Pacific Rocky Intertidal Monitoring: Trends and Synthesis
Haliotis cracherodii (Leach 1814)
Phylum Mollusca, class Gastropoda, order Archaeogastropoda
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Shell exterior is dark blue, dark green, or black, smooth and usually epiphyte-free; up to 20cm in length. There are usually five to nine open respiratory pores sitting flush with the shell’s surface. Shell interior is pearly, with pink and green iridescence. The mantle and tentacles are black (Morris et al. 1980).
Habitat and Geographic Range
Black abalone inhabit suitable rocky substrate from the high intertidal zone to 6 m depth but are most abundant in the mid-low intertidal. Their current range is from about Point Arena in northern California to Southern Baja California (MARINe). Black abalone are rare north of San Francisco and south of Punta Eugenia, Baja with unconfirmed sightings reported as far north as Coos Bay, Oregon (Cox 1962, Morris et al 1980).
A smooth dark shell and 5-9 round, flat shell holes distinguishes black from other abalone species. H. rufescens, which sometimes occurs intertidally, has a reddish to pink colored shell, commonly overgrown, and 3-4 open shell holes which are externally raised. Pink (H. corrugata) and green (H. fulgens) abalone occasionally occur in the intertidal. Pinks are dull green to reddish brown, highly corrugated, with 2-4 large, elevated holes, and the edge of the shell is usually quite scalloped. Greens are olive green to reddish brown, with numerous, broad, flat-topped ribs, and 5-7 small, circular, slightly elevated holes.
Black abalone are typically found clustered in crevices, under boulders, or on the walls of surge channels along exposed shores. Little is known about the requirements of newly settled black abalone, but they are believed to inhabit cryptic locations such as narrow crevices, undersides of boulders, and the interstices of mussel beds (Tissot 1995, Dan Richards pers. com.). Juveniles appear to be fairly motile, and likely graze on crustose coralline algae and micro flora, such as diatom films (Garland et al. 1985, Shepherd and Turner 1985); adults tend to be more sessile, feeding primarily on drift algae, especially brown kelps (Bergen, 1971, Blecha et al.,1992, Leighton and Boolootian1963, Morris et al. 1980). Growth varies with size, location, and other environmental factors, but does appear to be slow—about 20mm in the first year, and 10-20mm per year over the next several years, then slowing at sizes of approximately 100mm (Leighton and Boolootian1963, Morris et al. 1980). Black abalone become reproductively mature at around 45mm (between 3-7 years old) (Ault 1985). Absolute longevity has not been determined, but ages greater than 30 years appear likely based on tagging and other population studies (e.g. VanBlaricom 1993). Before recent catastrophic declines (see below), black abalone were very abundant and could occasionally be seen stacked on top of each other, reaching densities of more than 100/m² (Douros 1987, Richards and Davis 1993).
Black abalone are broadcast spawners, and fecundity (number of gametes produced) is directly related to adult size, with older, larger individuals producing significantly higher numbers of gametes than newly mature, smaller animals. Because gametes are released into the turbulent intertidal environment, close proximity of male and female abalone is assumed to be necessary for successful fertilization to occur (Prince et al. 1987, Miller & Lawrenz-Miller 1993). The requirements of black abalone larvae are not well known, but based on studies of other abalone species, it is thought that they spend 3-10 days in the water column before settling (e.g. McShane 1992). Larvae are thought to be relatively passive, and recruitment is believed to be generally localized, particularly in areas with offshore features such as kelp beds, which have been shown to retain larvae of other abalone species (McShane et al. 1988, Prince et al. 1988, Hamm & Burton 2000, Chambers et al. 2006). Other abalone species require the presence of crustose coralline algae to induce settlement (Morse et al 1979), and it is assumed (although not verified) that black abalone share this requirement.
Although once an important human resource, the black abalone fishery was closed in 1993 due to massive population declines. Intense fishing may have been responsible for declines in southern California, but the primary cause of decline throughout much of the black abalone’s range is a fatal disease called withering syndrome (WS). WS is caused by a gastrointestinal Rickettsiales-like prokaryote that interferes with digestion and results in the shrinking of an animal’s foot and eventual weakening so it can no longer attach to the substratum (Friedman et al. 2000). Declines were first documented on the Channel Islands in 1985 and subsequently spread to the mainland in 1992 (Lafferty & Kuris 1993, Richards & Davis 1993, Altstatt et al. 1996, Raimondi et al. 2002). Now a federally listed endangered species, H. cracherodii’s decline continues to spread north, with little to no signs of recovery at impacted sites. WS tends to move rapidly during El Niño events, when ocean temperatures are warm; hence cooler water sites to the north may only be temporarily protected (Raimondi et al. 2002). The general pattern of mortality once die-offs begin is that the population decreases by > 95%, leaving a few scattered individuals. This scarcity of survivors is a serious threat to black abalone recovery, because individuals are often too distant from one another for successful fertilization to occur (see reproduction above). We have seen virtually no recruitment of new individuals to mainland sites that have been impacted by WS, and, only recently, minimal recruitment to island sites. Ironically, successful recruitment into areas impacted by mass mortality events may be dependent on the presence of healthy, conspecific adults. These large, long-lived grazers may maintain suitable conditions for recruitment of conspecifics by preventing colonization of other organisms by pre-empting space on the substratum and dislodging newly settled larvae or algal spores through their movements and grazing (Cox 1962, Leighton & Boolootian 1963, Blecha et al. 1992, Richards & Davis 1993, Miner et al 2006). Our work suggests that following mass mortality events, communities that formerly supported large numbers of abalone change from open areas dominated by crustose coralline algae and bare rock to habitat encrusted with increased cover of sessile invertebrates and fleshy algae (Miner et al. 2006).
In addition to WS, other sources of mortality include: smothering by sand burial, dislodgment by storm waves, and predation by octopus, sea stars, fishes, and sea otters (Morris et al. 1980; VanBlaricom 1993). Oil impacts are not well known, but black abalone mortality was documented following an oil spill in Baja California (North et al. 1965). In response to the mass mortalities along the coast of California, black abalone are now protected under the USA Endangered Species Act. An Abalone Recovery Management Plan was adopted by the state of California in 2005. In October 2011, the National Marine Fisheries Service designated critical habitat for black abalone. Various projects are in place to monitor the species’ status, better understand WS, increase knowledge about the requirements for successful reproduction and recruitment, protect and restore (where appropriate) black abalone habitat, and minimize illegal harvest.