The horse eye bean is a fascinating plant part often found washed up on tropical coastlines. This article explores its characteristics, habitat, and safety aspects.
Identifying Horse Eye Beans
The term “horse eye bean” generally refers to the seeds of several Mucuna species and sometimes Dioclea reflexa, all belonging to the legume family (Fabaceae). Mucuna sloanei and Mucuna pruriens are two common species whose seeds are known by this name. The seeds are typically oval or kidney-shaped, often exhibiting a shiny, smooth surface.
These beans range in color from pale to dark brown, brick-red, or even black, sometimes with speckles. A distinguishing feature is a prominent “eye” or hilum, which is a scar where the bean was attached to its pod. This hilum is often a black band, sometimes encircling a significant portion of the seed. They are generally large, ranging from approximately 2 to 3 cm in length and 1.5 to 2.5 cm in thickness.
Where Horse Eye Beans are Found
Horse eye beans originate from tropical and subtropical regions across the globe. Mucuna sloanei are native to tropical Africa, including countries from Sierra Leone to the Democratic Republic of Congo, and extending south to Angola. They are also found in tropical America, including parts of Mexico, Central America, South America, and the Caribbean, with some presence in South Florida. These plants thrive in environments such as rockland hammocks, beaches, and disturbed uplands.
Mucuna pruriens is native to tropical Asia and Africa, and it has become widely naturalized and cultivated in other tropical and subtropical areas, including the West Indies and parts of the southeastern United States. Dioclea reflexa is native to tropical West Africa and parts of Asia, flourishing in rainforests, wetlands, riverbanks, and coastal forests. The buoyant nature of these seeds allows them to be dispersed by ocean currents, leading to their common appearance washed ashore on beaches worldwide.
Toxicity and Safety Concerns
The primary safety concern with horse eye beans, particularly those from Mucuna species, stems from compounds within the seeds and on the plant’s pods. Mucuna pruriens pods are notorious for bearing stiff, irritating hairs that contain mucunain and serotonin, which cause intense itching, redness, and small bumps upon contact. If these hairs enter the eyes, they can potentially lead to blindness, necessitating immediate medical attention.
The seeds themselves, especially raw ones, contain antinutritional factors and bioactive compounds, including L-DOPA (levodopa), saponins, alkaloids, tannins, and cyanogenic glycosides. L-DOPA is a precursor to dopamine and can be present in significant concentrations, particularly in Mucuna pruriens (3-7% dry weight). Ingesting raw Mucuna seeds can cause adverse effects such as nausea, vomiting, cramping, and even hallucinations at high doses. Due to the L-DOPA content, consuming Mucuna pruriens can interact with certain medications, particularly those for Parkinson’s disease or depression, and may lower blood sugar, posing a risk for individuals with diabetes. Consumption is advised against during pregnancy and breastfeeding, and while processing can reduce certain antinutrients, L-DOPA content remains a significant concern, making raw consumption unsafe.
Traditional and Other Uses
Despite potential toxicity, horse eye beans have various traditional and practical uses. Historically, the seeds of certain Mucuna species, particularly Mucuna pruriens, have been utilized in traditional medicine systems like Ayurveda. They address neurological disorders, male infertility, and act as an aphrodisiac, though scientific validation is limited and requires caution. The plant’s ability to fix nitrogen also makes it valuable as a cover crop and green manure in agricultural settings.
Beyond medicinal and agricultural applications, the durable and attractive seeds are popular for crafts and jewelry making. Their buoyancy also leads to their use as decorative “sea beans” found along coastlines. In some cultures, processed horse eye beans, after extensive preparation to reduce toxic compounds, have been used as a minor food crop or soup thickener.