The Buzz on Platypus Bills- My Favourite Tissue!

A Brief Introduction to Platypuses

 The platypus (Ornithorhynchus anatinus) is one of five species of Monotremes, or egg laying mammals (UCMP Berkley 2013). It lives in lakes and streams in eastern Australia and Tasmania. Contrary to popular belief, the platypus is not very big, and ranges in size from 50-60cm long (Britannica Concise Encyclopedia, National Geographic) . It appears to be a mishmash of many different animals; it has a tail like a beaver, and webbed feet and a bill of a duck, and the body and fur of an otter (National Geographic).

Figure 1: Platypus swimming 

(Source: http://www.amamoorlodge.com.au )

Platypuses are underwater bottom feeders; eating close to the equivalent of its own body weight in insects, larvae, shellfish, and worms. They hunt with their eyes and ears sealed shut with a watertight skin fold, and their nostrils sealed to prevent water from entering (National Geographic). In addition, most hunting activity is done at night and in turbid water (Gregory 1988).

If this is the case, how do platypuses find their prey?

Platypus Bills: Special Somatic Senses

Figure 2: A clip from National Geographic's "World's Deadliest": Platypus Hunts with Sixth Sense 

(Source: http://www.youtube.com/watch?v=i7_l_FdIuLs)

Structure

Platypuses have 2 main types of sensory receptors in the skin of their bills:

1)  Electroreceptors (Sensory Mucus Glands)

2) Mechanoreceptors (Push Rods)

Electroreceptors: Sensory Mucus Glands

(Source: Proske et al. 1998)

There are 3 main regions in the sensory mucus gland:

Figure 3: Sensory Mucus Gland in the Platypus. 

(Proske et al. 1998)

1) Sudermal mucus secreting area (Purple):

-This is the deepest structure, lying in subdermal tissues. 
- It is the main secretory area; its walls are made of secretory cells, which actively secrete mucus.
- The mucus helps conduct electricity.

2) Ascending duct (Red):

- Originates in the dermis, and runs up to the surface of the skin. 
- The lumen is lined with keratinocytes. At the surface of the skin, the keratinocytes form a "blossom-like" pore, which opens up when the platypus is underwater. 
-The ascending duct runs through the innervated region, called the papillary region.

3) Papillary Region (Blue)

-An envaginated part of the epidermis, which expands into a bulb-like structure. 
- The bulb-like structure is insulated by 2 layers of cells: loose packed cells with large intercellular spaces (closest to the inside), and a layer of flat cells with tight junctions. 

-This is the innervated portion of the receptor; afferent fibres go from here, to the Central Nervous System. 
- The terminal axons penetrate through the flat cell layer, into the loose cell layer, but never actually reach the ascending duct. The sensory nerve endings form a "daisy chain" network of nerve endings around the papillary region. 
- The axons also have bulbous terminal expansions, filled with mitochondria, suggesting that there is a large amount of metabolic activity, and may be the area where resting activity (characteristic of all electroreceptors) is generated. 
- Most nerve fibres project to the cerebral cortex. 

Distribution of the Sensory Mucus Glands:

Sensory Mucus Glands are found on the upper and lower jaws, on the inside and outside of the bill, and on the shield. The glands are arranged in a series of parasagittal stripes. For the exact distribution, see the picture below:

Figure 4: Distribution of mucus electroreceptors in platypus bill (Source: Pettigrew 1999)

Note: Bi=Upper outside bill, Bii=Upper inside bill, Biii=Lower outside bill, Biv=Lower inside bill

-There are a total of about 30,000-40,000 sensory mucus glands on the platypus bill

-Each gland is innervated with up to 30 myelinated sensory nerve fibres

The result? A substantial amount of innervation!

Mechanoreceptors: Push Rods
(Source: Proske et al. 1998)

The push rod is rigid and compact; this is due to a column of flattened spinous cells with many tight junctions between cells. The spinous cells are filled with tonofibrils. Individual push rods are separated from each other by dermal papilla; this means that the push rod can move independently of other rods and of surrounding tissue. 
Sensory structures within the rod (described below) are deformed and send signals to the brain when pressure is applied to the tip of the push rod.

Figure 4: Push rods from platypus bill: Original image and image coloured to highlight parts of the receptor discussed.

There are 4 types of nerve endings associated with the push rod (all supplied by myelinated stem axons): 

1) Central axonal vesicle chain receptors
2) Peripheral axonal vesicle chain receptors
3) Merkel Cell Complexes
4) Lamellated corpuscles


1&2)  Axonal Vesicle Chain Receptors 

-Strings of bead-like enlargements along the axons located in the core of the push rod.
-There are 2 types: Central (red), and Peripheral (blue)
-The central axonal vesicle chain receptors terminate just a few cell layers from the surface
-There are 7-13 strings of central axonal vesicle chain receptors, which are supplied by 5-8 axons
-The peripheral axonal vesicle chain receptors are located about 3 cells layers deep from the edge of the rod, and are supplied by about 20 axons. 
-The peripheral vesicle chain receptors form a concentric circle around the central ones. 

3&4) Merkel Cell Complexes and Lamellated Corpuscles
-At the base of the push rod
-Pressure sensitive receptors
-There are about 12 Merkel cells, which are supplied by 1 or 2 axons
-Lamellated corpuscles are larger than Merkel cell complexes. There are 3-6 per push rod, which lie in different directions. 

Distribution of the Push Rods
(Source: Proske et al. 1998, Pettigrew et al. 1998)

Push rods are scattered throughout the bill; the edge of the bill is estimated to have the highest density of receptors. When looking through a stereomicroscope, they look like "small, bright domes" (Pettigrew et al. 1998). Like the electroreceptors, they open up underwater and are closed when on land. There are a total of about 46, 500 on the platypus bill! (Proske et al. 1998)

Figure 5: Distribution of mechanoreceptors (push rods) in the platypus bill. (Source: Pettigrew 1999)

Note: Ai=Upper outside bill, Aii=Upper inside bill, Aiii=Lower outside bill, Aiv=Lower inside bill

AN IMPORTANT NOTE: Push rods are sensitive enough to detect water movements of prey (the threshold displacement of the push rod is 20 micrometres), therefore physical contact with the prey is not necessary in order for the platypus to detect it! (Pettigrew 1998)

Putting it all together: Using the Bill for Prey Detection

(Sources: Pettigrew et al. 1998, Biology 3202 notes, Sally Goddard 2013)

Figure 6: Platypus catching a crayfish (Source: Sydney Sea Life Aquarium)

-The mechanoreceptors and electroreception systems appear to be coupled to help the platypus find prey.

Figure 7: Temporal Differences in Mechanical and Electrical Stimulation help determine Location and Direction of Prey (Source: Pettigrew et al. 1998)

-Prey in the water, such as a shrimp, will make both a mechanical disturbance by making movement in the water, and an neuromuscular electrical impulse by moving its body. Both of these are detected by the platypus.

-Pressure waves and electric currents travel at different speeds underwater, so the different receptors receive the stimulus from prey movement at different times. 

-Some neurons are bimodal and can respond to both mechanical and electrical stimulation. These neurons are sensitive to the time delay between pressure waves and electric currents.

-Platypuses can use this difference in arrival time of the mechanical and electrical stimuli to measure prey distance, speed, and direction of travel. 

Works Cited

NOTE: Academic papers only. All websites are hyperlinked.

Gregory, J.E., Iggo, A., McIntyre, A.K., and Proske, U. (1988). Receptors in the bill of the platypus. Journal of Physiology, 400, 349-366. 

Pettigrew, J.D. (1999). Electroreception in monotremes. The Journal of Experimental Biology, 202, 1147-1454.

Pettigrew, J.D., Manger, P.R, and Fine, S.L.B. (1998). The sensory world of the platypus. Philosophical Transactions of the Royal Society B: Biological Sciences, 353, 1199-1210.

Proske, U., Gregory, J.E., and Iggo, A. (1998). Sensory receptors in monotremes. Philosophical Transactions of the Royal Society B: Biological Sciences, 353, 1187-1198.