EFFECTS OF SEA URCHINS (PARECHINUS ANGULOSUS) ON RECRUITS AND JUVENILES OF ABALONE (HALIOTIS MIDAE)

Abstract

Much current research on community ecology concerns indirect effects, cascading trophic interactions, and the role of top‐down vs. bottom‐up processes, as well as subsidization of matter and energy between different systems. We addressed all these topics by exploring interactions between the urchin Parechinus angulosus and both recruits and juveniles of the abalone Haliotis midae, in the light of the fact that juvenile abalone shelter beneath the urchins. In field experiments at six sites, urchins were eliminated from removal plots to determine the degree to which juvenile abalone (individuals 3–35 mm in length) depend upon urchins. Comparisons were made with undisturbed and disturbance‐control plots. We hypothesized that after removal of urchins: (1) there would be an immediate decline of juvenile abalone, since they are intimately associated with the urchins; (2) abalone recruits (recent settlers <3 mm in length) would be unlikely to suffer immediately or directly because, unlike juveniles, they are only weakly associated with the urchins; (3) foliar algae would proliferate due to decreased grazing by urchins; (4) there would be a consequent reduction in crustose coralline algae due to overgrowth by foliar algae; and (5) the decline of crustose corallines would reduce recruitment of abalone due to a loss of settlement and nursery habitat. In urchin‐removal plots, juvenile abalone declined dramatically, becoming virtually extinct locally. Their disappearance was slowest in crevices compared with exposed habitats, but even there they failed to survive longer than five months. Contrary to expectations, abalone recruits also disappeared from urchin‐removal plots, possibly because the cover of sediment almost doubled there. Also contrary to our hypotheses, neither foliar macroalgae nor encrusting corallines responded to urchin removal. The explanation of these results is that P. angulosus was able to gather drift kelp when this food source was available, rather than acting as an active grazer. This feeding pattern has profound consequences and explains why urchins failed to influence algal community composition. By sheltering beneath urchins, juvenile abalone gain protection from predators and may also benefit from an enhanced food supply. Bottom‐up subsidization of urchins by drift kelp is thus responsible for the feeding behavior of the urchins, and indirectly facilitates their role in harboring juvenile abalone. This role is of immense importance for the lucrative abalone fishery, and depletion of urchins could lead to a collapse of abalone populations. Human exploitation of top predators that consume urchins, such as rock lobsters and some fishes, has conceivably exaggerated the role of urchins in sustaining juvenile abalone.

Recruitment failure and shifts in community structure following mass mortality limit recovery prospects of black abalone

“…In particular, changes in community structure at Stairs were largely driven by an increase in the number of urchins, which may compete with abalone for resources (Davis et al 1992, Day & Fleming 1992, but may also maintain appropriate habitat for abalone recruitment through grazing of foliose algae and increasing the cover of crustose coralline algae (McShane 1992). Additionally, abalone recruits might benefit from the presence of sea urchins as a source of shelter from predation, as well as an enhanced food supply (Rogers-Bennett & Pearse 1998, Day & Branch 2002b, Day & Branch 2002c, Tomascik & Holmes 2003, although this relationship has not been demonstrated for Haliotis cracherodii and the relatively short-spined Strongylocentrotus pupuratus, which predominates in this intertidal region. Therefore, in the narrow crevices at Stairs it might be argued that habitat suitable for abalone recruitment persisted following the mass mortalities, yet there was still no abalone recruitment, supporting the contention that inadequate larval supply is limiting black abalone recovery.…”

Intensity of herbivory on kelp by fish and sea urchins differs between inshore and offshore reefs

“…However, a study at a nearby location also found low rates of grazing, although densities of sea urchins were markedly higher (5.8 m -2 ; Vanderklift & Wernberg 2008). There is increasing evidence that, although grazing by sea urchins undoubtedly modifies ecosystems at some places, density of sea urchins alone does not lead to high rates of grazing, as many species assume a drift-feeding behaviour (Castilla & Moreno 1982, Harrold & Reed 1985, Day & Branch 2002a, Shears et al 2008.…”

Historical isolation and hydrodynamically constrained gene flow in declining populations of the South-African abalone, Haliotis midae

“…Because of its size, meat quality and market value, it is considered as one of the most soughtafter and valuable marine molluscs of the African continent. Consequently, it suffers from overexploitation and together with ecological factors such as changes in habitat quality and predation pressure, has declined significantly over the last few decades (Hauck and Sweijd 1999;Day and Branch 2002). The species has also been listed in CITES Appendix III in 2007 but has since been removed.…”