Do you know what those devilish-looking seed heads are?
The sharp nut-like items found along the lighthouse trail are the seed of the water chestnut (Trapa natans)–an non-native, invasive aquatic plant species. The seeds seem diabolical if you are walking barefoot and happen to step on one. In many respects, they are a “pest,” but squirrels and muskrats eat the seeds. For a complete description and photo of the plant, refer to the following website:
http://www.invasive.org/eastern/biocontrol/3WaterChestnut.html