Repair and remodelling in the shells of the limpetPatella vulgata

Abstract

Limpets and other molluscs rely on shells to protect them from physical damage, predation, dehydration, etc. If the shell becomes damaged, this may significantly impair its function. In this work, experiments were carried out to investigate the effect of damage on the strength of shells of the common limpet () and their ability to repair this damage effectively. Shells were damaged in three ways: (i) low-energy impacts; (ii) abrasion of the outer layer; and (iii) creation of a small hole in the apex of the shell. Shells were left to repair for several time periods (0, 10, 30 and 60 days). The mechanical strength was evaluated by impacting the shells with a weight dropped from a known height. The damage reduced the strength (defined as impact energy to failure) by 50-70% depending on damage type. After 60 days, limpets in all three groups had repaired their shells significantly, bringing their strength to 79-91% of the control value (in each case, samples were statistically indistinguishable from their control counterparts). Measurements of the thickness of the shell at the apex suggest that the main effect of low-energy impact and abrasion is reduction in thickness, which correlates linearly with the impact energy needed for failure. The method of repair is believed to be by the growth of fresh shell material on the inside of the shell, though we could not identify this new material specifically. Even after 60 days, the shells were still statistically thinner than the controls. Consequently, there may be some other strengthening mechanism at work. This work has demonstrated the remarkable ability of limpets to detect the mechanical weakening of their shells caused by relatively subtle forms of damage and to take appropriate action to restore shell strength.

Mechanical fatigue fractures bivalve shells

“…mussels' sensitivity to the micro-damage associated with fatigue is unknown. Other mollusks have been shown to repair fatigue damage (LaBarbera and Merz, 1992;O'Neill et al, 2018), and to patch internally in response to external wear (limpets: Cadée, 1999;O'Neill et al, 2018;pteropods: Peck et al, 2018). The ecological threat of fatiguing forces will depend on the mussel's ability to sense and repair fatigue, as well as the time required to repair the shell, the cost of repair, and the mechanical properties of the shell once repaired.…”

Bivalves rapidly repair shells damaged by fatigue and bolster strength

“…Research in other mollusks has documented repair of holes beginning instead with formation of a non-periostracum collagenous ( Su et al, 2004 ) or brownish chitinous layer ( Cho and Jeong, 2011 ). Although repair visible by microscopy could not fully explain the mechanical results, repairing limpets show a similar mismatch between timelines of mechanical evidence of repair and extent of internal thickening ( O'Neill et al, 2018 ). In the case of our mussels, other methods of visualizing repair might reveal patterns of direct response not visible with calcein staining.…”

“…The timeline of repair in the California mussel exceeds expectations. Other hard-shelled mollusks can take weeks to months to repair fatigue damage ( LaBarbera and Merz, 1992 ; O'Neill et al, 2018 ). In response to perceived threats, many mollusks will alter shell form by, for example, thickening or strengthening the shell as a whole or in localized regions such as the aperture of snail shells; these alterations can take 2–3 months and have primarily been examined in response to environmental or chemical cues ( Appleton and Palmer, 1988 ; Bourdeau, 2010 ; Bourdeau and Padilla, 2019 ; Currey and Hughes, 1982 ; Leonard et al, 1999 ).…”

“…Scars from damage to the shell's edge are common, documenting survived encounters with predators and continuing growth ( Blundon and Vermeij, 1983 ). Many mollusk species also patch drill holes ( Meenakshi et al, 1973 ; Sleight et al, 2015 ) or thicken internally in response to external wear or damage ( O'Neill et al, 2018 ; Peck et al, 2018 ). Although an initial repair response has been documented within hours or days ( Mount et al, 2004 ; Reed-Miller, 1983 ), studies focusing on mechanical properties of gastropods have not found full repair for weeks to months ( LaBarbera and Merz, 1992 ; O'Neill et al, 2018 ).…”

“…Many mollusk species also patch drill holes ( Meenakshi et al, 1973 ; Sleight et al, 2015 ) or thicken internally in response to external wear or damage ( O'Neill et al, 2018 ; Peck et al, 2018 ). Although an initial repair response has been documented within hours or days ( Mount et al, 2004 ; Reed-Miller, 1983 ), studies focusing on mechanical properties of gastropods have not found full repair for weeks to months ( LaBarbera and Merz, 1992 ; O'Neill et al, 2018 ). Notably, most experimental studies of shell repair focus on fractures or holes that fully pierce the shell, in contrast to the accumulating fracture hypothesized to be associated with repetitive loading.…”