Laboratory Exercise Using Digital Images of Specimens from the Peabody Museum

Developed from a workshop held at the Yale Peabody Museum, July 2013

Sponsored by the National Science Foundation and hosted by the Division of Invertebrate Zoology in conjunction with Peabody Museum Public Education

Morphological Variation in the Asian Shore Crab, Hemigrapsus sanguineus

General Data Gathering

• Distinguish between males and females by examining the underside of the crabs

• Make carapace measures using the dorsal images of the crabs

• Using the chelae (claw) images, measure “hand height” and “hand length” of each

• Enter raw values into Excel; distinguish between individual crabs by referencing catalog number embedded in picture

Sex Determination in Crabs

• Examine the underside of a crab (image)• Males have narrowed abdominal plate; the

sides are often somewhat concave; plate does not cover entire underside

• Females have a broadly rounded abdominal plate; lateral margins convex; plate covers most or all of underside

Underside comparison of male versus female crabs

Carapace measures, variable 1 and 2: Carapace Length (CL) and Carapace Width (CW)

Chelae measures, variables 3 and 4: Chelae Height (ChH) and Chelae Length (ChL)

Rules to remember

• To get accurate results one must be consistent in measuring

• Strive for perpendicular measures• Lengths and widths should reflect absolute

longest/widest distances

Measurement Techniques

• Use marks on paper held to screen image to measure a distance

• Compare distance to scale bar; small gradations are millimeters

• Record length to nearest 0.5 millimeter• Most accurate measure will be from the edge

of one gradation to another (black edge to black edge)

Measuring with pencil mark on paper held against image on computer screen

Compare measurement to scale bar (21 mm in this example)

Alternative measurement technique:Calipers, reversed to hide numbers (avoids confusion)

Adjustable compass or dividers can also be used for measuring

Data Capture• Students, working in teams, should first enter raw data

manually on the data worksheet provided• Data are next consolidated into an Excel spreadsheet, see next

panel for example• Note that all measures are recorded individually, before the

ratios are derived; allows for auditing of data and results• Consider highlighting in a different color

questionable/irregular variables such as indeterminable sex or left chelae substitutions (when right was missing); allows for isolation and removal from analysis if necessary

Worksheet for manual collection of data

Data Capture – Excel Worksheet 1

Analysis format – Excel Worksheet 2

Data Analysis dialog window

Data Analysis output worksheet

Data Analysis

• Simplest example will be to calculate the ratio of chelae height versus chelae length

• Comparison will be made between males and females, i.e. which has the more robust claw?

• Carapace length versus width is another simple metric to measure and compare between males and females – who is stouter?

• Refer to instructional document for details

Additional Considerations

• Many more variables can be added:• Width between the eyes (orbital width)• Length of major section of one or more legs (“femur

length” or second article)• Dactyl length (i.e. moveable “finger”) on chelae• Consider testing data without the left-hand claws –

does this make a difference?

Further Questions to Consider

• How would collecting more specimens affect the analysis?

• What other comparisons may be useful, such as temporal and geographical variables, and what would the results potentially indicate?

• What would more robust claws in one sex potentially indicate, and why?