Parvulastra parvivipara

Lore & Background

Parvulastra parvivipara is a very small species of starfish in the family Asterinidae. Adults can grow to a diameter of about 1 cm (0.4 in) and are an orange-yellow colour. They are the smallest known starfish. The species is endemic to the coast of South Australia, where it is found within 200 km of the Eyre Peninsula. Its distribution is limited to intertidal rock pools on granite rocks. Some seemingly suitable pools contain none of these starfish, while others have large numbers. Starfish favoured pools with little wave action, but with a considerable degree of biodiversity. They also preferred pools low down the beach rather than high-level pools.

The lifecycle of P. parvivipara is very unusual for a starfish. The adults are self-fertilising hermaphrodites and the eggs are brooded within the gonads. No planktonic larval stage is seen, and the directly developing juveniles are cannibalistic, feeding on other embryos and juveniles while in the brood pouch. When mature enough, they are released into the water in batches of up to 20, where they continue their lives, quite probably in the same rock pool as their parents. Most starfish disperse to new habitats during the planktonic larval stage, but P. parvivipara is unable to do this. It likely can move to a new pool only by chance when an individual is swept there by a wave. Because of the inability of this species to disperse in a typical starfish manner, it may experience some adverse circumstances in any particular pool, be exterminated there, and perhaps recolonize the pool later.

Reader's Guide

Parvulastra parvivipara holds significance as the smallest known starfish and for its unusual reproductive strategy. Its viviparous nature—giving birth to live young—and self-fertilising hermaphroditism are rare among starfish. The absence of a planktonic larval stage limits its dispersal, making it vulnerable to local extinction in individual rock pools. The main identified threats to the species are habitat deterioration and destruction through sedimentation, eutrophication, and coastal development. As P. parvivipara uses mostly small boulders that are particularly susceptible to disturbances that are increasing with rising sea levels, future ecological restoration effects to protect this rare species may be required. Its limited distribution within 200 km of the Eyre Peninsula in South Australia, combined with its preference for specific rock pool conditions (low wave action, high biodiversity, low on the beach), underscores its ecological specialization. The species' lifecycle, including cannibalistic juveniles and seasonal breeding variations (most breeding in late spring and summer when brood sizes are smallest and juveniles largest), reflects an adaptive strategy to maximize offspring survival in a constrained habitat.

Did You Know?

• Adults of Parvulastra parvivipara can grow to a diameter of about 1 cm (0.4 in), making them the smallest known starfish.
• It is a viviparous species that gives birth to live young, with no planktonic larval stage.
• The species is endemic to a limited area of South Australia within 200 km of the Eyre Peninsula.