Overcoming the constraints of spiral growth: the case of shell remodelling

Abstract

As animals grow in size, their relationship to the physical environment necessarily changes, but molluscs and brachiopods whose accretionary skeletons expand at one end of a hollow cone conform to logarithmic‐spiral growth and retain a constant shape. Dissolution and remodelling of previously formed parts of the skeleton can alleviate the constraints of strict logarithmic‐spiral growth. How, when, where and in which clades mineral skeletal resorption has evolved are important questions because they relate to the conditions and history of skeletal formation and to the way in which ocean acidification in the past influenced that history. A synthesis of data on mineral dissolution in shells shows that resorption from the inner surface of bivalve shells occurs under temporarily anaerobic conditions within the closed shell, but functional remodelling is unknown in bivalves. Resorption and functional remodelling occur in brachiopods, gastropods and terrestrial hermit crabs, and to a lesser extent in scaphopods and cephalopods. Internal whorl resorption leading to a more compact visceral mass has evolved at least ten times in gastropods. Contrary to expectations stemming from patterns in the availability of calcium, gastropod remodelling is a phenomenon of warm, calcium‐rich environments and not of cold acidified conditions. There is therefore no evidence that internal whorl resorption increases calcium‐use efficiency. Resorption is one of several mechanisms that have enabled animal skeletons to become more dynamic and adaptable during ontogeny.

The ammonite septum is not an adaptation to deep water: re-evaluating a centuries-old idea

“…This follows from the observation that shell repair of the phragmocone is generally not possible [44], and remodelling and resorption are uncommon in cephalopods outside of resorption of some external ornamentation that occurs in the whorl overlap region [45]. This is unsurprising since the septa limit the rear movement of the soft body and prevent large-scale remodelling of the previous whorls, as seen in some gastropods [46], as well as any potential repairing of the phragmocone. However, there is no positive correlation between the proportion of shell injuries and septal complexity [47].…”

Deep resilience: An evolutionary perspective on calcification in an age of ocean acidification

“…Fine-scale sampling approaches will be needed to isolate calcifying fluids, and to identify different communities within skeletons, which have the potential to be highly heterogeneous (Marcelino et al, 2018). Perhaps the most important factor influencing deep resilience over time is an organism's power budget, the amount of energy and time available to an organism to carry out the many competing functions of life (Vermeij., 2020). There have been demonstrated increases in maximum power over time in calcifying as well as other organisms including plants (Vermeij., 2020), implying that more resources have been devoted to the construction and maintenance of external skeletal protection over time.…”

“…Perhaps the most important factor influencing deep resilience over time is an organism's power budget, the amount of energy and time available to an organism to carry out the many competing functions of life (Vermeij., 2020). There have been demonstrated increases in maximum power over time in calcifying as well as other organisms including plants (Vermeij., 2020), implying that more resources have been devoted to the construction and maintenance of external skeletal protection over time. Energetic costs of calcification may be relatively low, and are calculated to only increase modestly (~10%) under near-term ocean acidification scenarios (Spalding et al, 2017).…”

“…Studies using archival museum collections to study historical trends in calcifiers have identified multiple examples in molluscs and brachiopods where shifts in mineral layering and/or shell thickness have compensated for increased exposure to ocean acidification (Cross et al, 2018;Bullard et al, 2021;Telesca et al, 2021;Mayk et al, 2022). Finally, the ability of some calcifiers to regulate shell dissolution-such as those that perform bioerosion or those that dissolve parts of the shell to generate new shapes (Vermeij, 2020)suggests that unknown genetic controls of decalcification exist in some lineages. How species are capable of performing these changes in mineralogy, and which groups are capable of performing them, are largely unknown.…”

Sodium molybdate does not inhibit sulfate-reducing bacteria but increases shell growth in the Pacific oyster Magallana gigas

“…Sulfate reducing bacteria are considered the most likely candidates for aiding in the shell growth of molluscs such as oysters, potentially colonizing the extrapallial fluid between the shell and mantle and/or fluid-filled spaces within the shell itself [ 22 ]. Oysters also undergo periods of anoxia when their valves are shut [ 23 ], which potentially provides additional opportunities for anaerobic sulfate reducers. Still the degree to which sulfate reducers drive carbonate precipitation, particularly in animals, remains controversial.…”