Scale Thickness Predicts Skin Puncture-Force Resistance in Three Pleuronectiform Fishes.

In the ray-finned, or actinopterygian, fishes, the skin and scales form the integument, which mediates the interaction between the animal and the external environment. Because ray-finned fish species live in a wide variety of environments—pelagic (open-ocean), benthic (bottom-dwelling), and in some cases even terrestrial (on land) habitats—scales have been modified to serve distinct purposes. In some fishes, such as Lepidopsetta polyxystra (northern rock sole), scales may enhance crypsis (Ryer et al. 2004). For many pelagic species, scales reduce the friction drag experienced as a fish swims through the water (Fish 1998). In benthic species, scales may serve to increase friction between the animal and the substrate (Spinner et al. 2016), a feature that could improve a fish’s ability to entrap substrate particles on top of the body, thus disguising its presence. Beyond the functions listed above, scales and skin must resist mechanical forces that could puncture the fish’s integument. Among fishes, flatfishes are unusual because they are bilaterally asymmetrical and spend their lives resting on the bottom, with one side of the body in direct contact with the substrate. Flatfishes hatch as symmetrical larvae, but one eye migrates over the top of the head to the opposite side of the body during metamorphosis. As a result, in flatfish adults, one side is “eyed” (two eyes) and the other side is “blind” (eyeless) and a flatfish typically rests on the bottom with the blind side in direct contact with the substrate. Adult flatfishes are asymmetrical in the jaws and paired fins, and flatfishes are also typically unpigmented on the blind side of the body. Most flatfish species have either embedded (under the epidermis) cycloid scales or overlapping ctenoid scales, but occasionally they have protruding, tuberculate scales, as seen in the genus Platichthys (Roberts 1993). Flatfishes are a useful model system in which to study functional aspects of scales because the environment can potentially impose different selective forces on the eyed vs. the blind side of the body. When burying in the substrate, flatfish cyclically oscillate the entire body in close proximity to the granular medium that comprises the substrate. Thus, as a normal part of a flatfish’s life history, the ventrally-oriented blind-side is repeatedly forced against, rough, potentially damaging particles. In addition, the blind side of a flatfish is vulnerable to a variety of parasites or predators that live in the benthos and attack the fish from below, while the eyed side of the fish faces a different set of challenges, including free-living aquatic predators and a distinct cadre of parasites that may attack fish from the water column. Indeed, the scales of some flatfish species are markedly different on one side of the body versus the other. For example, Citharichthys sordidus, the Pacific sanddab, has ctenoid scales on the eyed side, which could serve to reduce friction drag with the water flowing over the fish, but possesses cycloid scales on the blind side where the body is in direct contact with the substrate (Batts 1964). In contrast, although Pleuronectes platessa, the European plaice, possesses cycloid scales on both the eyed and blind sides of the body, the eyed-side scales have been demonstrated to generate greater frictional forces when interacting with the sediment, relative to blind-side scales (Spinner et al. 2016). Thus, the scales on the eyed vs. the blind side are functionally different from one another in P. platessa, even though they are of the same type. The ability of the integument to resist puncture forces is a function of the material properties of the cells and connective tissues that form the skin and scales (Ikoma et al. 2003), in combination with properties of the structure that is attempting to penetrate the integument. For example, shark teeth (Whitenack and Motta 2010), insect proboscises (Oka et al. 2002), or cactus spines (Crofts and Anderson 2018) all have been demonstrated to pose distinct mechanical challenges to biological materials. Indeed, material scientists have documented the properties of offensive and defensive structures and used this information when designing microneedles (Oka et al. 2002), flexible armors (Zhu et al. 2013), and other human-serving devices (Chen et al. 2012). Puncture resistance of defensive structures such as fish scales is likely to be influenced by the three-dimensional shape of the scale: length, width, thickness, and overall topography as well as intrinsic material properties. Of the shape parameters, scale depth is perhaps likely to be the most important, as puncture resistance has been hypothesized to be strongly influenced by scale thickness (Ikoma et al. 2003; Whitenack and Motta 2010; Bergman et al. 2017). Assuming that scale thickness varies in proportion to animal size (i.e., assuming scale thickness is directly proportional to total fish length), then resistance to puncture forces is expected to vary in direct proportion with body size. Under this assumption, we would predict that, as a fish doubles in length, the ability of the animal’s integument to withstand puncture forces will also approximately double. However, we note that it is possible that scale thickness varies among species, with some species having markedly thicker scales, or that the relationship between fish size and scale thickness is not consistent (varies) among species. In this study, we quantified the relationship between body size, scale morphology, and puncture-force resistance in three species of pleuronectid (family Pleuronectidae) flatfishes from the Northeastern Pacific Ocean. To investigate how scale type may influence morphological (e.g., thickness) and functional (e.g., puncture-resistance) characteristics, we chose three species with different scale types. Parophrys vetulus (English sole) has cycloid scales, Isopsetta isolepis (butter sole) has ctenoid scales, and Platichthys stellatus (starry flounder) has tuberculate scales (Roberts 1993; Batts 1964). Because Platichthys have prominent, physically pointed, tuberculate scales, which suggests that they are, in some sense, “armored,” we predicted that Platichthys would have the greatest puncture-resistance of the three species considered here. However, we also expected that puncture resistance would be heavily influenced by scale thickness, because the thickness of a scale represents the dimension of a scale that contains the material that resists compressive forces applied to the body. By quantifying body size, scale dimensions, and puncture resistance in three flatfish species with different scale types, we sought to answer the following questions. (1) Do scales of a given flatfish species demonstrate the same morphological dimensions and functional properties on the eyed vs. blind sides? (2) Does the relationship between scale morphology/performance vary with body size in a consistent manner across species? (3) Do scales of different types (cycloid, ctenoid, tuberculate) appear to have different abilities to resist puncture forces? (4) Alternately, is the ability to resist puncture-forces best explained by simple morphological parameters—particularly scale thickness? To answer these questions, we examined a size range individuals representing three flatfish species to enable us to calculate intraspecific scaling (proportional) relationships for body size, scale dimensions, and performance parameters. A scaling-based approach enables us to determine if key characteristics of flatfish scales, such as thickness, remain consistent within a species individuals grow larger and if the relationship between size and morphology/function is the same in the three flatfish species considered here.