Acidic Apple Snails: Effects of Climate Change on the Mechanical Properties of an Invasive Gastropod.

Climate change can directly and indirectly affect species distribution. Warming may allow for invasive species, such as apple snails, to migrate to higher latitudes where temperatures are more conducive to their survival and invasion success. Higher temperatures and lower pH ranges have been previously documented to affect the form and function of calcium carbonate shells, which serve many functions, including protection from predators and thermoregulation. This study aimed to quantify differences in the morphology and mechanical properties of invasive apple snail, Pomacea maculata, shells after altering temperature and pH. We mechanically tested shells among three five-week treatments: control, higher temperature, and lower pH. Ultimate Strength increased in shells that were exposed to higher temperatures, but Young's Modulus and Peak Load did not differ among control, temperature, and pH treatments. Apple snails in higher temperature tanks increased their shell length over the five-week trials. Although snail morphometrics did not differ between sexes, male shells exhibited a higher Peak Load, Young's Modulus, and Ultimate Strength compared to female shells. Our findings are consistent with previous gastropod studies, in that a lower pH is associated with a decrease in shell size, and higher temperatures yield larger snail shells with a higher ultimate strength. Peak Load did not significantly differ among treatments, which suggests that the cross-sectional area is relatively important when considering this species mechanical performance today and in future climates. Due to the intense nutritional and calcium demands of egg production, female snails may be more susceptible to weakened shells due to low pH environments caused by climate change.

Caudal Spine Morphology and Puncture Performance of Two Coastal Stingrays.

A diagnostic characteristic of stingrays in the family Dasyatidae is the presence of a defensive, partially serrated spine located on the tail. We assessed the contribution of caudal spine morphology on puncture and withdrawal performance from two congeneric, co-occurring stingrays, the Atlantic stingray, Hypanus sabinus, and the bluntnose stingray, Hypanus say. Spines exhibited a high degree of morphological variability. Stingray spines were serrated along 50.8% (H. sabinus) or 62.3% (H. say) of their length. Hypanus say had a greater number of serrations along each side of the spine (30.4) compared with H. sabinus (20.7) but the pitch did not differ between species. We quantified spine puncture and withdrawal forces using porcine skin as a model for human skin. Puncture and withdrawal forces did not differ significantly between species, or within H. say, but withdrawal force was greater than puncture force for H. sabinus. We incorporated micro-computed tomography scanning to quantify tissue mineral density and found that for both species, the shaft of the spine was more heavily mineralized than the base, and midway (50%) along the length of the spine was more heavily mineralized than the tip. The mineralization variability along the spine shaft may create a stiff structure that can fracture once embedded within the target tissue and act as an effective predator deterrent.

Dermal modifications of the red lionfish, Pterois volitans.

Vertebrates have evolved flexible and hard dermal armor, but there is often a tradeoff between mobility and protection. Skeletal modifications include fish scales, the shells of tortoises and turtles, and the cranial projections that are found in marine iguanas and lionfish. Sexual dimorphisms of cranial structures are prominent in fishes; we hypothesized that cranial projections of Pterois volitans will change over ontogeny and are sexually dimorphic. The goal of this study is to quantify differences in the cranial armor of P. volitans over a range of sizes (juveniles to adults) to understand allometric growth and possible sexual dimorphisms using specimens from invaded ranges. We found no difference in the total number of cranial spines between sexes, suggesting that the number of cranial spines is not a sexually dimorphic trait in P. volitans. We found positive allometry for total cranial spine number, and length and width of the three largest cranial spines, with respect to the fish total length. Here, we also document another skeletal modification in P. volitans, which is the addition of spines on the caudal fin. Overall, these data show that P. volitans' develop robust dermal modifications as they grow larger, and we suggest that phenotypic plasticity of this species should be explored in their native and invaded ranges.

Mechanical properties of the venomous spines of Pterois volitans and morphology among lionfish species.

The red lionfish, Pterois volitans, an invasive species, has 18 venomous spines: 13 dorsal, three anal and one on each pelvic fin. Fish spines can have several purposes, such as defense, intimidation and anchoring into crevices. Instead of being hollow, lionfish spines have a tri-lobed cross-sectional shape with grooves that deliver the venom, tapering towards the tip. We aimed to quantify the impacts of shape (second moment of area) and tapering on the mechanical properties of the spine. We performed two-point bending at several positions along the spines of P. volitans to determine mechanical properties (Young's modulus, elastic energy storage and flexural stiffness). The short and recurved anal and pelvic spines are stiffer and resist bending more effectively than the long dorsal spines. In addition, mechanical properties differ along the length of the spines, most likely because they are tapered. We hypothesize that the highly bendable dorsal spines are used for intimidation, making the fish look larger. The stiffer and energy-absorbing anal and pelvic spines are smaller and less numerous, but they may be used for protection as they are located near important internal structures such as the swim bladder. Lastly, spine second moment of area varies across the Pterois genus. These data suggest there may be morphological and mechanical trade-offs among defense, protection and intimidation for lionfish spines. Overall, the red lionfish venomous spine shape and mechanics may offer protection and intimidate potential predators, significantly contributing to their invasion success.

Performance of teeth of lingcod, Ophiodon elongatus, over ontogeny.

Fish teeth can play several roles during feeding; capture, retention, and processing. In many fish lineages teeth may be present on non-jaw cranial bones that lack opposing teeth, such as the vomer and palatine. We hypothesized that teeth on different bones have different functions, and that the function of a set of teeth may vary over ontogeny. In this study, puncture, and draw performance of in situ vomerine teeth are compared to premaxillary teeth of the piscivorous lingcod, Ophiodon elongatus. The force required to pierce prey and to draw prey out of the mouth once the teeth were embedded was measured in ten individuals ranging from 205 to 836 mm SL to test for ontogenetic effects. Vomerine teeth in juvenile lingcod required proportionally less force to puncture prey items than adult lingcod, while premaxillary teeth showed the opposite trend. Draw force required to remove prey from the grasp of both toothed bones show the same shift with ontogeny. These results suggest that there is a shift in tooth function from vomerine to premaxillary teeth over ontogeny of lingcods. In juvenile lingcod, vomerine teeth function more effectively during initial puncture. In contrast, the premaxillary teeth pierce more effectively in adults. Juvenile lingcod are expected to use the premaxillary teeth while adult lingcod are expected to use the vomerine teeth to retain prey due to the larger force required for the prey to escape. The curvature of vomerine teeth increases over ontogeny suggesting increasing functional performance in retaining prey.