Statocyst Function and Compensatory Eyestalk Responses in
Crayfish
Objectives: After completing this exercise you will be able
to:
- define the terms: sensory neuron, mechanoreceptor,
proprioception, sensory stimulus, and modality
- compare the anatomy and functional properties of hair cells in
the acoustic system and the static or dynamic equilibrium system
- understand how virtually identical sensory neurons can respond
to such fundamentally different stimuli of sound, balance and
motion
- characterize the compensatory movements of walking legs,
claws, antennae and eye stalks as animals are passively tilted in
various planes
- measure the position of eye stalks relative to that of the
body as animals are turned
- become used to working with circular variables (measured in
angular degrees)
Discussion and Pre-Lab Preparation: Arguably the most
ubiquitous sensory capacity in animals concerns the detection of the
body's orientation relative to ground (ie. relative to the pull of
gravity). Despite considerable diversity in the particular anatomy of
sensory organs concerned with static equilibrium, most systems are
organized around the same basic principle.
In this lab exercise we use crayfish (species to be named, Family
Astacidae) and you may familiarize yourself with their
external
and internal anatomy, their taxonomic placement within the
arthropod,
crustacean,
malacostracan,
and
decapod
lineage, or browse the many crayfish-related links at the
Crustacean
neuroscience page
Fig 1. Interior of a statocyst gravity
receptor, a common equilibrium organ of invertebrates. A
fluid-filled vesicle lined with mechanoreceptors
(hair
cells) encloses one of more dense objects (eg. sandy or
stone-like elements) - the statoliths. As gravity pulls these objects
down, activity of sensory cells below them increases when their cilia
are sheared. From this, the central nervous system can extract
information about the direction of gravitational pull. When the
animal molts it loses the lining of the statocyst and with it the
sand grains. After molting the animal rebuilds the otoliths with
surrounding materials.
A crayfish passively tilted performs righting responses to regain
its primary orientation with obvious compensatory movements of its
various appendages. In this lab exercise we wil focus predominantly
on changes in the position of the eye stalks.
Exercise 1:
Question: Does an animals' eye stalk position depend on the
passive tilt in the roll plane? null hypothesis (Ho): No,
eyestalk position is independent of body orientation. alternate
hypothesis (Ha): Yes, eye stalk position is a function of the
body's orientation. Experiment: measure the position of
eyestalks relative to that of the body as the individual is tilted
about its long axis.
Materials: per student pair:
- Crayfish (ca. 20g) with eyes covered by small pieces of
aluminum foil (this handicap will be shed along with the old skin
at the next molt)
- Aquarium
- Crayfish holder and carapace clamp
Preparation:
- Pick up an animal firmly at the back of its carapace and
settle your grip over the tank. Don't be surprised if the animal
tailflips. Don't let go and drop the animal onto the floor - even
if it manages to pinch you. Place your hand (with the animal
attached) back into the water container and it will again release
its grip shortly
- Identify the
external
anatomical features of a crayfish.
- Make qualitative observations of the relative position of
legs, claws, antennae, and eye stalks as you slowly turn the
animal in various planes. 1. Rostrum up; 2. Rostrum down; 3. Left
side up by 30deg.; 4. Left side down by 30deg.
- Familiarize yourself with the added challenges of a circular
variable. Pay attention to the orientation of angles; while 360 =
0 = -360 degrees, -10 is not equal to 10 degrees, -10 = 350
degrees etc. Use the transparent disk at the front panel of the
aquarium (with degrees engraved on it) to confirm that angles of
10 degrees and -10 degrees are of equal size but that their
vectors will point to very different places of the room. Decide on
an appropriate way to deal with left and right turns, and collect
the data according to your criteria in a consistent fashion.
Procedures: Carefully attach the
crayfish to the carapace clamp and mount it on the stand as pictured.
Using the central knob on top of the holder you can now roll the
animal around its long axis (gamma). For each 20 degree turn note the
position of the left (alpha L) and right (alpha R) eye stalks
relative to the body high axis (see figure below). The position of
the stalks can best be observed through the front pane. A movable,
transparent disk engraved with angular degrees is attached to this
pane and you can rotate this disk so that its origin matches the
turning angle of the animal. Now use the movable hands attached to
this disk to indicate the orientation of the eye stalks and note down
the angles of the eye stalks from its origin.
|
Sample work sheet for angles
|
left eyestalk
alpha L
|
body
gamma
|
right eyestalk
alpha R
|
|
+
|
0
|
-
|
|
+
|
20
|
-
|
|
+
|
60
|
-
|
|
+
|
80
|
-
|
|
+
|
100
|
-
|
|
+
|
120
|
-
|
|
+
|
140
|
-
|
|
+
|
160
|
-
|
|
+
|
180
|
-
|
|
+
|
200
|
-
|
|
+
|
220
|
-
|
|
+
|
240
|
-
|
|
+
|
260
|
-
|
|
+
|
280
|
-
|
|
+
|
300
|
-
|
|
+
|
320
|
-
|
|
+
|
340
|
-
|
Data Analysis: Graph the relationship between the
independent axis - body position (gamma) - and the two dependent
variables - left and right eye stalk position (alpha L and alpha R).
What form should this graph take if there is no relationship between
positions of body and eye stalks (under Ho)? Does your data provide
evidence for dropping Ho in favor of Ha?
Discussion: What particular strategies does the nervous
system apply to the orientation of eye stalks when it copes with
tilt? It is probably not surprising that walking legs respond to the
passive tilt of animals which are frequently subjected to
displacement from waves and currents. However, what may possibly
explain a link between orientation of body and eye stalks?
Exercise 2:
Question: Do statocysts play a role in the animals' sense
of equilibrium during passive tilt in the roll plane? null
hypothesis (Ho): No, statocysts play no role in these righting
responses. alternate hypothesis (Ha): Yes, righting responses
are dependent on functional statocysts.Experiment: Repeat
experiments described above (Exercise 1) and compare the compensatory
and righting responses of normal crayfish with those of animals which
have had their statocysts ablated.
Materials: per student pair:
- Crayfish (ca. 20g) with eyes covered by small pieces of
aluminum foil and lesions to one or both of it statocysts
- Aquarium
- Crayfish holder and carapace clamp
In crustaceans, the statocysts are paired sensory organs located
at the base of the antennules. They have been ablated in several
specimens by cautery with a soldering iron. In some specimens, only
one statocyst, has been ablated, whereas, in others, both statocysts
have been ablated.
Preparation and Procedures: see exercise 1
|
Sample work sheet for angles following statocyst
lesion
|
left eyestalk
alpha L
|
body
gamma
|
right eyestalk
alpha R
|
|
+
|
0
|
-
|
|
+
|
20
|
-
|
|
+
|
60
|
-
|
|
+
|
80
|
-
|
|
+
|
100
|
-
|
|
+
|
120
|
-
|
|
+
|
140
|
-
|
|
+
|
160
|
-
|
|
+
|
180
|
-
|
|
+
|
200
|
-
|
|
+
|
220
|
-
|
|
+
|
240
|
-
|
|
+
|
260
|
-
|
|
+
|
280
|
-
|
|
+
|
300
|
-
|
|
+
|
320
|
-
|
|
+
|
340
|
-
|
Data Analysis: Graph the relationship between body position
(gamma) and the position of the eye stalks (alpha L and alpha R) -
the same way as you have done in Exercise 1. How should this graph
look if there is no contribution from statocysts (under Ho)? Does
your data provide evidence for dropping Ho in favor of Ha?
Discussion: What role do statocysts play in a crayfish'
sense of equilibium? In what way may the two statocysts be
interacting in the intact animal?
Reference: Davis, W. J. (1968) Lobster Righting Responses
and their Neural Control Proc. Roy. Soc. B 70: 435-456.
