Citrus australasica, the finger lime or caviar lime, is a thorny understorey shrub or small tree of lowland subtropical rainforest in the coastal border region of Queensland and New South Wales, Australia. It has edible fruits which are grown as a commercial crop.
Citrus australasica is a shrub or small tree to about tall with sharp spines up to long in the . The leaves are light green in colour, obovate or rhombic in shape, and arranged alternately along the twigs. They measure up to wide and long, but may reach long, and they may be faintly (scalloped) towards the apex. Like many other plants in the genus, the leaves contain numerous oil glands and are aromatic when crushed.
The flowers are either solitary or paired and are set on peduncles about long. The sepals are tiny, about long, the white petals are up to long. Stamens number between 20 and 25.
The fruit is a botanical berry, cylindrical and tapered at both ends, slightly curved and about long. The skin is rough with numerous oil glands, and greenish yellow to pink. They contain numerous pale seeds about long.
It is thought that for at least 60,000 years, First Peoples living along the east coast of Australia have been eating finger limes.
The fruit is high in vitamin C and the skin can be used to heal cuts and wounds.
Early settlers consumed the fruit and retained the trees when clearing for agriculture. Colonial botanists suggested that they should be cultivated, due to the lack of citrus alternatives.
The finger lime has been popularised as a gourmet bushfood. The globular juice vesicles (also known as pearls) have been likened to a "lime caviar",[1] [2] which can be used as a garnish or added to various recipes. The fresh vesicles have the effect of a burst of effervescent tangy flavour as they are chewed. The fruit juice is acidic and similar to that of a lime. Marmalade and pickles are also made from finger lime. Finger lime peel can be dried and used as a flavouring spice.
Commercial use of finger lime fruit started in the mid-1990s with boutique marmalades made from wild harvested fruit. By 2000 the finger lime was being sold in restaurants, and exported fresh.
The finger lime is grown on a commercial basis in Australia in response to high demand for the fruit. There is an increasing range of genetic selections which are budded onto citrus rootstock. With the sudden high market demand for the fruit the primary source of genetic material for propagation has been selections from wild stock.
The CSIRO has also developed several Citrus hybrids by crossing the finger lime with standard Citrus species. These hybrids have created many cultivars which generate finger limes in many different colors ranging from light pink to deep blue-green. Finger lime is thought to have the widest range of color variation within any Citrus species. The color of the pulp (juice vesicles) comes in shades of green or pink including pale lime-green, pale pink, coral, and scarlet.
This species was first described by Australian botanist Ferdinand von Mueller, and was published in the first volume of his massive work Fragmenta phytographiæ Australiæ in 1858. American botanist Walter Tennyson Swingle, in a 1915 review of the genus Citrus, placed all four Australian species in a new genus Microcitrus, based on morphological features which he said gave "[...] these plants a very different aspect from the commonly cultivated species of Citrus." However, in a paper published in 1998, British botanist David Mabberley discussed the mix of morphological features present throughout the subtribe Citrineae (genera Clymenia, Eremocitrus, Fortunella, Microcitrus and Poncirus) and the ease with which species can be crossed, and subsequently restored the finger lime to Citrus.
, this species has been assessed to be of least concern by the International Union for Conservation of Nature (IUCN) and by the Queensland Government under its Nature Conservation Act.
The finger lime is susceptible to a range of insect pests, including scale insects, katydids, larvae of various moths and butterflies, some bugs, grasshoppers and others. However, it is not a host to the Queensland fruit fly Bactrocera tryoni, saving Australian growers the extra burden of treatment before export.
Research conducted since the 1970s indicated that a wild selection of C. australasica was highly resistant to Phytophthora citrophthora root disease, which has resulted in a cross-breeding program with finger lime to develop disease-resistant citrus rootstock. In 2020, researchers began working with C. australasica to develop solutions for Citrus greening disease.[3]