31 ponds with confirmed records since 1985. However some of these are outside the HLS agreement area of the New Forest and were therefore ignored. This left 22 locations within or very close to the Keepers bounds.

Denise Eccles of the Forestry Commission undertook to produce Constraints maps of the supplied locations and with her help the potential locations were identified from aerial photographs. Aerial photographs of each area were inspected for features that stood out as either darker patches against the background or as different apparent vegetation types against the surroundings.

This resulted in a number of additional locations to be identified when the supplied grid reference did not match any feature on the aerial photograph that appeared to be a pond. Both summer and winter photographs were examined in order to attempt to correlate the six figure grid reference with a feature on the ground. In some cases this proved impossible and a number of nearby features were identified as potential candidates for the reference. As a result a total of 28 locations were identified to survey during the dry phase.

Locations

The 22 locations (based on 6 figure grid references) are mapped below. The 28 locations (identified as potential sites from aerial photographs) are listed in the table that follows.

The grid references are taken from a hand held Garmin GPS 60 for the purposes of relocating the features in the field on any subsequent visits and are only accurate to within 5 to 10 metres.

A major problem with working from six-figure grid references in attempting to locate transient features such as these seasonal ponds is that each location represents an area of 100 x 100 metres on the ground which must be searched for potential ponds. As became obvious during the winter (wet phase) surveys, some ponds had been missed as they were only apparent on the ground when filled with water, the aerial photographs having not indicated their presence. Consequently a number of new locations were added during the winter visits which are marked in the results table by an asterisk.

Map 1. Location map of ponds with post 1985 records with Keepers beats highlighted.

Table 1. Historical locations of Fairy shrimp ponds within HLS area.
NB LOCATION DATA IS SENSITIVE AND PROTECTED BY DATA PROTECTION POLICIES

Methods

Ex situ egg hatch

The sites were visited between 23rd July and 6th August 2013 and soil samples from each of the 28 features collected. Each subsample was of approximately 10 cm3 collected using a cylinder corer and trowel from the dry base of the pond and repeated up to 10 times and the subsamples combined. Each sample was spread out over the base of a white polypropylene tray (30x42x8cms) and allowed to dry thoroughly for 2 weeks (this drying is thought to be a requirement for breaking the diapause and allowing the eggs to hatch).

Each of the samples were then mixed thoroughly and a subsample of approximately one third placed in a plastic aquarium (20x20x34cms) and 10 litres of rain water added. The aquaria were inoculated daily with a dilute fish fry food to encourage the growth of any microorganisms present which would serve as food for any hatching Fairy shrimp. Once signs of life are spotted in the tanks (usually Cladocera or Ostracoda are the first visible signs), food supply is increased. Aquaria were examined twice daily to search for the presence of Fairy shrimp, which were counted when seen.

The aquaria were maintained under natural light and temperature regime for a period of six weeks. The aquaria were inspected daily. At the end of the period, the aquaria were drained down (through a plankton net to capture any larger microorganisms) and the substrate allowed to dry in situ for 2 weeks. They were then rewetted and observed as before. The microorganisms from each tank were inspected under the microscope for hatchling Fairy shrimp and other crustacea.

Field survey

The sites were visited between 6th and 14th November 2013 in order to sample ponds for any Fairy shrimp that had hatched during the onset of Autumn rains. These ponds were visually inspected for Fairy Shrimp and then netted using both a plankton net and a pond net.

Mature adult Fairy shrimp are relatively visible in clear water, however the netting was carried out in case juveniles were present which are less easily observed.

An appropriate date for such a visit is difficult to predict as it depends on local weather (the ponds must have been filled for at least two weeks). A follow-up visit on 12th December 2013 found three out of four ponds near Burley dry just a month after they were apparently full. Ideally a series of visits would be carried out over Winter and Spring which would catch ponds at the appropriate moment within the wet phase. Therefore the schedule for the project which was due to finish at the end of December 2013 was extended to the end of February which allowed further visits in January and February 2014.

Results

There are no published data or methods for estimating population levels of freshwater Crustacea from egg hatching data or field surveys as far as a literature search showed. Therefore the estimates were derived as follows :-

Table 2. Population estimates based on abundance

In order to account for sample size bias the area of the pond was used as a weighting factor, as in the following table :-

Table 3 Weighting factor for pond size

This is required to take into account the proportion of the pond that has been sampled. Although subsamples were spread out over the whole of the pond area and suitable bare patches selected preferentially, a larger pond should potentially hold a greater population than a smaller pond given the same number of eggs hatch from each. Therefore, the weighting factor is applied to the Fairy shrimp count accordingly.

Ex situ egg hatch

Table4. Results of ex situ egg hatch with population estimates for successful ponds.

Positive hatching results were found across the whole range of conductivities with was associated with more neutral pH (6-7) with one exception at 5.5. This suggests that Fairy shrimp will not be expected to be present in peaty bogs and mires but would be likely to turn up in neutral or slightly acid sandy or gravelly ponds or those on a mineral substrate, even if vegetated.

Field survey

The field survey consisted of a slow circumnavigation of the pond searching for adult Fairy shrimp swimming in open water usually near to vegetation. Once an adult had been spotted a timed count was then begun (5 minutes) and this was used as the basis of the population estimate using the same criteria as in Table 2. No account of pond size was made.

If no adults were seen, then the pond was netted, first with a plankton net in open water and then with a pond net in vegetation. The plankton net samples were examined in the laboratory under a microscope for juvenile Fairy shrimp, but none were found. The pond net samples were emptied into a white tray in situ and examined by eye, again no Fairy shrimp were found.

Suitable conditions for the field visits were lack of rain and wind as these disturb the water surface making visual inspection difficult. Therefore all the field visits took place on dry days with little wind.

Table 5. Results of field survey with population estimates (ponds not sampled for ex situ egg hatch are marked in red, ponds not identified from aerial photographs are marked with an asterisk).

Conclusions

Fairy shrimp

Nine of the twenty-eight locations successfully hatched Fairy shrimp, one of which had not been recorded since 1987 (pond 8 at South Weirs). Some of the recently recorded locations have failed to produce.

Six of the thirty-four ponds visited held adult Fairy shrimp, of which one was a location not sampled for the ex situ egg hatch in the summer (pond 12a Little Castle Common). The timing of the visits during November was a little early as some ponds held little or no water. Repeat visits in December were less successful as the intervening period was almost completely dry and most of the ponds had reduced water levels or had dried up completely. Therefore further visits after a prolonged period of rain and allowing some weeks for development were carried out (late January through to February).

In total, Fairy shrimp were found in eleven ponds, with 4 ponds successfully hatching from soil samples as well as from field visits. These are tabled below.

Table 6. Successful sites for Fairy shrimp 2013 with population estimates.

Tadpole shrimp

Tadpole shrimp, Triops cancriformis, are freshwater crustaceans that have a similar natural history to Fairy shrimp. They are however much rarer with only two sites known from the whole country, New Forest and Solway Firth, Scotland. Within these sites, the New Forest has two known ponds and Solway Firth some 6, recently discovered and within a hundred metres of the original, now lost pond.

Although their life cycles are similar, Tadpole shrimp appear to be more demanding in terms of water chemistry than Fairy shrimp and are thus far more restricted in distribution.

The two historical sites within the New Forest are also known for Fairy shrimp and were therefore sampled as part of this project. They successfully hatched during the ex situ processing as well as a third, previously unrecorded, site.

Tadpole shrimp Triops cancriformis

Discussion

It is important to place these results in a context as both Fairy shrimp and Tadpole shrimp are unlike most target species for a survey. They exhibit large natural seasonal and annual variation in population numbers across different locations depending on a number of environmental factors but primarily upon the hydroperiod of the particular pond under investigation.

In any one year, only a subset of the viable populations are likely to hatch and breed successfully, both within an individual pond and across the site as a whole. This means that survey results from a single season are almost certain to be underestimates of the true population. This can be seen within these results here as a greater number of sites successfully yielded Fairy shrimp from the ex situ egg hatch than from the field visits.

The same problem extends to population estimates in that they will vary naturally over orders of magnitude within each pond over a number of seasons depending on how many eggs have successfully broken diapause in that particular pond in that particular season.

A review of recent historical records is useful. The table below gives records of positive Fairy shrimp sightings from field visits only.

Table 7. Comparison of historical records.

This suggests a fairly constant number of ponds are observed with adult fairy shrimp over the whole Forest each year, although the actual ponds differ between years. Of course, this data is biased by the limited number of interested observers who probably return to the same locations so it is neither systematic nor random. However it illustrates the transient nature of the populations within each pond. An example is of the South Weirs pond which was last recorded to have Fairy shrimp in 1987 before this year’s sightings. Undoubtedly they were present but unrecorded in the intervening years.

A similar caveat should be levelled at the population estimates in that there is inherent variability in the size of the population each year. Thus a High population estimate does not equate to any level of certainty that the population will manage to persist, a better guide being consistency of sightings between years. Furthermore, each population estimate must be regarded as highly provisional as there is no accepted methodology for such an estimate and there is clearly some discrepancy between egg hatch derived estimates and field derived observations.

Recommendations

The current management regime is the most likely to sustain the Fairy Shrimp if present because regular poaching by grazing animals does expose new bare soil and grazing keeps the vegetation suppressed.

It is important not to interfere with the hydroperiod of seasonal ponds by attempting to deepen them as this may lead to them becoming permanent and therefore unsuitable for Fairy Shrimp. Similarly no attempts to drain them should be allowed. A number of ponds have formed alongside roads and take drainage form the road surface. Although this is not ideal as this water can contain hydrocarbons and heavy metals, it is not recommended that these are altered in any way as this may lead to unsuitable changes to the hydroperiod. (ponds 4a, 4b, 7, 12a, 22b, 22c).

Scrub encroachment is a potential problem for seasonal ponds as it encourages water loss through additional transpiration. Although shade is not necessarily seen as a problem for Fairy shrimp, it does cause the pond to have a lower water temperature and therefore may increase development time for the Fairy shrimp. Additional leaf litter entering the pond can cover bare soil and make the habitat less suitable for egg laying. Scrub should be cut back from pond edges on affected ponds (ponds 8, 12) but this should be phased so as not to cause excessive change in one go. It is recommended that no more than 50 % scrub is removed in any one year.

The most significant negative factor affecting seasonal ponds is the presence of Crassula helmsii, a non-native invasive species which, once introduced, tends to form a mat of vegetation which masks and excludes native plants. It is problematic to control and experiments are underway within the New Forest by Hampshire Wildlife Trust with dye to exclude it from a pond at Abbey Well. A number of ponds are badly affected (ponds 2, 3a, 17, 18), although control measures are in place for some.

The following table summarises management actions recommended.

References

Adams,C.E., Griffin, L.R., Benzies, E., Aherne, D., Bean, C.W., Dodd, J., Fairlamb , D. and Maitland, P.S. (2014) Short range dispersal by a rare, obligate freshwater crustacean Triops cancriformis (Bosc). Aquatic Conservation: Marine and Freshwater Ecosystems. 24(1), 48–55.

Baird, W. (1850) The natural history of the British Entomostraca. Ray Society, London.

Bratton, J.E. and Fryer, G. (1990) The distribution and ecology of Chirocephalus diaphanus Prevost (Branchiopoda: Anostraca) in Britain. J. Nat. Hist. 24, 955-964.

Feber R., Hilton G.M., Hutchins E., Griffin L., Ewald N., Pain D., Biggs J., Hughes I., Macdonald D.W. (2011). Ecology and conservation of the tadpole shrimp Triops cancriformis. British Wildlife 22: 334–341.

Friday,L.E.. McCall,R.A., Elson, H.L.G. & Walters, M.G. (1996) Temporal and spatial patterns of temporary pond organisms. Nature in Cambridgeshire, 38, 3-15.

Acknowledgements

Acknowledgement and grateful thanks are due to HBIC for supplying the historical data for Fairy Shrimp in the New Forest.

Naomi Ewald of the Freshwater Habitats Trust for all of her data and helpful discussion.

Ruth Feber of WildCRU Oxford and WWT for her recent Fairy shrimp records.

Denise Eccles of the Forestry Commission for help with aerial photographs and site locations.