Trace element homogeneity in bottlenose dolphin skin
By Colleen Bryan, MSc, PhD student, Medical University of South Carolina, Lori Schwacke, PhD, NOAA National Ocean Services, and Steven Christopher, PhD, National Institute of Standards and Technology

Little is known about the distributions, concentrations, or types of trace elements in cetacean skin. Cetacean skin has physiology that is unique from other marine mammals, allowing for distinct trace element accumulation patterns. Trace elements mainly accumulate in the multilayered epidermis and dermis. The high lipid content in blubber prevents the deposition of trace elements since many elements are not lipophilic. Previous studies have shown that some elements, such as mercury, are not homogeneous in skin on a micro-scale. We are referring to micro-homogeneity as homogeneity between dermal layers. No studies have examined trace element homogeneity on a macro scale, where trace element homogeneity in skin is assessed across an animal's entire body and this is the focus of the present work. Skin may be useful for assessing anthropogenic trace element contamination and population stocks. High variability in the trace element concentrations among different geographic areas and regional patterns may be used to delineate stocks.

Skin is a non-lethal sampling tissue being considered for monitoring of trace elements in wild free-ranging bottlenose dolphin populations. Currently published data are deficient for a broad suite of trace elements in bottlenose dolphin skin. Skin is always collected when wedge and dart biopsies are taken and has the potential to allow for the assessment of various trace elements. Many published studies do not indicate a standardized location when skin is obtained from a live or stranded animal and dart biopsies have a greater potential for location variation. This study addresses whether or not trace element levels in bottlenose dolphin skin are homogenously distributed across an animal's entire body, or if there is severe heterogeneity that would preclude the use of skin as a reliable non-lethal monitoring compartment.

Bottlenose dolphin skin samples were collected from twelve standardized locations from the body from two freshly dead animals to assess skin for trace element distribution and across body homogeneity (see figure below). The sample sites on the animals were divided into two types of transects: lateral and longitudinal. The lateral transects, depicted by letter codes B, C, D, and E in the figure, were oriented vertically dividing the animal into planes around the girth anterior to posterior. The longitudinal transects, depicted by number codes 1, 2, and 3 run lengthwise, dividing the animal into planes dorsal to ventral.

Skin samples were analyzed for Al, V, Mn, Cu, Zn, As, Se, Rb, Sr, Mo, Cd, and Pb by inductively coupled plasma mass spectrometry (ICPMS) and Hg by atomic fluorescence spectrometry (AFS). Zinc had the highest (›100,000ng/g wet mass) concentration in dolphin skin; indicating skin could be a target organ for zinc deposition. Vandium, Mo, Cd, and Pb had the lowest (‹10ng/g wet mass) concentrations in dolphin skin. Relative standard deviations of less than 20% between sample sites on each animal for Cu, As, Se, and Hg demonstrate that deposition of these elements may be tightly regulated in skin tissue. Multifactor mixed-effect analysis of variance analyses (ANOVA) showed significant (p‹0.05) effects laterally (anterior-posterior) for As and longitudinally (dorsal-ventral) for Mo, Ru, Se, V, and Zn, indicating that standardized sample location collection is needed for accurate evaluation of these elements between animals due to non-homogeneous deposition in skin. Use of a standardized skin collection region will assist with comparing trace element data between cetacean studies.

Researchers should consider whether cetacean skin samples are coming from live or dead animals. If samples are collected from a dead animal, skin should be collected from a freshly dead animal to ensure that the skin has not begun to decompose, and comments on the skin condition should be put forward when reporting concentration data. Decomposing skin may not be representative of true trace element concentrations since layers would begin to slough off from the full thickness of the skin. Future studies will include correlation analyses between blood and skin to determine the degree of correlation between these tissue types for certain elements. A separate study will examine dart biopsy samples. Dart biopsies provide limited skin sample mass that may not be representative of bulk skin and the sample size may restrict determination of trace elements present at ultra low concentrations due to analytical instrumentation detection limits. Support for this project has been provided by NOAA Fisheries and the National Institute of Standards and Technology.