Abalones at risk: A global Red List assessment of Haliotis in a changing climate

Alaska to Baja California, Mexico, on the Atlantic coast, a paucity of suitable rocky habitat probably accounts for their absence as far south as the Carolinas. Even then, just a single spe- cies, H. pourtalesii, occurs thereafter south along the Atlantic coast of Central and South America. Apart from the Gala´pagos Islands and Cocos Island of Costa Rica, the entire length of coastline of Pacific South and Central America is similarly devoid of abalones despite the presence of rocky shorelines that would seem to offer suitable habitat. Areas of distribution vary widely between species, with some, such as H. rubiginosa, found only in the confined waters of Lord Howe Island in the Tasman Sea off the east coast of Australia. Some species, however, occur across the length of the Indian Ocean and Western Pacific, such as H. ovina, found from the Maldives in the west to the Society Islands in French Polynesia in the east, a distance of approx. 14 900 km. Abalones are broadcast spawners with separate males and females. They typically reach maturity within five to eight years and release their gametes into the water column on a cue standard to each species. These cues may include a sudden variation in sea-surface tempera- ture (SST) or tidal height [43], lunar cycle, and even typhoon events, as in the case of H. diver- sicolor [44]. Once released, for successful fertilisation the eggs must rapidly encounter high densities of sperm. This demands closely packed assemblages of animals for reproductive suc-

cess. Experiments with greenlip abalone (H. laevigata) of South Australia have shown that when the distance between male and female is greater than two metres fertilization failure ensues [45]. Known as the Allee Effect, this is a common biological phenomenon [46] that pos- itively correlates population density with individual fitness. It is also one of the most significant influencers in the likelihood of recovery of a population decimated by overfishing or mass mortality, where remnant populations may become too widely dispersed and the individuals eventually age and die without reproducing successfully. This has been demonstrated in the case of white abalone (H. sorenseni), where a general model was developed that determined that too few remain for successful reproduction throughout its range in the wild [5]. The larvae of abalones are lecithotrophic and form the first shell, the protoconch, while still planktonic. Settlement of the larvae occurs after one or two weeks and is induced by a chemical signal from crustose coralline algae (CCA) [47, 48]. Abalones are mainly sedentary, although some species move more than others in search of food. Young juvenile abalones typically for- age and feed at night returning during the daytime to their cryptic habitat under boulders and in crevices. Older juveniles and adults may have a home site, or remain in one place to capture drift algae or longer strands of attached algae [49]. Although most species feed exclusively on algae, a few, including those occurring in Australian waters, also consume seagrass and their

epiphytes [50]. Abalones’ exceptionally powerful adhesion to rocky surfaces through a pedal foot enables them to occupy environments with strong currents and wave activity. Their remarkable ability results from setae and nanoscale terminations enabling capillary and van der Waals forces to seal the interface between foot and rock on a range of surface conditions [51]. When disturbed, abalones retract their epipodium and tentacles that line the margin of the shell and clamp the shell fast to the rock with their foot, becoming virtually immovable. In common with other molluscs, abalone haemolymph does not coagulate, and so removal must be accomplished without damage to avoid death, although trials have shown that a degree of survival is possible [52]. To achieve this, abalones have to be quickly hand-levered by a diver using a specially rounded abalone iron. In deeper water, SCUBA or hookah i.e., sur- face-supplied air, are used to fish for abalones. Once harvested, abalones may be sold whole, shucked, canned or dried. Although most of the animal can be eaten, the foot muscle with the end trimmed is preferred, but it requires slicing and tenderising to make it palatable and ready for cooking. The abalone fishery worldwide peaked in the 1970s at nearly 20 000 mt but by 2020 the wild fishery had collapsed to 4500 mt [53, 54]. Today, aquaculture production has far PLOS ONE Abalones at risk: A global Red List assessment of Haliotis PLOS ONE | https://doi.org/10.1371/journal.pone.0309384 December 23, 2024 4 / 25