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Old 11/02/2006, 10:24 AM   #1
Gonodactylus
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So how long will it take for my stomatopod to molt and what is it doing?

One question that I'm frequently asked is how long does it take for my stomatopod to molt. As is typically the case with stomatopods, the answer depends on the species and the size of the individual. I thought it might be useful to briefly describe the process - without a lot of the endocrinology and physiology. Hopefully this will help you understand what your stomatopod is doing.

The molt cycle is made up of a series of steps. During the intermolt not much happens initially, but then the animal starts to lay down a new cuticle under the old one. As it approaches the molt, minerals are pulled from the old cuticle and stored in special glands. You sometimes can see these as chalky looking structures inside the ventral edge of each segment. The animal will then start to prepare for the molt and its behavior will change. It may become increasingly aggressive, undertake major reconstruction of its home, and finally stop eating. At this point, things really start to happen. The old cuticle starts to lift up creating a fluid filled space between the new and old cuticle, the animal may (depending on the species) seal itself in its cavity or burrow, and the sixth, seventh and eight thoracic tergites (dorsal plates) start to weaken on the dorsal midline.

As the actual molt (ecdysis) begins, the animal increases its fluid volume and starts to contract opposing sets of muscles increasing the internal hydrostatic pressure causing the thoracic tergites to rupture. The animal jack-knifes and begins to force itself through the dorsal thoracic opening. Interal pressure is rupturing other suture lines of the cuticle so that eyes, limbs and gills can be pulled free. If everything works according to plan, the animal will pull completely free of the molt skin in a mater of minutes leaving behind a perfectly articulate molt skin with all pieces connected except for the carapace which falls free. As the animal frees itself, it maintains high internal pressure and everts its new setae. These had developed inside the body like the fingers of a glove that had been pushed inside out into the center of the glove. The process is like taking a rubber glove, weakly inflating it and pushing the fingers into the interior of the glove. Then when you blow into it to increase the pressure, the fingers (setae) pop out.

The actual ecdysis is the time of greatest danger for the stomatopod. If any of the old cutlicle is stuck to the new due to wounds or disease, it will be unable to pull free. This is often fatal particularly if the gills remain covered suffocating the stomatopod. Commonly, the raptorial appendages become stuck. This occurs because of their great size and the small oriface that they have to be squeezed through. When this happens, the animal may be able to tear off the offending appendage to free itself - but at a cost since it will take three or four additional molts to regenerate a new one.

As the molt skin is shed, the animal begins the tanning process hardening the cuticle and redepositing salts. Any appendage that is stuck or deformed at this point will now remain in this condition. The cuticle will quickly strengthen sufficiently to allow normal movement (but not a strike). Usually the softer parts of the old cuticle are eaten providing nutrients and additional minerals although it may take a couple of days before the jaws are sufficiently hardened to allow the stomatopod to eat the harder parts. Some species such as odontodactylids may bury the old cuticle away from the burrow and then dig it up later to eat it. As the cuticle continues to harden, the animal will eventually open its cavity or burrow and when it can strike, beginning to forage. Many species will initially take soft prey, but smashing specialists will often remain sealed until they are able to strike with sufficient force to break snails and crabs.

So how long does all this take? There is no simple answer because it depends on the species as well as the size of the animal. You can view the molting process in stomatopods as an ever increasing spiral. Each 360 degree turn is one molting cycle and the circumference is the time it takes to complete that cycle. For example, a juvenile Neoonodactlylus wennerae 8 mm long will molt very couple of weeks and the actual molt from closing to opening may be only a day. Six months later at 18 mm, the animal molts about once a month and remains sealed for two or three days. At one year, a 25 mm animal will molt every one to two months and close or 3 or 4 days. At 3 years, a 40 mm N. wennerae will molt every 3 or 4 month and may remain closed for over a week, etc.

In general, heavily armored species for a given size take longer to complete a molt cycle. However, it is hard to generalize. Some species like G. chiragra and G. platysoma spend a very long time sealed in their cavities. It is not unusual for an 8 cm G. platysomal to remain closed for three weeks. An Odontodactylus scyllarus the same size would close for less than a week.

***** Added data
The molting process starts weeks or even months (in large animals) before the actual ecdysis (shedding of the old molt skin). The must lay down an entire new molt skin underneath the old one. (You can sometimes see the setae of the antennal scales growing into the antennal scales. They grow inside out and at the time of the molt pressure causes them to "pop out" much like the fingers of a rubber glove that is inside out and you blow into it.) As the molt nears, minerals are withdrawn from the old cuticle and stored in glands in the body. You can sometimes see these as while glands in each segment of the thorax and abdomen along the ventral lateral surface. Closer still to the molt, the old cuticle begins to raise up and a fluid filled space opens between the old and new cuticle. A few hours before the molt, suture lines begin to weaken. You can see these as lines on the dorsal mid-line of the thoracic segments. They run completely across the tergite plates. When it is time to molt, the animal increases fluid in the body (osmotic change) and violently contracts its body muscles to greatly increase internal pressure. This causes the thoracic tergites to split and the animals body is forced into the newly created opening. The animal is now in a jack-knife position working its way out of the old molt skin back first. It is sort of like a breach birth and any loss of hydrostatic pressure or muscular contraction will stop the process and the animal will become stuck and die. Note that during this time the gills are not functioning well until they pull free and are exposed to water. Once free, the animal is so soft that the muscles have nothing to work against except each other and initial movement is by hydostatic pressure changes. However, within minutes the new cuticle begins to "tan" and harden and over the next few days minerals will be added to harden the cuticle to its old functional stiffness. Until that happens, the animal cannot strike and if it tries, it will literally tear the appendage apart.

I won't go into all the things that can and do go wrong, but I think you can see that this is the weak link in the stomatopod's life cycle and a typical gonodactylid will have to do this 25-35 times. Most stomatopods that are not eaten or badly wounded die during the molt simply because of the energetic demands that potential for physical malfunctions.

When we keep stomatopods in an aquarium, we often see them die during a molt and assume that something is terribly wrong. It may not be. Like all organisms, stomatopods die and typically death comes during a molt.

***** End added data

I hope this provides a little help in understanding the molti and the variables that are involved in setting the length of the cycle and the relative duration of the various components of the molting process.

Roy



Last edited by traveller7; 05/15/2007 at 12:39 PM.
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