Kerry Quarry, Bonnie Bain, Fredric Govedich, Stephen Shuster

1
Sexual Dimorphism in Sea Spiders
College of Engineering and Natural Sciences
Kerry Quarry, Bonnie Bain, Fredric Govedich,
Stephen Shuster
Department of Biological Sciences, Northern
Arizona University, Flagstaff, AZ 860111
Additional Results
Abstract Pycnogonida (sea spiders) show varying
degrees of sex role reversal. Males are smaller
than females and possess specialized limbs
(ovigers) for offspring care. Sex differences are
thought to have arisen via sexual selection,
making the Pycnogonida well suited for
investigating this process, although sexual
dimorphism in sea spiders is poorly known. We
documented morphological
Introduction Sea spiders (Arthropoda, Class
Pycnogonida), are marine chelicerates found
throughout the world in many different habitats
including intertidal zones to the abyssal depths.
Although spider-like in appearance, they are not
true spiders, and are instead more closely
related to the Acari, which includes ticks and
mites. Sexes are separate in sea spiders and
mature gonads in both males and females extend
into the walking legs, usually to the distal end
of the femur. Ammothea hilgendorfi (Cole, 1904)
is a common west coast pycnogonid species,
inhabiting intertidal zones from Tijuana, Mexico
to San Francisco, California where it is usually
found clinging on to the byssal threads of
mussels or walking around on hydroid beds.
Mature Female
Immature Female
Area
differences in male and female walking legs in a
common California pycnogonid, Ammothea
hilgendorfi. Like most sea spiders, females in
this species carry mature ova in the first 3
segments of their walking legs. The bulk of the
ova lie within the femur. The male gonad is
located in these same segments, but
Both mature and immature female femurs had
longest mid-femur widths and coxal end widths
were longer than tibial end widths.
occupies a much smaller volume than the female
gonad. We measured femur length and 3 different
femur widths (coxal end, midfemur, tibial end)
for each specimen (n 12) and calculated the
area of each segment. Male and female femur
lengths did not differ but females had longer
midfemur/coxal end width and midfemur/tibial end
width, and thus larger femur area compared to
males. Immature and mature females had similar
femur dimensions. Thus, while females with larger
femurs may produce more offspring than females
lacking such a modification, we cannot eliminate
the hypothesis that selection has not also
favored narrower femurs in males.
Egg size and egg number did not show a
significant relationship to femur size.
Results

• Methods
• 209 specimens were collected over a period of
  one year near Mission Bay, San Diego, CA, USA.
  Sex ratio was determined to be 5050 based on the
  entire collection.
• All specimens from one collecting trip (n 75)
  were used to prepare whole mounts for further
  study. One walking leg per individual was made
  into a whole mount using standard techniques. Of
  the 75 specimens, four were males and 71 were
  females.

• Conclusions
• Mature females and males are sexually
  dimorphic.
• Femur area is larger in females than in males
  due to greater female femur width than males
  femur lengths were not significantly different
  between the sexes.
• Females had longer midfemur/coxal end width and
  midfemur/tibial end widths compared with males.
  Male femurs are more tapered at each end than
  female ones.
• Both immature and mature females had similar
  femur measurements, indicating female femur shape
  is independent of sexual maturity. Both mature
  and immature females had relatively long midfemur
  widths and a longer coxal end width than tibial
  end width.
• Egg size and egg number did not have a
  significant relationship with femur size,
  demonstrating that increased femur size does not
  increase fecundity.

Femur Measurements

• Mature and immature females were separated based
  on the number and size of eggs within the femur.
  Four mature and four immature females were
  randomly selected for comparison to the four
  males.
• Photographs were taken with Motic Educator.
• Femur length (coxal end-tibial end) and width
  (midfemur) were measured with ImageJ
• Area was determined by length x width.
• For 4 males, 4 mature, and 4 immature females,
  additional width measurements were taken (coxal
  end width tibial end width)
• Female femurs contained ova in 2 size classes
  (large, small)
• Egg circumference was measured in 21 females all
  visible femur eggs were measured (max. 21 eggs
  per femur) and radius was derived from
  circumference using circumference 2piradius

References Cole, L. J. 1904. Pycnogonida of the
West Coast of North America. Harriman Alaska
Expedition, 10 249-330. Bain, B., 1991. Some
observations on biology and feeding behavior in
two southern California pycnogonids. Bijdragen
tot de Dierkunde, 61 (1) 63-64.