Sandy Beach Monitoring: Sand Crabs

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Sandy Beach Monitoring Sand Crabs
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Sandy Beach Monitoring

• Long-term project to gather baseline data
• Time commitment required by the teacher and the
  students
• Training session
• Up to three monitoring days per year

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Objectives

• What is the community of sand crabs? Described
  in terms of the crabs size, gender, and
  reproductive status.
• What is the abundance of sand crabs? Described
  in terms of density.
• What is the distribution of sand crabs along a
  transect?
• How does the community of sand crabs change
  throughout the year?

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Sand Crabs (Emerita analoga)

• Also known as the Pacific mole crab
• Live in sandy beach, a few centimeters beneath
  the sand.
• In the swash zone, head up, facing the waves
• Move with water and the tide
• Burrow backwards
• Live in aggregations
• Filter feed on plankton with large second
  antennae as the waves recede

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• Emerita analoga live along the Pacific coast
• from Alaska to Baja California in the northern
  hemisphere and
• between Ecuador and Argentina in the southern
  hemisphere.
• Emerita resemble another sand crab, the spiny
  sand crab (Blepharipoda occidentalis). The spiny
  sand crab is larger (up to 6 cm) and lives in the
  subtidal zone. Blepharipoda feed on dead
  Emerita.

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Life History of Sand Crabs

• Reproduction from February through October,
  mostly in the summer
• Males are half the size of females
• Females produce one clutch per month of 50 to
  45,000 eggs
• Larvae spend 4 1/2 months as plankton ...

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Pelagic Larval Sand Crabs
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Settlement and Life on the Beach

• Crabs settle on a beach at about 4 mm in size and
  are considered recruits until they are 9 mm in
  size.
• When crabs are recruits you cannot determine if
  they are male or female.
• Females live about 2-4 years, males live about 1
  year.
• High density on beach in April through October.
• Low winter density
• Mortality
• Offshore migration to sand bars

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Predators

• Coastal birds Surf scoters
• Shorebirds Sanderlings, Willets
• Fish Surfperch
• Sea Otters
• Crabs may cause death to its predators
• carry parasites which can kill birds, fish, and
  sea otters
• accumulate toxic chemical from their food
  (diatoms) which can kill birds

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Prior to Monitoring you must

• Prepare equipment
• Learn sand crab natural history
• Learn how to set-up a transect and collect sand
  crabs
• Learn how to measure crabs
• Learn how to identify males, recruits, females,
  and females with eggs
• Learn how to complete data sheets

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Sampling Scheme
shore
swash zone
5 random transects Samples collected every 0.5 or
1 m
ocean
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Equipment

• Sampling tubes
• Sieves
• Calipers (metric)
• Survey line
• Survey flags
• Measuring tape (metric)
• Compass
• Data sheets

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Goals for First Day at the Beach

• Collect sand crabs
• Practice technique
• Identify males and females
• Measure length
• Determine sampling interval
• Establish station location
• Complete data sheets

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Length Measurement
carapace length
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Male or Female?
Males Smaller (up to 22 mm)
Females Larger (up to 35 mm)
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Data Analysis

• Database in Excel
• Graph the size frequency
• Graph the distribution of crabs along a transect
• Compare distribution between transects

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Extensions

• Share results with other schools that are
  monitoring the sandy beach
• Post results on the Internet
• Create a poster for display at school
• Present the project at a conference
• Create a kiosk at your site

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Future Additions to the Project

• Parasite study
• Dissect crabs to look for parasites
• Domoic acid study
• Send the crabs to a lab to look for the presence
  of domoic acid