Small macrobenthic invertebrates affect the mortality and growth of early post-settlement sea urchins and sea stars in subtidal cobble habitat

Abstract

Early post-settlement events can have a large impact on the successful recruitment of benthic invertebrates. A field caging experiment was conducted in 2007 to examine whether predation by, and/or competition with, small macrofauna affects mortality or growth of recently settled sea urchins Strongylocentrotus droebachiensis or sea stars Asterias spp. Kelp was added to half of the sea urchin cages to test whether the addition of this food source (including its associated biofilms) altered growth or survival. Sea urchins survived 25% better in cages where the other organisms were removed, indicating that predation or bulldozing likely plays an important role. The growth of sea urchins was greatest in cages without other organisms and lowest in cages with other organisms and with food, possibly indicating competition that affects sea urchin behaviour. Sea stars showed the opposite trend in survival. A greater proportion of sea stars survived in cages where the other organisms were present (31.3 vs. 11.5%), presumably due to them being a food source for the sea stars, indicating that starvation, cannibalism and/or competition for food are likely important for recent settlers of this taxon. The declines in abundance in the cages were greater than those in the natural environment for sea urchins, but similar for sea stars, suggesting that caution is required when extrapolating experimental results to the field. These results indicate that multiple factors, which differ between these species, affect early post-settlement growth and mortality. KEY WORDS: Strongylocentrotus droebachiensis · Asterias spp. · Mortality · Growth · Predation · Competition · Early post-settlement · Natural declinesResale or republication not permitted without written consent of the publisher 90% mortality in juvenile limpets due to interspecific competition, an amount comparable to predation.However, competition does not have to be fatal, and may be more likely to have sublethal impacts on processes such as growth or fecundity (Birch 1957, Tomas et al. 2005. Intraspecific competition can reduce growth at high densities (e.g. Marsden 2002, Silina 2008 or reduce growth rates for small individuals forced to compete with larger ones (e.g. Kautsky 1982). Reduced growth rates can prolong the time it takes to reach recruitment (if defined by size) and sexual maturity.When predation and competition are examined together, the presence of predators often results in a greater reduction in abundance, fecundity and growth than does the presence of competitors (e.g. Aukema & Raffa 2002, Beal 2006. However, few studies have examined the effects of both predation and competition on juveniles. In the sea urchin Evechinus chloroticus, juveniles had higher densities when predators were excluded, regardless of the presence of adult conspecifics (Andrew & Choat 1982). Beal (2006) found that, while both predation and intraspecific competition had significant impacts on the survival of juvenile clams Mya arenaria, predation accounted for 45% of the variabi...

Opsin4,Opsin5, andPax6significantly increase their expression in recently settled juveniles of the sea urchinStrongylocentrotus intermedius(Echinodermata: Echinoidea)

“…This convincingly indicates the importance of metamorphosis in the formation of photosensory elements. Survival and growth of recently settled sea urchins would be significantly affected by the presence of predators (Scheibling & Robinson 2008;Jennings & Hunt 2011). Photosensory elements are probably essential for early post-settlement sea urchins to avoid predation and consequently to have a significant impact on their successful recruitment, although relevant experimental evidence is limited to adult urchins.…”

Cannibalism among green sea urchins Strongylocentrotus droebachiensis in the laboratory and field

“…Cannibalism by larger juvenile sea urchins was hypothesized to be a possible mechanism of mortality in a field experiment where 1 to 3 mm TD S. droebachiensis settlers were placed in cages along with the suite of organisms (chitons, scale worms and larger sea urchins) found on the cobble bottom surrounding the cages in the Passamaquoddy Bay, New Brunswick (Jennings & Hunt 2011). In cages with the suite of organisms, sea urchin mortality was 25% higher than in cages without the suite (Jennings & Hunt 2011, Jennings & Hunt 2014. A follow-up laboratory experiment by Jennings & Hunt (in press) found that survival of 1 to 3 mm S. droebachiensis in cages with potential predators was lowest (70% after 8 wk) in the juvenile conspecific (mean ± SD size: 8.8 ± 2.2 mm TD) treatment.…”

“…Small juvenile S. droebachiensis (3−6 mm test diameter [TD]) are also vulnerable to predation by rock crabs (Cancer irroratus), juvenile lobsters (Ho marus americanus), and juvenile sculpins (Myoxocephalus octodecemspinosis), which can significantly reduce the abundance of recruits in the laboratory (Scheibling & Hamm 1991). Although predation by carnivores is the most frequently cited cause of post-settlement mortality for sea urchins, omnivorous grazers have recently been recognized as a potentially important source of mortality (Jennings & Hunt 2011). A field experiment in the Bay of Fundy, Canada (Jennings & Hunt 2011), and subsequent laboratory experiment (Jennings & Hunt in press) suggested that juvenile S. droebachiensis are a source of mortality for smaller conspecifics, though it is unclear whether the mortality occurred through 'bulldozing' (inadvertent crushing), or ingestion.…”

“…Although predation by carnivores is the most frequently cited cause of post-settlement mortality for sea urchins, omnivorous grazers have recently been recognized as a potentially important source of mortality (Jennings & Hunt 2011). A field experiment in the Bay of Fundy, Canada (Jennings & Hunt 2011), and subsequent laboratory experiment (Jennings & Hunt in press) suggested that juvenile S. droebachiensis are a source of mortality for smaller conspecifics, though it is unclear whether the mortality occurred through 'bulldozing' (inadvertent crushing), or ingestion. Cannibalism has been reported in several taxa of marine benthic invertebrates, including crabs (Luppi et al 2001, Moksnes 2004, Long et al 2012, lobsters (Jones 2009), gastropods (Kelley 1991, KingsleySmith et al 2005, Chiu et al 2010, polychaetes (Herringshaw et al 2010), the sea urchins Strongylocentrotus purpuratus (Sonnenholzner et al 2011) and Lytechinus variegatus (Richardson et al 2011), and, occasionally, in sea stars (Witman et al 2003).…”

Small macro‐benthic invertebrates influence the survival, growth and behaviour of juvenile green sea urchins Strongylocentrotus droebachiensis in the laboratory

“…For example, in field experiments, early post‐settlement sea stars and sea urchins survived better when the suite of small animals had been removed from the substrate (Keesing & Halford ; Keesing et al . ; Jennings & Hunt , ). Keesing & Halford () and Keesing et al .…”