Mucuna--Sea Beans

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Bat-Pollinated Mucuna Flowers
© W.P. Armstrong 22 April 2010 (updated 1 Dec 2013)
The Source Of Tropical Sea Beans

The genus Mucuna includes tropical lianas with flower clusters on
long, rope-like stalks that hang down below the rain forest canopy.

See The Bat-Pollinated Sausage Tree
See Article About Tropical Drift Seeds
See Article About Tropical Drift Fruits
See The Article About Botanical Jewelry
  See Assorted Drift Seeds Called Sea Beans  

Note: My previous identifications of M. rostrata from the Monteverde Cloud Forest are probably M. sloanei.

The enormous Legume Family (Fabaceae) contains many species of tropical vines, but some of the most interesting belong to the genus Mucuna. There are many species of Mucuna throughout tropical regions of the world, including M. urens, M. pruriens and M. sloanei. Most species of Mucuna are climbing woody vines called lianas that twine through the rain forest trees like "botanical boa constrictors." Their bat-pollinated flowers and pods are produced on long, rope-like stems that hang from the forest canopy. The seed pods are covered with microscopic velvety hairs (called trichomes) that can be extremely painful if they get into your eyes. In the Caribbean region and Central America, the hairs were stirred into honey or syrup as a remedy to dispel intestinal parasites. The dense covering of irritating hairs may help to discourage seed predators, particularly when the seeds are soft and vulnerable. At maturity, each pod produces several hard, marble-like seeds. The seed is called "ojo de buey" because of its striking resemblance to the eye of a bull. The seeds are also known as "sea beans," because they are commonly carried by rivers into the ocean. The closely-related genus Dioclea includes woody vines which also produce seeds called sea beans. The pods of Dioclea generally do not have the stinging trichomes. The seeds of both genera are commonly polished and made into seed necklaces.

Mature pods of Mucuna (cf. M. sloanei) in the Monteverde Cloud Forest of Costa Rica. The pods are covered with dense, whiskerlike, stinging hairs (trichomes) that discourage seed predators.

Pods and seeds of Mucuna (cf. M. sloanei) from the Monteverde Cloud Forest of Costa Rica. The pods are covered with dense, whiskerlike hairs (trichomes) that discourage seed predators. One seed shows an attachment stalk that encircles the seed along its hilum. The scar (hilum) where the stalk was attached produces the characteristic layered appearance (hamburger seed).

According to Dr. Daniel Janzen, noted authority on the Costa Rican rain forest, developing seeds of Mucuna are nearly free from seed predators. In addition to the stinging trichomes, the pods are rich in the bitter, potentially toxic amino acid L-dopa. [L-dopa, precursor of the brain neurotransmitter dopamine, is given to patients suffering from Parkinson's Disease.]

Dense lianas of Mucuna near Golfito on the humid Pacific coast of Costa Rica. The pods hang from the rain forest canopy on rope-like vines. They are covered with whiskerlike, stinging hairs.

Pods and seeds of Mucuna near Golfito on the humid Pacific coast of Costa Rica. The pods are covered with dense, stinging hairs. Also note the transverse ridges on the pods. The black attachment scar (hilum) on the seeds give them a layered appearance resembling a miniature hamburger. They are commonly called "sea beans" and they drift on the world's oceans, often washing ashore on the beaches of distant continents and tropical islands.

A. Pods and seeds of Mucuna (cf. M. sloanei) from the Monteverde Cloud Forest of Costa Rica. One seed has an attachment stalk (funiculus) that encircles the seed along its hilum. B. Pod and seeds of Mucuna near Golfito on the humid Pacific coast of Costa Rica. The pods are covered with dense, stinging hairs. These may be the same species.

One of the hazards of handling the seed pods of Mucuna species are the numerous irritating hairs (trichomes) that readily penetrate your fingers. These hairs can be especially painful and troublesome if they get into your eyes.

Magnified view (400x) of stinging hairs (trichomes) from the seed pods of Mucuna (cf. M. sloanei). Each hair is a modified epidermal cell which presumedly serves to discourage seed predators.

  See Stinging Hairs Of Mala Mujer  

Two of the most common species washed ashore on beaches of the Gulf states and southeastern United States are M. urens and M. sloanei. Seeds of the two species are difficult to distinguish, although the latter species appears to be more common. Dr. Gerald Sullivan (The Drifting Seed Vol. 14 (1) May 2008) has devised an ingenius method of separating these two species. Sea beans have a conspicuous hilum that encircles the seed. At one end of the hilum is a small pin point depression called the micropyle where the pollen tube entered the ovule during fertilization. At the opposite end is a peculiar scar (smiley) that resembles a smiling face. It is this "smiley" that separates seeds of the two species. The two types of smileys are shown in the following image.

Seeds of Mucuna species showing two distinct "smileys." A. Seed from a pod collected in Golfito, Costa Rica. Originally identified as M. urens, the smiley is more similar to M. sloanei. B. Drift seed of M. urens collected on a beach in Belize. Note the thinner, upcurved smiley and reddish coloration. C. Drift seed of M. sloanei from Belize showing a thicker, blocky shaped smiley. The latter species also occurs in the Monteverde Cloud Forest of Costa Rica.

According to the "Flora of Guatemala" by P.C. Standley and J.A. Steyermark (Fieldiana Vol. 24, 1946), "male" and "female" seeds from another rain forest liana Mucuna argyrophylla are carried by natives to prevent hemorrhoids. Those that sink in water are called "hembras" (female) and those that float are "macho" (male). They are also called ojo de venado (deer eyes) and "sea beans" because they often become drift seeds.

Velvety pods of a sea bean (Mucuna) hang from long, ropelike stems in the rain forests of Belize. The hard, black seeds are called "ojo de buey" (eye of the steer) and "ojo de venado" (eye of the deer) by local residents. I originally concluded this was species Mucuna argyrophylla; however, an updated flora of legume species in Belize lists M. holtonii rather than M. argyrophylla.

Mucuna holtonii from Belize. The seeds are more flattened than other Central American species of Mucuna.

Clusters of Mucuna blossoms hang down from the forest canopy where they can easily be accessed by bats. They are pollinated by night-flying bats that sip the sweet nectar and transfer pollen from one plant to another. After pollination, the ovary of each flower develops into a legume pod containing several large seeds resembling miniature hamburgers. They have a hard, thick, woody seed coat which makes them impervious to water. Internal air cavities also make them buoyant in water. The conspicuous, dark, central attachment scar (hilum) produces the layered appearance, and their superficial resemblance to a miniature hamburger. Sea beans are washed down gullies and creeks where they are carried into rivers that eventually flow into the Atlantic or Pacific oceans. The buoyant seeds drift for months (or years) at sea, eventually washing ashore on the sandy beaches of a distant continent or tropical island. They are often collected and polished by natives and made into lovely necklaces and bracelets.

Flower cluster of Mucuna (cf. M. sloanei) in the Monteverde Cloud Forest of Costa Rica. The long, rope-like stalk hangs below the forest canopy where night-flying bats can easily access the fragrant blossoms.

Guinea creeper (Mucuna bennettii), an interesting liana from New Guinea with bright red, pea-shaped flowers. Photographed at Fairchild Tropical Garden in Coral Gables, Florida.

Several Mucuna species from Central America: A. M. urens. B. M. holtonii (cf. M. argyrophylla). C. M. fawcettii. D. M. sloanei. E. cf. M. urens. F. cf. M. urens showing attachment stalk that encircles seed along the hilum (seed was removed from the pod above). The scar (hilum) where the stalk was attached produces the characteristic layered appearance (hamburger seed).

Sea bean relatives that belong to the tropical Old World genus Strongylodon and the New World genus Oxyrhynchus: A. Strongylodon siderospearus from lowland forests on the island of Reunion off the east coast of Madagascar. B. Oxyrhynchus trinervius, a high-climbing woody vine in tropical rain forests of Central and South America.

Jade vine (Strongylodon macrobotrys) native to the Philippines. In growth habit it resembles the Guinea creeper (Mucuna bennettii).

A pendant from Costa Rica made from polished seeds of Oxyrhynchus trinervius and a gray nickernut (Caesalpinia bonduc).

   Sea Beans In The Hawaiian Islands      Maui Sea Beans 2012        Maui Seabeans 2013  

There are at least three species of sea beans that grow wild on some of the Hawaiian Islands, Mucuna gigantea ssp. gigantea, M. sloanei and Dioclea wilsonii (D. violacea). Like other species of sea beans from throughout the world, the seeds are polished and made into beautiful leis. According to the revised Manual of the Flowering Plants of Hawaii (Volume 1, 1999) by W.W. Wagner, D.R. Herbst and S.H. Sohmer, M. gigantea ssp. gigantea and D. wilsonii may be indigenous to some of the Hawaiian Islands. The seeds may have drifted to these islands naturally, long before the voyages of ancient Polynesians. Two varieties of M. sloanei are also recognized in the revised Flora of Hawaii (1999), indigenous var. sloanei on Kauai, Oahu and Maui, and endemic variety persericea on the east coast of Maui. Unlike M. sloanei, the seed pods of Mucuna gigantea do not have transverse ridges, alhough the pods of both species have irritating trichomes.

Mucuna gigantea is widespread from East Africa, India, China, Malaysia, and tropical Pacific Islands. The woody seeds are well adapted for ocean dispersal. Known as "peka'a" by early Hawaiians, the seeds were used medicinally as a violent cathartic and were strung into necklaces and leis. The seeds of M. gigantea ssp. gigantea resemble Dioclea wilsonii, except they are more rounded in outline and slightly thinner (more disk-shaped). In addition, the hilum consists of a single black band that is slightly indented (grooved). Dioclea seeds generally are thicker with a flattened side that superficially resembles the bottom of a purse. [In fact, Dioclea seeds washed ashore on beaches are often referred to as "sea purses" by beachcombers.] Also the black hilum of Dioclea wilsonii has a distinctive yellowish-brown border. The flowers of Mucuna gigantea ssp. gigantea are greenish-yellow, while those of Dioclea wilsonii are purplish. Mature flowers are a fairly reliable characteristic to separate these two species; however, they are often unavailable (out of reach) or out of season.

Like Mucuna, the seeds of Dioclea wilsonii are well adapted for ocean dispersal. This species is native to rain forests of Central America, often under the name of D. violacea. It is not certain whether this species is truly native to the Hawaiian Islands, or whether it is an escape like so many other species introduced by people. It was collected as early as 1825 on these islands and its seeds are superb ocean drifters. According to C.R. Gunn and J.V. Dennis (World Guide To Tropical Drift Seeds and Fruits, 1976), the related species D. reflexa can remain buoyant and viable for two years. With sufficient time, D. wilsonii could have arrived on the Hawaiian Islands without the aid of humans.

  • Wilmot-Dear, C.M. 1989. "A Revision of Mucuna (Leguminosae-Phaseoleae) in the Pacific." Kew Bulletin 45 (1): 1-35.
  • Mucuna gigantea ssp. gigantea (above) and Dioclea wilsonii (below). The following photo shows the mottled seeds of Mucuna gigantea ssp. gigantea from the Hawaiian Island of Oahu.

    Seeds of Mucuna gigantea ssp. gigantea from the Hawaiian Island of Oahu. They have mottling similar to Dioclea wilsonii, but the hilum is thinner and consists of a single black band that is slightly indented.

    Left: Pendent flower cluster of Mucuna gigantea ssp. gigantea hanging from the rain forest canopy on the east coast of Maui. Right: Seeds of M. gigantea ssp. gigantea. Populations of this species on Maui produce brown or tan seeds without mottling. The mottled seed at the top of photo and the seeds in necklace came from the island of Hawaii.

    Sea beans (Mucuna gigantea ssp. gigantea). These flattened "hamburger" seeds are produced on a vine (liana) high in the rain forest. I have seen this species along the rain-soaked Hana coast at the east end of the island. The seeds from Maui are typically tan-colored without mottling. Two varieties of M. sloanei are also recognized in the revised Flora of Hawaii (1999), indigenous var. sloanei on Kauai, Oahu and Maui, and the endemic variety persericea on the east coast of Maui. Nikon D-90.

    Why Is A Sea Bean Sometimes Called A "Hamburger Seed"?

    A sea bean Mucuna gigantea ssp. gigantea collected on a West Maui beach on 22 November 2012. It truly resembles a miniature hamburger, especially when compared with an order of fries! I must confess that I greatly reduced the size of the plate and fries. The sea bean is only 22 mm in diamter (just under one inch). Nikon D-90.

    Dioclea wilsonii (D. violacea) in the Hawaiian Islands

    Dioclea wilsonii (D. violacea), a pantropical, purple-flowered liana that grows wild in mesic coastal forests on the islands of Hawaii and Kauai. This species may have drifted to these islands naturally prior to the Polynesians, and if so, may be considered indigenous. The polished necklace in photo is made from seeds of another species of sea purse (possibly Dioclea megacarpa).

    Pods and seeds of Dioclea reflexa collected in the dense rain forest along the New River of Belize. According to an updated checklist for Belize, D. megacarpa is the correct synonym for D. relexa var. grandiflora. The attractive seeds of this pantropic species wash ashore on tropical beaches throughout the world. The pods of Dioclea species typically do not have the stinging trichomes of Mucuna species.

    Another Species Of Sea Purse (Dioclea)

    A gourd mask from Brazil and a rosary from El Salvador. The eyes of the mask and the seeds of the rosary are from a tropical species of Dioclea, possibly D. megacarpa.

    Gigasiphon macrosiphon: A Seed That Resembles Mucuna
    Suggested pronunciation:  jy-ga-SY-fon  mak-roh-SY-fon

    Gigasiphon macrosiphon is a rare leguminous tree known only from moist, lowland and coastal forests of Kenya and Tanzania. According to Encyclopedia Of Life (EOL), destruction of East African coastal forests and the use of this tree for firewood, tools, charcoal and lumber has reduced its population to 33 known mature individuals in the wild. The International Union For Conservation of Nature (IUCN) assessed G. macrosiphon with "Red List Endangered Status" in 1997, and in a report in conjunction with the Zoological Society of London (ZSL) places this species on their list of 100 most endangered species in 2012. It is included here because of the remarkable resemblance of its seeds to those of Mucuna species.

    The Kew Plant Index includes four accepted species in the genus Gigasiphon: G. amplus, G. gossweileri, G. humblotianum and G. macrosiphon. Seeds of G. humblotianum, a species native to Madagascar, have been verified by B. Verdcourt on beaches of east Africa (Kew Bulletin Vol. 36 No. 4, 1982). According to Verdcourt, the illustration of G. humblotianum seeds on page 161 of World Guide to Tropical Drift Seeds and Fruits is misidentified as Mucuna: "It also matched a drawing in C. R. Gunn & J.V. Dennis' book on drift seeds (World Guide to Tropical Drift Seeds and Fruits, New York,1976, fig. 66/A-D), also incorrectly captioned Mucuna sp." The caption reads "Beaches of Canton Island." This is a South Pacific island approximately half way between Hawaii and Fiji. G. humblotianum is clearly a drift seed species capable of long ocean voyages.

    Gigasiphon macrosiphon: The shape and thick hilum of the seeds resembles those of sea beans (Mucuna). Unlike Mucuna seeds, the brown, smooth hilum does not have a thin line or groove along the center. The tough, indehiscent seed pod contains two seeds and they appear to be adapted for drifting in water. According to the Foster Botanical Garden in Honolulu, Hawaii, this unusual tree may be extinct in the wild. Several web sites postulate that only 30-40 trees survive in its native habitat. Ann Robertson (The Drifting Seed May 1998) has collected drift seeds of Gigasiphon on the East African coast of Malindi, Kenya. Her seeds were apparently from a different species (G. humblotianum) native to Madagascar. See: B. Verdcourt, B. 1982. "Gigasiphon humblotianum (Leguminosae: Caesalpinioideae -- Bauhinieae) as a Drift Seed." Kew Bulletin 36 (4): 659-660.

    Although the seeds of Gigasiphon macrosiphon resemble Mucuna species, the large blossoms are very different from the pea-shaped (papilionaceous) flowers of Mucuna (subfamily Papilionoideae = Faboideae). Gigosiphon actually belongs to a different subfamily, the Caesalpinioideae. It is more closely related to Bauhinia than Mucuna. In fact, it belongs to the subtribe Bauhiniinae that includes the genera Bauhinia, Barklya, Brenierea, Gigasiphon, Lysiphyllum, Phanera, Piliostigma, Schnella, and Tylosema. See: Wunderlin, R.P. 2010. "Reorganization of the Cercideae (Fabaceae: Casesalpinioideae)." Phytoneuron 2010-48: 1-5.

      Bauhinia In Palomar College Arboretum  
    Caesalpinioideae: Bauhinia variegata
    Caesalpinioideae: Bauhinia species (1)
    Caesalpinioideae: Bauhinia species (2)

    Assorted Drift Seeds Called "Sea Beans"

    Identification of Sea Beans in the Above Image
    See This Sea Bean Assortment On A T-Shirt

    Legume Family (Fabaceae):

    A.  Entada species, probably the Old World E. phaseoloides.
    C.  Entada rheedei. Drift seed from tropical Africa.
    D.  Erythrina variegata. Coral tree from the island of Hawaii.
    E.  Mucuna holtonii (cf. M. argyrophylla). Sea bean with velvety pods native to Belize.
    F.  Oxyrhynchus trinervius. Drift seed from Costa Rica.
    G.  Dioclea reflexa (cf. D. megacarpa). Drift seed from New River of Belize.
    H.  Mucuna (cf. M. sloanei). Sea bean from the Monteverde Cloud Forest of Costa Rica.
     I.   Mucuna urens. Sea bean from Golfito, Costa Rica.
    J.  Caesalpinia ciliata. Yellow nickernut, drift seed from the Caribbean.
    K.  Canavalia rosea. Beach bean from the Caribbean (syn. C. maritima).
    L.  Entada gigas. Sea heart from huge vine in Golfito, Costa Rica.
    M.  Caesalpinia major. Brown nickernut, drift seed from the Caribbean.
    N.  Mucuna sloanei. Drift seed from the Caribbean.
    O.  Caesalpinia bonduc. Gray nickernut, prickly shrub from island of Antigua.
    P.  Dioclea wilsonii. Sea purse from the island of Hawaii (syn. D. violacea).
    Q.  Mucuna gigantea Sea bean from Hawaiian Islands: Tan, unmottled bean from Maui.
    R.  Gigasiphon macrosiphon. African tree planted on Hawaiian island of Oahu.
    S.  Mucuna fawcettii. Caribbean drift seed. Hilum thicker than other Mucuna species.
    T.  Canavalia nitida. Cathie's bean named in honor of author & naturalist Cathie Katz.

    Morning-Glory Family (Convolvulaceae)

    B.  Merremia discoidesperma. Mary's bean from mountains near Golfito, Costa Rica.

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