Re-examination of the effects of food abundance on jaw plasticity in purple sea urchins

Abstract

Morphological plasticity is a critical mechanism that animals use to cope with variations in resource availability. During periods of food scarcity, sea urchins demonstrate an increase in jaw length relative to test diameter. This trait is thought to be reversible and adaptive by yielding an increase in feeding eiciency. We directly test the hypotheses that (1) there are reversible shifts in jaw length to test diameter ratios with food abundance in individual urchins, and (2) these shifts alter feeding eiciency. Purple sea urchins, Strongylocentrotus purpuratus, were collected and placed in either high or low food treatments for 3 months, after which treatments were switched for two additional months between February and September, 2015 in La Jolla, CA (32.8674°N, 117.2530°W). Measurements of jaw length to test diameter ratios were signiicantly higher in low compared to high food urchins, but this was due to test growth in the high food treatments. Ratios of low food urchins did not change following a switch to high food conditions, indicating that this trait is not reversible within the time frame of this study. Relatively longer jaws were also not correlated with increased feeding eiciency. We argue that jaw length plasticity is not adaptive and is simply a consequence of exposure to high food availability, as both jaw and test growth halt when food is scarce.

Tooth Mg/Ca ratios and Aristotle's lantern morphometrics reflect trophic types in echinoids

“…A. lixula and P. lividus alter the size of their mouth appendages in relation to the size of the test, while S. granularis and D. setosum exhibit variation regarding the elongation index of the teeth. Food availability has long been hypothesized to affect the relative length of the demi‐pyramid and Aristotle's lantern in some echinoid species (e.g., Black et al., 1982 ; Ebert, 1980 ; Ebert et al., 2014 ; except deVries et al., 2019 ). The results regarding P. lividus and A. lixula might reflect food scarcity for these species in winter in Pagasitikos gulf, which is also supported by another study (Petihakis et al., 2005 ) (Figure 4 ).…”

“…However, this is not necessarily a plastic response to food scarcity but might be related to different growth rates due to variation in food availability, as shown by deVries et al. ( 2019 ). Hagen ( 2008 ) proposed that the enlargement of the lantern is a functional specialization for durophagy.…”

Supplemental Information 4: An explanation of the data file and the code used in this study.

“…In patches of kelp forests, sea urchins are often at low densities and primarily employ a risk-averse passive-grazing strategy on detrital (hereafter, 'drift') kelp (Filbee-Dexter & Scheibling, 2014). However, in sea urchin barrens where drift is limited, sea urchins alter their behavior, emerge from the refuge space of crevices, and actively graze at high densities on nutrient-poor encrusting algae species (Harrold & Pearse, 1987;Steneck et al, 2002;DeVries, Webb & Taylor, 2019). Therefore, the success and survival of sea urchins in these two distinct habitats may be strongly dependent on the ability of sea urchins to modify resource allocation from reproduction (i.e., gonad development) to survival (i.e., metabolic maintenance).…”