The first specimens of Najas marina were collected at the beginning of the 19^th century and stored in the herbarium of Vilnius University (WI). Only a few specimens are labelled with an indication of the place and time of collection, which is sometimes questionable.

One herbarium sheet contained two specimens of plants at an early stage of development with three handwritten labels (Fig. 1A). One label with the inscription 'Najas species nova' indicated the locality as "Kryžiokų Lake near Gulbinai" and the date of collection as "11 July 1823". The other label gives the plant name "Naias intermedia mihi" and a detailed morphological description. A third small label noted that the plants with fruit were found in the same lake on 15 August 1824. As the handwriting on the labels differs, they may have been written by more than one person, possibly at different times. One more sheet contained one plant specimen with a label bearing a morphological description and a note about the date of collection of the fruiting plants.

One of the specimens, judging by the handwritten label in Polish, was collected by Gorski in Nava Lake near Aukštadvaris on 03 August 1823 and named as Najas monosperma W. Two other labelled specimens, collected by Gorski at the same location and time, were named Caulinia fragilis Willd. Previously, they were found in the N. minor folder and re-identified as N. marina (Sinkevičienė 2001). One herbarium sheet shows tufted plants without developed fruits, while the other is presented by fruiting plants. Specimens that are morphologically very similar, however, without labels, are placed in approximately 30 unlabelled herbarium sheets. They were most likely collected for exchange, as similar specimens with a morphological description of "Naias intermedia mihi" and without any indication of locality were found stored in the National Herbarium of Ukraine (KW). A morphologically very similar herbarium specimen to that collected in Nava Lake was selected as the lectotype of N. intermedia Wolfg. ex Gorski (Bräuchler 2015) (Fig. 1B). The specimen labelled as N. intermedia Wolfg., with morphological description and indication of three localities: "Kryžiokų Lake near Gulbinai", Nava Lake near Wysoki-Dwor (Aukštadvaris) and "pond near Strėva". The label was possibly handwritten by Wolfgang in 1826. This date at the bottom of the label probably indicates when the label was written, but not when the plant was collected. We did not find any herbarium specimens collected in the above-mentioned vicinity of Strėva. The later publication by Gorski (1830), in which the species N. intermedia was validly published, did not refer to these three exact locations, but indicated "one lake near Vilnius and many other lakes in the Trakai district".

The publication by Wolfgang (1824) reported on a new species of Najas (with toothed leaf sheaths), which was collected by A. Jankiewicz in Lake Kryžiokai near Gulbinai. However, the discovery of a new Najas species in Nava Lake in 1823 was not included in the account of the rare and new species found that year, reported in the same publication.

Zelencow (1892) incorrectly reported the locality "pond near Strėva" as the locality of Najas major var. intermedia "in Strėva River". This erroneous information about locality was repeated by Snarskis (1954), Šarkinienė (1963a), Sinkevičienė (1992) and Sinkevičienė (2021). We did not find any herbarium specimens that could confirm the locations in Samanis Lake (Lazdijai District) reported by Šarkinienė (1961), Balevičius (1998) and lakes Tautesnis and Girininkija (Zarasai District) reported by Galinis (1958), Šarkinienė (1961) and Galinis (1962). Therefore, we cannot confirm which species, N. marina or N. major, was found in these lakes.

During the revision of the specimens in both WI and BILAS Herbaria, 68 were identified as Najas marina. The occurrence of species was confirmed in 33 waterbodies. It was mostly found in lakes, but it has also been recorded in two reservoirs and one waterway connecting lakes. The distribution of this species is restricted to the lakes of the Baltic Uplands, which extend from the south to the northeast of the country (Fig. 2).

Herbarium data confirmed that Najas marina can persist in the same waterbody for centuries. This species has been present in lakes Kryžiokai (now known as Balsys Lake) and Nava (the recent Aukštadvaris Reservoir) since the early 19^th century. The first location was situated close to the City of Vilnius and, therefore, easily accessible. The other location was also relatively close to the capital, near former manor houses where the researchers could stay during their fieldwork.

Before 1990, Najas marina was found in nine lakes. Judging by the herbarium collections, only the populations in Lakes Baltelė and Bivainėlis (Ignalina District) were abundant. In general, the species was considered rare and Red-listed (Sinkevičienė 1992). As N. marina s.l. was protected, it is unclear to which species sensu stricto the literature records referred.

In the period after 1990, Najas marina locations have increased significantly. The species was only found repeatedly in the Lakes of Pravalas (Vilnius District) and Dringis (Ignalina District). The earlier site at Dringis Lake was known from literature (Šarkinienė 1961, Šarkinienė 1963b). In the advisory years, the species was found for the first time in many lakes (Dūkštas, Žaltytis, Žuvintas, Viešintas), which have been surveyed several times in the past or are included in the state monitoring programme. In lakes, such as Kauknorėlis, Rašia, Žaltytis and Žuvintas, populations were found particularly abundant, especially in the muddy, shallowest areas of these waterbodies. N. marina was found in different lakes from N. major. Only in one bay of Ilgis Lake (Varėna District) were both species found growing together.

The oldest specimens of Najas major collected in Lithuania at the beginning of the 19^th century were also found in the Herbarium of Vilnius University (WI). We found one specimen bearing a detailed label with characteristic Gorski's handwriting (Fig. 3). The species name N. major All. (Najas monosperma Willd.) and the morphological description were written on one side of the label. The indication of locality 'in lake circa Skorbuciany districtum Trocensi...' [in lake near Skorbutėnai in Trakai District...] and the collecting year 1822 were written on the opposite side.

Four more samples, which were morphologically similar and possibly collected in the same place, were unlabelled. One specimen was named as Najas major All. (N. monosperma Willd.), with a label that was probably handwritten by Wolfgang, containing the same morphological description and location, but was dated 1826. This date likely indicates when the label was written, rather than when the plants were collected. This finding was also reported in literature by Wolfgang (1822), that student A. Meltzer had found N. monosperma W. and N. tetrasperma W. near Skorbutėnai. We suspected that the unnamed lake near Skarbutėnai might be Ilgutis Lake near Lygainiai. However, N. major was not discovered until 2002, when N. marina was found there.

The second locality from the 19th century reported by Jundzilł (1830) was "lakes near Papiškės" (recent Šalčininkai District). Herbarium specimens collected 24.08.1824 are stored in the Herbarium of the (Polish) Academy of Sciences in Krakow (KRAM) (Köhler 1995).

Very few specimens of Najas major were collected during the period from 1950 to 1989.

In general, Najas major was documented in 22 waterbodies, according to the revised data (Fig. 4). Lakes are its main habitat, but it has also been recorded in a river, a reservoir and a sand quarry pond. Most sites are concentrated in the south and east of the country, with only one known site in the west. These sites are mainly located in the Lakelands of the Baltic Uplands, an area that extends from the southern part of the country to the north-east.

The oldest labelled specimen of Najas minor, collected by S. B. Gorski in 1822, is preserved in the WI herbarium. Different 19^th century and later literature sources interpret the locations indicated on the specimen label differently. Information in the label is "In lacus Swinta circo Towiany, 8 miliaria a Vilna. Haciski et Meltzer primi inventores 1822" [In Lake Swinta, near Towiany, 8 miles from Vilna. Haciski and Meltzer, first inventors, 1822]. Wolfgang (1824) described the location in more detail. — …w iezierze pode dvorem Sędziego Towianskiego Szwinta zvanem" […in the lake called Švinta, near the manor of Judge Towianski]. Gorski (1830) re-interpreted information about locality as "sehr selten im See Godskischki, 8 Meilen von Wilna" [very rare in Lake Godskischki, 8 miles from Vilna]. At the end of the 19^th century, Zelencow (1892) incorrectly reported that Godskischki was located in Trakai County when it was in Vilnius County. This erroneous information was repeated by many researchers who failed to check the information on the herbarium labels. Later, the most important sources on the flora of Lithuania interpreted the name of the lake as Šventininkai Lake, located in the Trakai District (Snarskis 1954, Šarkinienė 1963a). This location was subsequently recorded in the Red Data Books (Sinkevičienė 1992, Sinkevičienė 2007). Although Sinkevičienė (2001) noted that the name of the Lake Szwinta (Šventas) cannot be interpreted as Šventininkai Lake, she incorrectly suggested that it could be Šventas Lake near Taučionys (Trakai District). The possible name of the Village Towiany, indicated on the specimen label, similar to "Towcany", made it difficult to identify the exact location, as it did not refer to a geographical name, but to the residence of the Towiany family (Anonymous 2025). The residence was in the settlement Antašventė. This fact helped establish the exact historical location of N. minor, which is Šventas Lake, situated in the Molėtai District (Fig. 5). The site mentioned by Gorski (1830) as "Godskischki" (recently Gačkiškiai) is located nearby.

During this research, the locality in Lake Šventas was not checked. The species was not found in Lake Skritelis, where it was recorded in 1961. Currently, Najas minor is known from five lakes: Alksnas, Dringis, Dūkštas, Sągardas (also known as Sangardas), Ažvintis and Zarasas. The location in Sągardas Lake has been known since 1998. The species continues to occur intermittently there to this day. In other lakes, N. minor has been recorded in recent decades. The species was first discovered in Lake Alksnas in 2006 and recent observations suggest that it is now spreading at the site. The species was first discovered in Lake Dūkštas in 2014 and is still present today. The species was recorded in Zarasas Lake twice, in 2015 and again in 2021. Although Dringis Lake was studied by several researchers in the 20^th century (Bagdonaitė 1962, Stepanavičienė 1991), N. minor was never recorded. The species was discovered in 2015 and remains abundant in the south-eastern part of the lake. The species has been listed on the Red List since 1992 and all known localities are supported by herbarium specimens (Sinkevičienė 1992, Sinkevičienė 2007, Rašomavičius 2021).

This study identified specimens of Najas minor in seven lakes. Although the oldest site in Šventas Lake is somewhat distant from the other localities, they are all concentrated within a relatively narrow area in the north-eastern part of the country, mainly in the Zarasai and Ignalina Districts.

We did not find 19^th-century specimens of Najas flexilis in any of the studied herbaria. However, botanical literature from the 20^th century (Dagys et al. 1934, Snarskis 1954) suggested that N. flexilis could be found growing in Lithuania. Triest (1988) recorded the locality of N. flexilis in Lithuania as "Lithuania, Lacus Switez, 1898, Dybowski s.n. (K, LY-Gand.)" [Switez Lake], which is actually located in Belarus.

The oldest labelled specimen was from Germantas Lake, collected in 1966. Several Najas flexilis plants were found within a sheet of macroalgae Chara aspera collected by I. Trainauskaitė (Sinkevičienė 2001). However, the species was never re-discovered in this lake.

Later, in 1998, the species was found at two different sites in Sągardas Lake. In one site, it was growing together with Najas minor. For a long time, findings of both species in Lake Sągardas were scarce. An abundant mixed population of both species was recorded in 2023.

Three new localities for the species were discovered during the 2020–21 period (Fig. 6). In 2020, the species was recorded at two sites in Dūkštas Lake. In 2021, it was found in Avilys Lake and a further locality was in Ažvintis Lake. This last lake is connected by a small channel to Sągardas Lake, where Najas flexilis was recorded previously, but the species was not found in Ažvintis Lake until 2021.

Except for a single, currently unconfirmed locality in the western part of the country, all the currently known records are concentrated in lakes in the north-eastern part of the country, which are relatively close together.

Both species have been known in Lithuania since the beginning of the 19^th century, but their distribution as separate species is poorly understood. Due to morphological similarities between species and their rarity, herbarium specimens collected between 1950 and 1990 were usually identified as Najas marina L. s.l., with no clear distinction between subordinate taxa (subspecies or varieties). Although two varieties (Snarskis 1954, Šarkinienė 1963a) or subspecies (Kuusk et al. 2003) have been mentioned in literature from this period, no clear morphological differences were indicated and the same locality was often recorded for both variations.

This study revealed a significant increase in the number of both Najas species locations over the last few decades. The diversity of waterbodies is also increasing. For instance, N. major was discovered in the Nemunas River and in a flooded sand quarry. However, both species are typically found in freshwater lakes, which are common in the southern and eastern parts of Lithuania. By contrast, in Latvia, N. marina is more common in brackish coastal lakes and the Gulf of Riga, but it is also found in freshwater lakes and gravel and dolomite quarries (Suško et al. 2022). N. major is widespread in natural freshwater lakes in south-eastern Latvia and it appears to be expanding its range northwards through the Baltic uplands.

The abundance and spread of Najas marina and N. major in bodies of water is not limited to Lithuania. The problems related to the spread of these species and their hybrids, as well as those regarding their delimitation, were studied in Germany (Rüegg et al. 2019).

The number of abundant populations of both species also increased during the last decades. Although the plants were not found in several waterbodies repeatedly, there are locations where they have survived for almost two centuries. Furthermore, both species, with one exception, were found only in separate waterbodies. Additionally, Najas marina was found more frequently, almost twice as often as the N. major. The distribution trends suggest that these species are not endangered and, therefore, do not require protection. On the contrary, their distribution and impact on aquatic plant communities should be monitored.

Although the increase in the number of known localities of N. minor and N. flexilis in Lithuanian waters over the last two decades could suggest an improvement in the state of the populations, this should be treated with caution. We argue that the current state of the populations should be used as a baseline for future evaluations of the species. The increase in the number of known localities is primarily the result of greater attention being paid to searching for new localities and the special research of protected species, particularly those of European Union interest. The inclusion of aquatic macrophytes in national water monitoring programmes also had a positive impact on the better understanding of the distribution of Red-listed species.

Therefore, although Najas minor has been known since the beginning of the 19^th century, and N. flexilis was only discovered in the mid-20^th century, both species remain amongst the rarest in Lithuania and are Red-listed (Rašomavičius 2021). N. flexilis is endangered worldwide and is listed in the Annexes II and IV of the European Council Directive 92/43/EEC. The native range of N. minor is temperate and subtropical Eurasia to North Africa, while N. flexilis is native to the whole Nordic-temperate Northern Hemisphere (Casper 1979, POWO 2025). Therefore, the ranges of both species overlap in the Baltic States, but N. minor is not found in Estonia (Kuusk et al. 2003). The previous inventory of the species of European importance (Kamaitytė-Bukelskienė et al. 2022, Sinkevičienė et al. 2023) and this research confirmed the occurrence of N. flexilis in four localities in Lithuania. They are concentrated in the north-eastern part of the country and are closely connected with the main localities in Latvia. The number of lakes in Lithuania is negligible compared to the 19 extant localities of this species recorded in neighbouring Latvia (Suško et al. 2022). In terms of the number of N. flexilis sites, Latvia is one of the richest countries in the European Union together with Great Britain, Ireland, Norway and Switzerland (Wingfield et al. 2004, Wingfield et al. 2006, Mjelde et al. 2012, Suško et al. 2022). The southernmost location of N. flexilis in Europe was recorded in Austria (Pall 2011). This species is extinct in Poland, which is located to the south of Lithuania (Zalewska 1999). Currently, N. minor is known from five localities in Lithuania (Kamaitytė-Bukelskienė et al. 2022). This is considerably lower than the 15 known localities in Latvia (Suško et al. 2022). N. flexilis was first discovered in Latvia at the beginning of the 20^th century, whereas N. minor was found much later (Suško 1991, Suško et al. 2022). This pattern may indicate that recent climate changes are favourable for the northward spread of N. minor, a thermophilous species that is more prevalent in southern regions.