White Spots on the Walls


One of the most common questions I get asked in my “Ask Dr. Ron” forum on Reef Central is, “What are these white spots on the walls of my aquarium?” Such questions often include a photo or two of a small circular or oval structure often containing one or several small white dots. My reply is that these are snail egg capsules. In aquaria, they are typically deposited by female Nassarius snails as well the females of a few other species. That is, of course, the short answer. In reality these capsules form a part of the snail’s life history that is really fascinating.

Here are links to a couple of the threads which have pictures of the egg capsules: LINK 1, LINK 2, LINK 3

Primitive mollusks were probably animals that had separate sexes which spawned by broadcasting their gametes into the water column much as do many marine animals today. Broadcast or “free” spawning is reproduction by simply releasing the sex cells, or gametes, into the water column. If these gametes, the sperm cells and eggs, are released in close enough proximity, the sperm will fertilize the egg, and embryonic development will begin. Subsequent to spawning and fertilization, there are a number of potential development patterns leading either directly to a small juvenile, or to a larva of some sort and thence to a juvenile.

The one thing all of these modes of development have in common is that they are unlikely to succeed. The odds against successful reproduction are exceptionally high. Consider that if a population of any animal is more-or-less stable through time, all any female has to do is to produce two eggs over her lifetime that successfully grow to reproductive adults. These two eggs would replace her and her mate. Many free-spawning animals live a long time, and reproduce yearly. Animals that possess this mode of reproduction such as sea urchins, sea cucumbers, some polychaete worms, such as the common aquarium fire worms, clams and some snails will often produce hundreds of millions of eggs for each spawning event. Over the course of their lifetimes, they may literally produce billions or even trillions of eggs. If only two need to survive until the next generation, it can be easily seen that the odds against survival is often billions or hundreds of billions to one.

Obviously, there would be great advantage to being able to maximize the success of such reproduction; the more successful the reproductive event, the more offspring are left, and the more successful is the species. Natural selection, with the subsequent evolution of the animals, has resulted in many ways to cut the odds. The first one is to have copulation and internal fertilization of the eggs. Eggs that are broadcast into the environment have little chance of fertilization, even if there is a male spawning nearby. Most eggs and sperm broadcast into the water probably never unite with a partner. Eggs may remain viable for periods of a few hours to up to a day. Sperm generally are much shorter lived and “run out of gas” within a couple of hours.

Within the mollusks several groups of animals, notably for aquarists, the chitons and the clams, have retained the primitive mode of broadcast reproduction. The snails, however, generally have abandoned it in favor of a process that is more likely to be successful. This more successful mode consists of copulation followed by internal fertilization. Such reproduction cuts the odds significantly, and these animals produce far fewer gametes over their lifetimes. However, to get from a broadcast spawning ancestor to an animal that can utilize internal fertilization to reproduce necessitates a lot of changes in both the female and male plumbing. Interestingly enough, probably the best group of animals to use to examine these changes is the vast Class Gastropoda of the Phylum Mollusca, or as it is more commonly known, the snails.

Some snails, including many of the so-called “turbo-grazers” found in reef aquaria are broadcast-spawning animals. At the complete other end of the spectrum are numerous snails, mostly living in fresh water, that give birth to live young. The actual number of snail species is unknown; the estimates range from a low of about 30,000 to a high of over 100,000 species. In either case, however, the huge number or species makes the snails the second most species-rich group of animals after the insects. However, not only are there a lot of snail species, there are a lot of different ways to make a snail, and many of these have different and unique ways to reproduce.

In my invertebrate zoology courses, I often use the Class Gastropoda as examples of how animals change to meet any contingency. Using this group as an example is particularly good because in many cases, it appears that intermediate forms have survived. In effect, the group is full of “missing links.” Only here they are not missing. This is particularly true when the changes in the reproductive system are concerned. Apparently, primitive snails had very simple reproductive plumbing; basically they had gonads and a duct to the great out of doors, and simply spawned into the water. Today, snails that are considered to be primitive, such as some of the group containing Turbo, still have that type of reproductive system. In the marine shelled snails, the other end of the continuum is seen in the predatory snails such as the whelks. In these animals, both sexes have highly modified reproductive systems. The simple duct to the outdoors has changed in the male to have internal sacs or vesicles to store sperm, glands to produce nourishment for the sperm, and glands to provide a fluid environment for the sperm. Additionally, the duct has developed externally into a penis. In the female, the duct has changed into a vagina, but additionally there are compartments to store sperm after copulation and glands to secrete nourishment for those sperm, as the female can often store sperm for several months prior to use. Additionally, the simple oviduct has been modified with glands to produce yolk for eggs, and food for developing embryos. Finally, in the middle of the female’s foot a gland is found that secretes an egg capsule.

The Ultimate in Egg Capsules

In this final extreme reproductive modification seen in the marine shelled snails, reproduction is a rather complicated procedure. Initially, during certain seasons, the males and females may aggregate or meet and copulate. Little is actually known about how they attract one another or the process that brings them together and if, for example, males will fight over females as often happens in other species. After this process, both partners often seem to go their own separate ways.

Several days to several months later the females are ready to spawn. During the intervening period the eggs have been “ripening” inside the gonad and the reproductive glandular tissue has been growing. The females seek out a place to spawn. This spawning behavior varies considerably between various species. In some species, the females find a spot where the deposited eggs will be hidden from potential predators such as fishes, sea urchins, or other snails. In other species, the female may seek out some specific site to deposit her egg capsules where some other animal or physical environmental structure may protect them. In yet other species, females often deposit their egg masses in large communal masses and then stay with the eggs and protect them from predators.

Figure 1. Whelks of the species Neptunea pribiloffensis depositing egg capsule masses near a sea anemone which will protect the egg masses from predation by sea urchins during the 12 to 13 months it takes for the eggs to hatch.
Figure 2. A single female Neptunea depositing her egg capsule mass (it is extending downward from under her foot). It will typically contain dozens of separate capsules fused into a corn-cob like mass. The sea anemone she was using as a baby-sitter was located just out of the picture to the upper left.
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In all of the above reproductive modes the females typically deposit several dozen to several hundred egg capsules and each of the capsules contains several thousand eggs. The capsules are very durable, but will eventually open, and from each, one to a few small snails will crawl away. Only a few embryos develop in each capsule, and as they develop they eat the other non-developing eggs. The length of time necessary for the development from a tiny egg, a thousandth of an inch in diameter, in a capsule to a small crawling snail, up to a third of an inch long, ranges from a few weeks to over a year, depending on the species.

This type of development is occasionally found in aquaria, generally by accident. When it occurs, it appears that the aquarist has added a piece of reef rubble to their tank that has on it or in it, several egg capsules. Some time after the rock has been added to the aquarium, the capsules hatch and all of a sudden the aquarist is forced to confront the situation that they have a plague of predatory snails in their tanks. Even if those predators are small and moving at a snail’s pace they may be able to do a lot of damage.

White Spots On The Walls

Many snails however, don’t go to the extreme seen in the predatory whelks. They have a reproductive mode that involves a combination of a transitory or less-durable capsule and a planktonic embryonic existence. This type of reproduction is not correlated with the diet of the animals. It is seen in some of the predatory and scavenging buccinid whelks such as Nassarius, and also in the herbivorous strombid conchs, but interestingly enough also in some of the venomous snails such as the cones and turrids.

Reproduction in these animals probably involves some sort of courtship behavior, although this has never been studied, resulting in copulation. As in their distant cousins, the whelks, sperm may be stored for quite some time. I studied reproduction in the cold water venomous snail species Oenopota levidensis, and had an isolated female lay egg capsules containing viable eggs for over two months, indicating that she had been able to store sperm from some time before I put her in her "spawning jar."

These animals typically deposit a clear, lenticular, round or oval capsule. The capsules are made of several layers of, presumably, proteinaceous material. Over the course of their development, diatoms generally do not grow on them, and they may have some sort of algicide in their outer layer. Often the upper surface of the capsule appears to have a long seam, and a plug in the center. The inside of the capsule is filled with a clear fluid and several eggs are deposited in it. The number of eggs varies with the species and with the mother’s size. Larger species and larger females within the species deposit more eggs.

Figure 3. A diagram of the type of egg capsule found on aquarium walls.

These capsules may be deposited on specific surfaces. The females of one population of Oenopota levidensis that I studied seemed to search out and deposit their egg capsules on clam shells. On the other hand, in aquaria, the Nassarius seem to deposit their eggs on the nearest vertical or near vertical surface. This nearby surface is often the aquarium wall, but it may also be a rock, or another snail. Nassarius capsules are typically small, as befits the small size of the mother, generally less than 1/32 of an inch across. The eggs are white and generally from one to about a dozen may be found in a capsule. On the walls of the aquarium such capsules are quite noticeable, as the eggs are shiny and white, but on the rocks they tend to be quite hard to see.

Figure 4. Left, an Oenopota levidensis female depositing an egg capsule on the side of a jar. The arrows indicate the edges of the egg capsule. The eggs are clearly visible in the capsule. The capsule is secreted by a gland located in the middle of the foot. Right, a similar egg capsule photographed in nature on a black rock. The white structures are calcareous worm tubes.

If the capsules on the aquarium wall are examined with a hand lens, or a good magnifying glass, all stages of early molluscan embryology may be seen. Right after deposition, the eggs have not yet begun to develop and are spherical. Over the next few hours they may be seen to start to change shape, looking occasionally lumpy and misshapen. This is the visible manifestation of some rather rapid and synchronous cell division. After about a day or so, depending on the temperature of the system, the “eggs” look to be spherical again, but in this case they now are an embryo of over 150 cells and they have reached the “blastula” stage, and form a hollow sphere. During all of this division the embryo has not grown, in fact, the mass of the embryo has dropped when compared to that of the egg, as yolk in the egg has been used up to fuel the development. The blastula is generally roughly the same size as the egg had been.

Figure 5. An Oenopota egg capsule that is a couple of days old. Although the edges of the embryos is blurry, the irregular shape indicates that significant larval development has occurred. The capsule is about 1/16th of inch across.

Shortly after this, the careful observer may begin to see some interesting changes. The small spheres, no longer eggs, but looking much as the eggs did, will often start to move. Generally, at first they may simply spin in place. Sometimes, however, they will move around inside the fluid contents of the capsule. This occurs as the surface of the embryo becomes covered with fine moving cilia. A number of other changes now start to occur, which may be quite rapid in some species or quite prolonged in others. In most of the capsules seen in my aquaria, the changes occur very rapidly.

On close examination, the embryo will be seen to become irregularly shaped. In fact, it is turning into a larva, but at the magnification of a hand lens, these changes are essentially impossible to discern. Primarily, it is growing some large lobes (large relative to the egg, the lobes are really pretty small), that it will use in feeding. Also, the blobby embryo is turning into a recognizable small snail, and a shell is starting to be secreted. This development from an embryo to a small larva contained in the egg capsule can occur in a day or less. Typically, each egg in the capsule will develop into larvae, but in some species only one or two will change into larvae. The others may decompose or be eaten.

Figure 6. Oenopota larvae developing inside the egg capsule.

After the change into a larva is complete, the plug on the top of the capsule (facing into the aquarium) will open, releasing the larvae into the water. Once the larvae are into water, they are called veliger larvae and they need to feed on phytoplankton.

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Figure 7. Left: An Oenopota veliger larva that is three days post hatching from the capsule. Right: this larva is ten days old. The green spot in the shell is algae in the gut, and indicates the animal was feeding.

If there is sufficient food, and provided nothing eats them, these little veligers grow and develop internally becoming quite complex small snails. Once more depending upon the species, the length of time that they remain as planktonic larvae varies. Some are feeding larvae for only a few days, others may be in the plankton for over a year. It appears some Caribbean snails may have planktonic veligers that exit the Caribbean with the Gulf Stream, travel over to Northern Europe, and back across the Atlantic on the Equatorial Current before they return back to the Caribbean.

Once the animal has developed sufficiently in the plankton, it will begin to search out an appropriate bottom substrate. Often they swim down toward the bottom and “bounce” along it, presumably “tasting” the bottom for the appropriate chemical cues. If the correct cues are found, they quit swimming, settle to the bottom and change into a crawling small juvenile snail.

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Figure 8. Newly settled Oenopota juvenile, after living for about 55 days in the plankton. The animal is about one mm, or 0.04 inches long.
Conclusion:

It is quite possible to raise these animals all the way from “white spots on the glass” to small snails. Many useful techniques are listed in my article and the book by Meg Strathmann cited as “useful references.” It is far easier to raise these little snails, however, in special isolated containers such as gallon jars or small aquaria rather than in reef aquaria, as in such containers it is easier to provide the food and care that are necessary to maintain the larvae.

The period from “white spots on the walls” to a small snail in our tanks is probably on the order of a month or so for the Nassarius, but may be more or less than that for other species. In reef aquaria, there is seldom enough food for the total cycle to be completed, and additionally there are often a lot of predators on these small larvae. Everything from fishes to corals will eat them. Nonetheless, I have received a few reports that seem to indicate that successful reproduction of such species has occurred in aquaria. These reports are also becoming more common as well, perhaps indicating we are being able to more closely mimic the natural conditions necessary for survival. If you have success in propagating these animals I would appreciate knowing about it.



If you have any questions about this article, please visit my author forum on Reef Central.

References

Shimek, R. L. 1986. The biology of the Northeastern Pacific Turridae. V. Demersal Development, synchronous settlement and other aspects of the larval biology of Oenopota levidensis. International Journal of Invertebrate Reproduction and Development. 10: 313- 337.

Strathmann, M. F. 1987. Reproduction and development of marine invertebrates of the Northern Pacific coast. University of Washington Press. Seattle. 670 pp.

 




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