Citrus australasica

Citrus australasica, thefinger limeorcaviar lime, is a thornyunderstoreyshrub or small tree of lowland subtropical rainforest in the coastal border region ofQueenslandandNew South Wales, Australia. It has edible fruits which are grown as a commercial crop.
Citrus australasicais a shrub or small tree to about 10 m (33 ft) tall with sharp spines up to 2.5 cm (1 in) long in theleaf axils. The leaves are light green in colour,obovateorrhombicin shape, and arrangedalternatelyalong the twigs. They measure up to 2.5 cm (1 in) wide and long, but may reach 5 cm (2 in) long, and they may be faintlycrenate(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 onpedunclesabout 3 mm (0.12 in) long. Thesepalsare tiny, about 1.5 mm (0.06 in) long, the white petals are up to 9 mm (0.35 in) long. Stamens number between 20 and 25.
The fruit is a botanicalberry, cylindrical and tapered at both ends, slightly curved and about 8 cm (3.1 in) long. The skin is rough with numerous oil glands, and greenish yellow to pink. They contain numerous pale seeds about 7 mm (0.28 in) long.
This species was first described by Australian botanistFerdinand von Mueller, and was published in the first volume of his massive workFragmenta phytographiæ Australiæin 1858.American botanistWalter Tennyson Swingle, in a 1915 review of the genusCitrus, placed all four Australian species in a new genusMicrocitrus, 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 botanistDavid Mabberleydiscussed the mix of morphological features present throughout the subtribe Citrineae (generaClymenia,Eremocitrus,Fortunella,MicrocitrusandPoncirus) and the ease with which species can be crossed, and subsequently restored the finger lime toCitrus.
The finger lime is native to the southeastern corner of Queensland and the northeastern corner of New South Wales, from the area aroundBrisbaneto theClarence River. It grows in coastal rainforest and extends into neighbouring open forest, often on soils derived from basalt. It is favoured by disturbance.
As of December 2024, this species has been assessed to be ofleast concernby theInternational Union for Conservation of Nature(IUCN) and by the Queensland Government under itsNature Conservation Act.
The finger lime is susceptible to a range of insect pests, includingscale insects,katydids, larvae of various moths and butterflies, somebugs, grasshoppers and others.However, it is not a host to the Queensland fruit flyBactrocera tryoni, saving Australian growers the extra burden of treatment before export.
Research conducted since the 1970s indicated that a wild selection ofC. australasicawas highly resistant toPhytophthora citrophthoraroot disease, which has resulted in a cross-breeding program with finger lime to develop disease-resistant citrus rootstock. In 2020, researchers began working withC. australasicato develop solutions forCitrus greening disease.
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 gourmetbushfood.The globularjuice vesicles(also known as pearls) have been called "lime caviar",and 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 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 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 ontocitrus rootstock. With the sudden high market demand for the fruit the primary source of genetic material for propagation has been selections from wild stock.
TheCSIROhas also developed severalCitrushybrids by crossing the finger lime with standardCitrusspecies,  creatingcultivarswhich produce finger limes in many different colours ranging from light pink to deep blue-green.  Finger lime is thought to have the widest range of colour variation within anyCitrusspecies.