Among the open bays, Reigi laht on the northwestern coast of Hiiumaa Island is a part of the open Baltic Sea, with a salinity of about 6-7%0.
Of the lagoons Kirikulaht on Hiiumaa Island receives fresh water from a stream but is also connected with Reigi laht by a strait.
In the mainland, the shallow lakes of Kaomardi laht and Kussa laht are completely overgrown with reeds and dense Chara, while the lakes of Sutlepa meri and Pikane jarv have also extensive but shallow open-water areas.
Four marine and brackish-water oligochaetes (Heterochaeta costata, Paranais litoralis, Amphichaeta sannio, and Lumbricillus sp.) were observed only in Reigi laht, the most typical marine site.
In the cores taken from the bay of Reigi laht, a very large polychaete worm, Hediste diversicolor, was abundant, accompanied with an unidentified gastropod.
Cyprideis torosa was the only ostracod at the most marine site, Reigi laht. The only true freshwater species in this list, Cypria exsculpta, was found once in the lake of Allikajarv.
The small (non-mermithid) Nematoda, all in all 22 taxa, were observed at 13 sites of 20, most abundantly in the bay of Reigi laht and in the lagoon of Kirikulaht connected with each other.
The share of eumeiobenthos, mostly nematodes, was very high in the open bay of Reigi laht. Without the open bays, the percentage of eumeiobenthos was lower: 34% of all animals (or 41% of the meiobenthic animals) in the lagoons, and 20% (or 27%) in the freshwater lakes (Table 1).
However, 93% of it was formed of macrozoobenthos, particularly larger Chironomidae (but Polychaeta in Reigi laht).
A rich benthic fauna of animals typical of the Baltic Sea was found in the cores taken from the open bay of Reigi laht: the polychaete Hediste diversicolor, the oligochaetes Heterochaeta costata, Paranais litoralis, Amphichaeta sannio, and Lumbricillus sp., the priapulid Halicryptus spinulosus, a poorly preserved and hence unidentified prosobranchian gastropod, the meiobenthic ostracod Cyprideis torosa, and a number of marine nematodes.