Published records of the solitary hydrozoan Protohydra leuckarti Greeff Reference Greeff1870 in the Plymouth area refer only to the River Tavy at Bere Ferrers and the estuary of the River Plym at Chelson Meadow (Baker, Reference Baker1912; Hickson, Reference Hickson1920; Lebour, Reference Lebour1930; Marine Biological Association of the United Kingdom, 1957). Its presence at Millbrook Lake, a shallow creek near the mouth of the River Tamar, is reported in this preliminary note. It was found by C.C.K. during the period March 1970 to February 1971 while sampling estuarine oligochaetes, and was still present at this site in June 2007 (National Grid Reference SX 4401 5245).

The surface sediment consists mainly of silt with small amounts of sand and is relatively firm when exposed. At high water marine conditions obtain, with the salinity of standing water in depressions at low tide, up to 35.45 ppm (determined by silver nitrate titration). After heavy rain, lower salinities were recorded (10.40–24.20 ppm).

In 1970–1971 the meiofauna was sampled at regular monthly intervals from a 5 m² fixed quadrat situated at about mid-tide level, by means of a simple coring device. Sediment samples were taken to a depth of 10 cm using a clear plastic tube (internal diameter 3.1 cm) with a piston for expelling the core. A 10 cm length of core was cut into 2 cm sections as it was extruded. Ten cores were taken at random within the quadrat, furnishing 50 samples which were preserved within 4 h of collection by adding 4% formaldehyde solution containing 0.2% rose Bengal. The meiofauna was extracted later by stirring gently with tap water, giving a suspension that was washed into a small sieve (250 µm). The residue was sorted in a flat glass dish under a stereobinocular microscope, and specimens stored in 70% ethanol (see Hulings & Gray (1971) in Fenchel, Reference Fenchel1978).

The meiofauna was dominated numerically by small worms: polychaetes (Manayunkia aestuarina Bourne and Streblospio shrubsoli Buchanan), and oligochaetes in smaller numbers (mainly Tubificoides (=Peloscolex) benedeni Udekem). Nematodes and copepods, upon which P. leuckarti often feeds (Heip & Smol, Reference Heip, Smol, Persoone and Jaspers1976) were less numerous.

Preserved P. leuckarti were approximately 1 mm long and deeply stained with rose Bengal. They were all in the top sections of cores (at 0–2 cm depth): see Table 1. In the course of 12 months 94 polyps were found in 1.8 l of sediment.

Table 1.

Protohydra leuckarti in intertidal mud samples from Millbrook Lake (all found at depth 0–2 cm).

The presence of P. leuckarti at this site was confirmed in June 2007 by a field excursion. Surface mud was collected at the same sampling station or next to it (10 separate samples, total volume 1.2 l). Suspensions of mud in seawater were filtered on a 250-µm sieve which retained sand grains together with strands of filamentous green alga (Enteromorpha sp.). This material was allowed to stand in Petri dishes and scrutinized at intervals under a stereobinocular microscope. Over a 24 h period five living polyps were found. They were identified by observing nematocysts under the microscope with the help of Dr Carina Östman. All were from a 300 ml sample collected within 25 m of the original site where the mud was firmer, possibly due to a slightly higher sand content. No other polyps were found, suggesting that P. leuckarti may be unevenly distributed over small areas (see also Table 1).

The author (I.A.K.) who found the live specimens is familiar with P. leuckarti and its habitat in the vicinity of the White Sea Biological Station of Moscow State University, at 66^o34′N 33^o06′E (see Bozhenova et al., Reference Bozhenova, Stepanjants and Sheremetevsky1989; Stepanjants, Reference Stepanjants, Koltun, Marfenin and Stepanjants1989). It is found there in muddy sand in the middle part of the intertidal zone (estimated proportion of sand at least 80%; grains of medium size). The location is marked by the presence of Arenicola marina, Macoma baltica and sometimes Mya arenaria. The characteristic meiofauna consists of harpacticoid copepods, nematodes, small gastropods (Hydrobia sp.), oligochaetes and polychaetes. The salinity is on average 18–24 ppm and may fall to ~5 ppm. Seawater temperatures range from 12–14^oC (summer) to below zero (winter). The White Sea polyps are usually observed adhering to organic particles by the basal peduncle but they can move interstitially by peristalsis. They readily catch and engulf prey (e.g. nematodes). In summer, the surface sediment of an area 20 × 20 cm may contain 10–20 polyps, among which it is not uncommon to find individuals undergoing transverse fission.

Protohydra leuckarti was first recorded in the White Sea at Seldjanaya Bay, an intertidal brackish habitat near a small stream, consisting of muddy sand (Stepanjants, Reference Sergeeva1989; Anokhin, Reference Anokhin2001; and personal communications).

The ecology and distribution of P. leuckarti are considered by McIntyre (Reference McIntyre1969) and Fenchel (Reference Fenchel1978) in general reviews of meiobenthos. Protohydra leuckarti appears almost cosmopolitain in its distribution (e.g. Omer-Cooper, Reference Omer-Cooper1964; Clausen, Reference Clausen1971; Anokhin, Reference Anokhin2001; Schuchert, Reference Schuchert2006; Sergeeva, 2006). It is recorded from a wide range of brackish marine sediments. It is euryhaline and tolerates pollution and low oxygenation. Its habitat preferences differ from those of cnidarian psammofauna (Teissier (1950) in Delamare-Deboutteville, Reference Delamare-Deboutteville1960; Swedmark, Reference Swedmark1964; Stepanjants, Reference Sergeeva1989). It occupies the top layer of sand or muddy sand, usually with detritus or else a surface diatom layer. In common with other vermiform interstitial species its robust burrowing behaviour and adhesive ability ensure survival.

The population ecology of P. leuckarti has been studied by Muus (Reference Muus1966, Reference Muus1967), Heip (Reference Heip1971) and Heip & Smol (Reference Heip, Smol, Persoone and Jaspers1976). It feeds voraciously on copepods, nematodes or small polychaetes, capturing passing prey by means of nematocysts. Protohydra leuckarti is a top predator and influences the structure of prey populations (Heip, Reference Heip1977). In favourable conditions it multiplies asexually (Muus (Reference Muus1967) records up to 20/cm²). In contrast, the life cycle of Protohydra remains largely unknown. Several authors have described gonads (ovaries with a single large oocyte, or testes) and although sexual individuals are not uncommon, fertilization and the course of embryonic development have not yet been observed (Nyholm, Reference Nyholm1951; Muus, Reference Muus1966).

Protohydra has an evolutionary history that is shrouded in mystery. Its simple form makes it very challenging to infer its relationship to other hydrozoans. Nevertheless, Protohydra has traditionally been classified (in its own family Protohydridae) close to Hydridae (e.g. Naumov, Reference Naumov and Salkind1960; Bouillon, Reference Bouillon1985; Schaefer, Reference Schaefer, Westheide and Rieger1996). Hydridae contains species of Hydra, which have made such excellent model organisms that more is known about their biology (from development to genomes) than that of any other cnidarians. If Protohydridae were shown to be the sister group of Hydridae, it would make an important point of comparison when assessing the generality of features studied in Hydra. However, Petersen (Reference Petersen1990) noted that Protohydra could conceivably share an ancestry with any number of different groups of capitate hydrozoans, given its relative lack of characters. More recently, Stepanjants et al. (Reference Stepanjants, Anokhin and Kuznetsova2000) suggested that Protohydra and Hydra may have independent origins from within Corymorphidae. Molecular data have since confirmed a close relationship between representatives of Corymorphidae and Hydridae, in the clade dubbed Aplanulata (Collins et al., Reference Collins, Schuchert, Marques, Jankowski, Medina and Schierwater2005, Reference Collins, Winkelman, Hadrys and Schierwater2006). Genetic data extracted from our new samples of Protohydra should allow the shroud to be lifted so that we might see a bit into the past of this fascinating animal.