Not to be semantically confused with : Fen Pondweed (Potamogeton coloratus) [a plant with similar name belonging to the same family]
Hybridizes with :
Bothnian Pondweed (Potamogeton vaginata ) (which apparently has no presence in the UK...!!!!) to produce Stuckenia × bottnica which nevertheless is found on the River Tweed in Berwicks and Cheviot and also on the River Ure (in Yorkshire) and the River Liffey (in County Kildare and County Dublin). Go reckon!
Lookee-Likees : Slender-leaved Pondweed (Potamogeton filiformis) which also has very thin narrow leaves, at 0.25 to 1.2mm wide much narrower than Fennel Pondweed but with ligules having an identical range of lengths: 5 to 15mm, but the fruits, at 2.5 to 2.8mm are smaller than those of Fennel Pondweed. This too is native but is found only much further north, in Scotland, North West Ireland and Orkney and Shetland Isles, but occurs in lowlands no higher than 390m. It thrives in shallow eutrophic water very low in oxygen or in brackish water and at the edge of lakes or reservoirs (although one cannot imagine why these are slightly salty)...
It is to be found in similar habitats as Slender-leaved Pondweed (Potamogeton filiformis) - namely in lakes, dykes, rivers, streams and sometimes in slightly salty (brackish) water but unlike that is frequently found over most of the British Isles.
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