Nitellopsis obtusa, known as starry stonewort, is an aquatic algae native to Europe and Asia, belonging to either the Characeae or Feistiellaceae families. It was first recognized as an invasive species following its migration to North America in 1978. N. obtusa was first discovered in North America in 1978 in New York. Since then, it has been documented in various lakes throughout the continent, extending as far north as Canada. Records indicate that N. obtusa is the sole surviving member of an evolutionary lineage of the Cretaceous-Tertiary boundary. As a native species, N. obtusa contributes to water clarity and supports aquatic food webs. The species is commonly found in low-flow lakes with alkaline-to-neutral pH and elevated conductivity. However, as it expanded into North America, populations in parts of its native range have declined. It’s extended, late-season growing may contribute to its invasive success by making it challenging for timing control efforts. Although many efforts have been made to control N. obtusa through physical and chemical removal treatments, there have been few systematic evaluations on the success of these attempts.
Taxonomy and classification N. obtusa was first classified within the genus Chara in 1810. Since then, it has been reclassified into four other genera, including Nitella, Lychnothamnus, Nitellopsis, and Tolypellopsis. Although it is currently accepted as a member of the Chareae tribe, its taxonomic placement has varied inconsistently between Chara, Lamprothamnium, and Lychnothamnus. Molecular phylogenetic analysis indicates that starry stonewort is more closely related to Lychnothamnus than to Chara or Lamprotheamnium.
Characteristics/Reproduction Starry Stonewort is most recognizable by its white star-shaped bulbils, which are seed-like structures located at the nodes of rhizoid structures that anchor its grass-like body to a substrate. Identifying N. obtusa is difficult due to the long internode cells and the node whorls intertwining. Proper identification of the species involves counting the number of upwardly radiating node whorl branches, which can be unclear when the nodes are entwined. N. obtusa must be composed of 5-7 upward radiating node whorl branchlets, which contain bract cells. Due to the algae containing both fragments and bubils, they can produce both sexually and asexually. However, studies have shown that native populations primarily reproduce asexually. Sexual reproduction becomes more prevalent during shorter growing seasons and in shallower water. According to a 2017 study, only male individuals of N. obtusa have been identified in North America. With no female individuals, the species relies on vegetative bulbil reproduction and fragmentation. This alga can survive at depths up to three times deeper than the Secchi disk transparency, requiring very little light. It’s also known for drastically improving water clarity by releasing substances that slow or stop the growth of phytoplankton and other algae. It has been reported that elevated temperatures can lead to the complete collapse of starry stonewort meadows. After dense stands of starry stonewort decline in shallow areas, the species often persists as less concentrated but still actively growing meadows in cooler zones. These remaining populations tend to follow a distinct temperature- and light-driven zonation pattern, extending into deeper parts of the littoral region before sharply dropping off at the outer edge of the slope. Water temperature has been shown to play an important role in the growth and reproductive development of N. obtusa, with studies indicating that gametangia formation is more likely to occur during warm, sunny growing seasons.
Habitat Starry stonewort is native to Eurasia, where it has declined across much of its native range even as it rapidly expands throughout North America. Once considered a highly beneficial plant in Europe and Asia, it now occupies a variety of freshwater habitats across both regions. The species typically inhabits deep, slow moving waters with sparse vegetation and can maintain permanent populations in freshwater with salinity levels up to 5%, while tolerating fluctuations of up to 17% for short periods. Nitellopsis obtusa is strongly associated with neutral to alkaline waters, elevated conductivity, and calcareous, nutrient‑rich sediments that support its growth and the formation of dense benthic mats. Starry stonewort can grow on both organic and inorganic substratesand is commonly found at depths ranging from 1 m to more than 6 m in lakes and slow moving rivers, though the article documents occurrences from 0.5 to over 14 meters depending on water clarity and light availability. It is most abundant in mesotrophic to eutrophic, low flow or still waters where its tall, filamentous structure can dominate the substrate. Starry stonewort is sensitive to disturbances such as drought and eutrophication, which can reduce its abundance and restrict it to deeper, continuously inundated areas. These environmental characteristics help explain both its decline in parts of its native range and its patchy but expanding distribution across North America.
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