Invertebrate Strippers

One of the more fun things that invertebrate zoologists do is to speculate on the relationships of the various groups of animals. One question, "Who evolved from whom?" has dominated and driven research about these animals for a very long time. Until recently many of the discussions about evolutionary relationships have been exercises in logic based on limited evidence. This has led to a lot of interesting and, generally, fun discussions about which character sets were most important in determining relationships (the important ones were always the ones "you" chose) and which weren't (always the other guy's). In essence, as we had no real way of independently evaluating the various lines of evidence and conjecture, such discussions were not really science, but more akin to science fiction.

Recently, however, with the advent of techniques allowing for direct comparisons of the genetic code of organisms, it has become possible to actually assess the degrees of genetic interrelationship amongst and between the various major animal groups. Additionally, the development of a particular set of statistical techniques and methodologies called "phylogenetic systematics" has allowed a separate independent way of assessing relationships based on evolved or derived characters shared between animal groups. In essence, we now have some tools that permit us to get answers to the same question from two independent methodologies. Consequently, we can use the two methodologies together to check on each other and to assure ourselves that the results are no longer in the realm of literature, but rather actually are "real" science.

Over the past ten to fifteen years, these two tools have allowed us to put together a relational framework of the animal kingdom that has overturned some long-cherished and well-defended ideas. These were ideas that seemed logical at the time, but were simply wrong; and in hindsight, one wonders how they could have been supported for so long. One of these cherished ideas held that the segmented annelid worms and the segmented arthropods were closely related simply because they both were segmented and had an internal morphology that was fundamentally different from the third great segmented group, the vertebrates.

One of the more striking discoveries of the new methodologies was that there are three main lineages or assemblages of animal groups. One lineage, called the deuterostomes, derives the Echinoderms (sea stars, sea urchins, and related animals) and Chordates (fishes, mammals, and birds) from the Cnidaria (corals, sea anemones and hydroids). This linkage was not too dissimilar to what had been proposed prior to the new methodologies and served, in some regards, to validate the logical methodology used through most of the nineteenth and twentieth centuries. However, the lineages presumed to link the rest of the animal kingdom, indeed the vast majority of the animal groups, was thoroughly juggled and rearranged. We now recognize one vast assemblage of many animal groups which is defined by the presence of a particular type of larva called a trochophore, or the presence of a crown of ciliated tentacles around their mouth. This crown of tentacles is called a "lophophore." This assemblage of animal groups, containing the mollusks, annelids, brachiopods, and numerous smaller groups is called the "Lophotrochozoa," a name derived from the names of the two major characters, the lophophore and the trochophore.

The final assemblage of animal groups, containing relatively few major groups but, by far and away, most of the animal species, is characterized by animals that have to molt to grow. These animals have either an inelastic cuticle or an "exoskeleton" that covers their entire body. The name for the group, the Ecdysozoa reflects their need to shed their outer covering as they grow. Ecdysozoa is derived from the words "ec-" (from the Greek word "ek-" meaning: "out of"), + "dysis" (from the Greek word, "dyō," meaning: "to put on clothes,") giving the New Latin word, "ecdysis," (meaning: "a getting out of clothing, molting.") +"zoa" (from the Greek word, "zōon," meaning: an animal or living being). So, these are the animals that molt or get out of their clothes or, in other words, they are strippers.

The Strippers…

Apparently the genetic machinery for the construction of a cuticle or inelastic covering, coupled with the need to shed such a covering to grow really evolved only once as all of these groups seem to have the same basic genome for the physiological "machinery" necessary for molting. There is a progression of forms from simple molted cuticles to quite complex molted "exoskeletons." The latter, of course, are found in the arthropods, such as crabs, insects, and spiders, whereas the simpler forms are found in several other animal groups or phyla. Complex, but simpler, cuticles are also found in nematodes or roundworms. Both of these groups are quite well represented in aquaria.

I will discuss nematodes and the arthropodan groups in some subsequent columns, but in this column I thought it would be fun to explore some of the lesser-known strippers. Molted cuticles are found on animals in many groups that are actually reasonably common on coral reefs, and it is likely that some of these animals have found their way into reef aquaria, even though aquarists may not have yet noticed them. Subsequent to discussing odd groups in this column, I have often been asked questions about representatives that have been recognized as living in someone's aquarium. Perhaps that will be the case here.

With only a few exceptions, these lesser-known animals are really quite small, and this contributes to the lack of information about them in the hobby. There are three groups of animals that I will be discussing in this column, the Priapulids, the Kinorhynchs, and the Loriciferans. Only the Priapulids are likely large enough to be commonly visible to aquarists. The Kinorhynchs would certainly be visible to those aquarists who periodically microscopically examine some of their systems' sediments. Loriciferates may or may not be present in aquaria; they should be there, but are rarely seen anywhere.


Priapulids are some rather odd-looking worms and take their common name from their taxonomic placement in the phylum Priapulida. These are generally small animals which, although they may appear annulated, do not have a body comprised of separate segments. There are about 20 extant species, and they are found in all oceans, although they are most diverse in coral reef areas. There are no fresh water species.

Figure 1. Priapulus caudatus, a widely distributed Priapulid worm.

The largest of them reach lengths of about four inches. They have three body regions. In the front is a spiny introvert or proboscis, which has occasionally been dignified with the name of "head," however these animals really don't have a defined head. This structure is covered with spines which are used to grab the prey and possibly also to aid in burrowing. The main part of the body is called the trunk, and it is annulated; that is, it appears to be comprised of rings of tissue. Off the back end of the worm are generally one or more structures referred to as caudal appendages. These may serve a respiratory function.

Here is a link to a diagram showing several species of Priapulids:

The body is completely covered with a cuticle, and as is characteristic of all of the animals discussed this month, this cuticle needs to be molted to allow the animal to grow. The cuticle is made of protein when the animal is young, but becomes chitinized as the animal grows. The polymer, chitin, is found in many invertebrate cuticles. It a very good structural material as it is very durable and chemically resistant, yet it is flexible enough to allow a wide variety of movements.

Here is a link to information about a Priapulid, Tubiluchus, that is different from the commonly illustrated Priapulus:

Priapulids have a simple nervous system, consisting of a simple ring around the esophagus, and a midventral nerve cord. There is no really well-defined brain, and little is known about their nervous system. The body wall is generally quite tough and muscular, and the muscles are striated, indicating that the animals are capable of quite rapid movements.

Little is known about their natural history, but they appear to live in burrows and perhaps strike out of their burrow to grab prey, which they seize and engulf with their proboscis. Some species may live head down in burrows foraging in sediments or in rocky crevices for prey. It appears that most coral reef species are found in the reef rocks or reef rubble, and these are the Priapulids which may be expected to appear in aquaria from time to time.

Figure 2. The mouth of Priapulus caudatus. Note the very "functional" teeth used to hang on to prey.

Quite surprisingly for a group of wholly soft-bodied animals, Priapulids actually have a significant fossil record. Although they are absent from large spans of the Earth's fossil history, they are prominent and dominant animals in some of the earliest good deposits of animal fossils, such as the Burgess Shale Fauna, and similar faunas from southeastern China. There are 11 species of presumed Priapulids found in the Burgess Shales, a fossil deposit dating from about 530 million years ago. Interestingly enough, Priapulids are more diverse in this deposit than they are in any present day marine habitat.

Here are some links to the Burgess Shale Priapulids especially Ottoia:


The next group of small strippers, is the Phylum Kinorhyncha; or as they are know to everybody who has seen one, the kinorhynchs. The name of this small group is derived from the Greek words, kineō meaning moving and rhynchos meaning a beak or snout. The name is very apt as the animals are continually moving and feeding by pushing their introvertable proboscis in and out. There are only about one hundred species of scientifically described kinorhynchs, although most experts seem to agree that the total species number likely significantly exceeds that rather paltry sum.

Figure 3. Two different species of Kinorhynchs. Left image. View from
above the animal. Right image: Lateral view.

All kinorhynchs live in marine habitats, and they are all small, generally less than about 1/25th of an inch, or 1 millimeter in length. They live in sand or sandy mud and are not uncommon in such environments throughout the world. They likely enter reef aquaria within wild collected live sands. Although, they have not been definitively reported from reef aquaria, I would expect them to be present in some tanks, and they may actually be quite common, if overlooked, reef tank inhabitants.

Here is a link to a labeled diagram and some information:

And another link showing some of the external features:

These are segmented animals, and generally have thirteen segments, referred to a zonites. The body and segments are often covered by spines, but they lack articulating appendages. The most anterior point of the animal is the mouth, located on an introvertable cone, which can be withdrawn into the body. The body is covered with a thick chitinous cuticle. There are three chitinous plates that comprise each zonite, one thick curved dorsal plate, and two thinner ventral plates. Depending upon the species, there are often adhesive tubules that open in the ventral region of the worm. These are used to "fasten" the animal in place when it is feeding.

Figure 4. Lateral view of a Kinorhynch.

Kinorhynchs have a number of interesting attributes, not the least of which is the fact that the epidermis underlying the cuticle is a syncytium. That is to say, it lacks cells, and basically is a mass of protoplasm with many nuclei in it, but no cell membranes. This layer overlies a highly muscular dermis with striated muscles. These are rapidly moving little worms and the relatively complicate musculature that furnishes that movement is comprised of longitudinal, oblique and dorsoventral muscles. The nervous system is relatively complicated, and built in the invertebrate pattern with a ring of nervous tissue surrounding the mouth. There are a wide variety of sensory structures, such as sensory spines called scalids, as well as ocelli, or eyespots, in some species.

The gut is complex and with a highly muscular throat region where the cuticle is often very thick. Kinorhynchs are often described as grazers on diatoms and other sediment dwelling algae. The muscular, cuticularized throat region likely acts as a gizzard, crushing the prey for subsequent easy digestion.

Kinorhynchs have separate sexes, and internal fertilization, although copulation has not been documented in the scientific literature. The female lays eggs which hatch to give a juvenile of eleven segments. These animals grow by molting and with each molt a segment is added until they reach the adult complement of thirteen segments. Any subsequent molts result in an increase in size, but no additional segments are added.

These small worms are likely reasonably common in some reef aquaria, but presently are unknown in reef tanks. Because of their small size, it will likely be necessary to examine the sediments microscopically to find them. If they are found in reef aquaria, they will likely contribute to the "clean up" crew, eating diatoms and moving through the sediments, thus facilitating the functionality of the sediment biological filter.


The last group discussed in this column will be the Phylum Loricifera. As with the other two phyla discussed this month, they have no common name and are referred to as Loriciferans. The name refers to the "Lorica" or "shelled" external body wall that they possess. They are also unlikely to be observed in aquaria; but who knows, perhaps they will be seen and recognized in some lucky soul's system.

As I have no images of my own for the Loriciferans, you should follow these links to find some nice images of them:

And this link will lead you to some labeled diagrams, so that you might appreciate just how strange are the structures in these animals:

Loriciferans were described as a phylum in 1983, although the animals were first seen about 20 years earlier. These are animals that are best described as bizarre, and yet they are structurally amongst the "neatest" looking invertebrates. They look rather like miniature mechanical constructions rather than as members of the animal kingdom. Only about a dozen species are known, and none have been seen alive for more than a few minutes. They are minute, and may be the smallest of metazoan animals.

The body is regionated, and as with both the Priapulids and the Kinorhynchs, they have an introvertable, quite spiny, head. There is also a large abdomen, enclosed in a cuticular shell or lorica consisting of several rows of plates and structures arranged in a pattern of sixes. The mouth is located at the tip of the "head cone" much as in the kinorhynchs, and there are protrusible stylets or spines in the cone. The throat region contains what appears to be a muscular pumping apparatus, and this region is also lined with a molted cuticle. Although the diet is unknown, it appears that the throat area could act as a gizzard to crush or mash any ingested material.

The nervous system is relatively quite large, and as in the Kinorhynchs and Arthropods, virtually every spine on the body appears to be enervated by sensory nerve fibers. These animals receive a lot of sensory input through their cuticle by means of the sensory spines.

Remarkably little is known about Loriciferans, even though they have been studied for about thirty years. They glue themselves to sediments and are commonly found in relatively coarse shell fragment sands near Floridian and other reefs, but the only way to get them out of the sediments is by shocking the sediments with fresh water. As many aquarists realize, such fresh water rinses kill most animals, and that is also the case with Loriciferans. However, such rinses also break them free of the sediments, and their corpses can be collect for examination and study. It is hard to do natural historical or ecological studies from dead animals, and this is compounded by the fact that these are exceptionally small animals, so it is particularly difficult to identify gut contents. Their presence around Floridian, and other reefs, argues for the likelihood of their presence in reef aquaria, but obviously they have never been documented from aquaria.

Loriciferans will only be noticed if an aquarist undertakes a particular search for them. Sediments will have to be removed from the aquarium, and rapidly rinsed in fresh water to remove adherent materials and animals. The material cleaned out of the sediments by the fresh water rinse has to be rapidly sieved through a fine screen, preserved, and then microscopically examined. As they seem to be relatively rare and they are very tiny, it will likely take a lot of looking before any are seen. Of course, if any are seen, it would be momentous enough to warrant publication in both the scientific and popular literature. If anybody participates in a Loriciferate safari, I heartily extend my good wishes for good luck! These are oddest of animals and it would be grand to find them in aquaria.

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


Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Here are some online sources of information:
A list of 856 articles on Priapulids:
A list of 354 articles on Kinorhynchs:
A list of 149 articles on Loriciferates:

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Invertebrate Strippers -