Autotomy and Regeneration
Hulali Alford & Rose Driscoll
Reed College Biology 342 Fall 2015
How does the behavior occur? In other words, what processes fill the gap between the stimulus and the final form of the behavior? How is the behavior triggered?
Autotomy occurs through different mechanisms in different species, but requires some external stimulus resembling a predator as a trigger. Autotomy is by definition internally controlled (for example, through the nervous or endocrine system) and occurs along a specific fracture plane. Species that practice economy of autotomy (see Adaptive Value) have multiple fracture planes, and can control which plane autotomy occurs at. Frequently, muscle contraction is involved in pulling a plane apart.
1. In the squid O. deletron, autotomy can occur anywhere along an individual’s arm. However, squid preferentially autotomize along one of several defined planes. Autotomy only occurs when there is resistance, as traction is required for the arm to be fully pulled away (Bush 2012).
Figure 1. Two fracture planes (indicated by arrows) in an O. deletron arm. The fracture plane on the left is where the arm autotomized. (From Bush, 2012.)
2. Lizards autotomize their tails along fracture planes. There are two main mechanisms of caudal (tail) autotomy in lizards (see also Phylogeny). First, there is intravertebral autotomy, where fracture planes cross each vertebra of the tail, with the exception of a few at the base and sometimes a few at the tip. When a portion of tail is autotomized, the segment that grows in its place is a cartilaginous tube with no fracture planes, so future autonomies must take place closer to the animal’s body. Species with this model show economy of autotomy (see Adaptive Value.) Second, there is intravertebral autotomy, where areas of weakness (not defined fracture planes) exist between each vertebra of the tail (Bateman & Fleming 2008). However, there are also some lizards (especially geckoes) have only one fracture plane, at the base of the tail (Arnold 1984).
3. Fracture planes are produced during a period of resegmentation in the vertebra when somites (structures which ultimately develop into vertebrae) do not fuse completely. (Evans 1981).
4. Autotomy in lizards involves voluntary contraction of muscles adjacent to the fracture plane, meaning that much more force is required to induce autotomy in a dead lizard than in a live one (or, in a less extreme example, more force is required in an unconscious animal than a conscious one, which reinforces the idea of voluntary control.) Valves in the veins and arteries of the tail reduce blood loss due to autotomy. (reviewed in Arnold 1984).
5. The stimulus required to induce autotomy in an individual lizard can vary greatly depending on context (reviewed in Arnold 1984). Porcelain crabs have extremely low threshold stimuli for autotomy to occur (Wasson et al 2002).
6. In blackworms (L. variegatus), autotomy may be induced by compression force (for example, pinching or biting by a predator) in the absence of any significant pulling force. The autotomy fissure appears just above the compression site. After the division occurs, both ends are sealed such that there is no observable blood loss. Autotomy is an all-or-nothing response in blackworms; either a full division occurs, or there is no division (Lesiuk & Drewes 1999).
7. Blackworm autotomy is controlled by the nervous system and may occur through muscular contraction. Neuromuscular signaling in annelids (segmented worms) is cholinergic, and autotomy was suppressed in worms treated with nicotine, a cholinergic antagonist (Lesiuk & Drewes 1999).
8. In a sea cucumber, Eupentacta quinquesemita, evisceration (autotomy of the digestive system) is facilitated by muscle contraction and is neurally controlled. Changes to connective tissues during autotomy are hypothesized to be the result of an as-yet unknown transmitter or neurally secreted hormone (Byrne 2001).
Figure 2. Hypothesized autotomy control pathway for the sea cucumber E. quinquesemita. Some evisceration factor, possibly a hormone secreted from an endocrine cell or a neurosecretory neuron, is released into the coelom (the body cavity) and causes the mutable connective tissue (MCT) to first soften and then rupture. (From Byrne, 2001.)
Figure 3. Anatomy of the sea cucumber E. quinquesemita showing autotomy structures. Three structures rupture during autotomy: (1) the introvert, an extendible portion of the body wall; (2) the P-L tendon, which links the pharyngeal retractor muscle (PM) to the longitudinal body wall muscle (LM); and (3) the junction between the large intestine (LI) and the cloaca (C ). (From Byrne, 2001.)
9. Figsnails (Ficus ficus) have a specialized body part, the autotomy mantle, which is used exclusively for autotomy. The mantle contains mostly muscle and connective tissue, but very low protein, lipid, and carbohydrate, so the mantle is not used as an energy reserve the way some animals’ autotomy tissues are (Liu 2002 and references therein).