1) Pseudocoelom filled with mesenchyme and loose organs- creating a functional acoelomate state

2) At least one pair of adhesive tubes and associated cement glands

3) Cuticle often possessing plates or spines

4) Ventral ciliation

5) Protonephridia and complete gut

6) No circulatory or respiratory structures

7) Hermaphroditic or parthenogenetic

If acanthocephalan worms are known for their bizarre forms of copulation, then gastrotriches may be regarded as a phylum with a sexual identity crisis

Gastricha-the Phylum studied by Edward Ruppert, who publishes a competing invertebrate zoology textbook.

There are two orders of Gastrotriches:

Macrodasyida- is marine only

Chaetonotida- freshwater and marine-Lepidodermella, which you saw in lab is a member of this order.

1) Although gastrotriches are considered pseudocoelomates, there’s not much space in the fluid-filled space to such an extent that they are effectively acoelomate. This may be a by-product of small body size. Nonetheless, this probable loss of a pseudocoel has caused evolutionary biologists to consider the pseudocoel as having limited phylogenetic importance and is offered as proof that pseudocoelomates are probably polyphyletic.

2) Gastrotriches have one or more pairs of adhesive tubes, from which an adhesive from the cement glands is extruded. The adhesive tubes form a forked tail in gastrotriches. The adhesive tubes are capable of grasping substrata in some groups

The cement glands contain an adhesive and another secretory product which dissolves the adhesive. In this way the structure is similar to the duo-glands of flatworms.

3) The outer cuticle has spines or scales covering a portion or the entire body. It is believed that this serves as an adaptation to particularly unstable sediment- since interstitial animals in sand, whether they are nematodes, loriciferans, gastrotriches, or kinorhyncs, tend to have knobby body walls or spines, or both.

4) Gastrotriches have ventral cilia situated in distinct rows that are used for moving across a substratum. This is how the phylum gets its name "stomach hair". The rows may be longitudinally or transversely arranged on the animal however.

5 & 6) Gastrotriches have simple protonephridia and a complete gut. Not surprisingly, they have no respiratory or circulatory system-which is unnecessary for a small animal usually less than a millimeter in length (some may reach 4 mm though).

7) Most gastrotriches are either hermaphroditic or parthenogenetic. Although, sex determination is more complicated than that in these pseudocoelomates.

SHOW GASTROTRICHES OVERHEAD (PAGE 283 BARNES)

Gastrotrichs sex:

Gastrotriches are hermaphrodites.

With a full gradation between-that is that often they have both male and female reproductive structures, but they are not developed at the same time.

Still others like Lepidodermella, may be parthenogenetic and males are very rarely found in a population.

Lepidodermella squamata was previously thought to be entirely parthenogenetic and have no functional males or sperm production among individuals.

Experiments in the lab have found that these females, if left in culture for extended periods of time, become male that produce sperm. The sperm are non-motile however-and it is unknown if they are functional.

These cultured lepidermella are kept alive a considerably longer period of time than they would in nature...so it is unknown if males would ever appear.

These parthenogenetic species, may be a case of protogynic hermaphroditism with the male portion supressed. Most gastrotriches live less than a year.

Chaetonotid Gastrotrichs produce few eggs, in line with small size and freshwater reproduction. They usually produce between 1 and 10 eggs.

Mating behavior has only been observed in two species of Macrodasyid gastrotriches- but what was seen is pretty cool.

First, most gastrotriches are believed to have internal fertilization.

There is a copulatory organ that is used for insemination. Some species have stylets and barbs on them which either suggests that hypodermic insemination may occur, or the barbs are used to remove another male’s sperm prior to insemination.

At least one species produces a spermatophore which is a packet of sperm surrounded by a mucous covering that is attached to a sand grain and picked up by another individual in female phase.

Some males attach the spermatophore directly to the body wall of a female where they are somehow absorbed through the cuticle to fertilize.

These are unusual modes of sperm transfer, most are directly through the copulatory organ of one to the female pore of another.

Turbanella cornuta was observed in typical gastrotriches mating....

Individuals in male phase would raise their posteriors and wave them in the water.

If no female responds, the male tries again later.

If a female responds, the male coils around the female forming a complex knot.

Other species are believed to be protandric, but whole populations are found in the male state only. This is weird if not unique in the animal kingdom.

Male-like gastrotrichs usually take about 20-23 days to become female-like.

The animals then separate.

Eggs are huge relative to body size, in some cases taking up collectively 80% of the body mass of the individual.

Some gastrotriches can pass the eggs through the female gonopore, some simply blow up with water -perhaps ceasing osmoregulation? and explodes sending eggs out.

There is no larval stage-individuals hatching out of eggs look like tiny adults.

1) Functionally acoelomate, but may have possessed one

2) Pharynx houses jaw apparatus

3) No excretory, circulatory, or gas exchange structures, but

4) Possess epidermal canals

5) Monociliated epidermal cells

6) Incomplete gut (vestigal anus)

7) Hermaphroditic

9) Spiral cleavage, determinate cell fate

1)Gnathostomulids are another weird group.- a phylum consisting of only 80 or so species. It is arguable if they are or are not pseudocoelomate animals.

Like gastrotriches and rotifers, they may have a reduced pseudocoel due to their small size. Most are less than one millimeter.

Originally described in 1956, they were classified as turbellarian flatworms. This early classification is reflected in many invertebrate zoology textbooks grouping them with Platyhelminthes. In 1969 they were assigned as an independent phylum.

They are different and have many advanced features.

2) They have a pharynx that houses jaws that are used to scrape algae and bacteria off of sand grains since they live interstitially. Gnathostomulida means literally "jaw mouth". They can be found in high densities in marine sand that is anoxic and rich in sulfide-suggesting anaerobic respiration in some of them.

3) They have no excretory, circulatory, or gas exchange structures, but possess epidermal canals that may have some function in one or several of these regards.

5) They have monociliated epidermal cells. You may remember that turbellarian flatworms have a ciliated epidermis-but they have multiple cilia on each cell used for locomotion. Gnathostomulids have only one per cell.

6) They have an incomplete gut, but there is strong evidence that they may have had a through gut at one time, but have secondarily lost it. since vestiges of a through gut remain.

7) They are hermaphroditic

Just when you thought you heard about bizarre sexual behavior in animals-here is some more freak sex among invertebrates.

Gnathostomulids are all hermaphroditic. They (Marlene Mainitz 1989) alternate periods of sexual maturity and sterile periods-kind of like your worst nightmare-reliving puberty over and over again.

The range in copulation and fertilization modes within this tiny phyla is astounding-just to give you a sample:

Some species have barbed stylets whereby they do the ol’ hypodermic insemination routine. These species have immobile sperm that are aflagellate so the sperm have to be injected into the right place (bursa) or no fertilization takes place.

Gnathostomulids are apparently unselective about where or whom they inject with sperm- since it may be found in the wrong body regions and organs and sterile phase individuals are just as likely to be inseminated as sexually mature ones.

Others have an adhesive penis that sticks to the body wall of another individual and the sperm whirl into a circle and bore into the body wall of the other individual.

The third method is by Conuli.

A conuli is either a giant sperm, or many small spermatophores.

This is the most poorly understood method. Apparently, males can only inseminate a female that are sterile but close to becoming sexually mature (again) phase. The reproductive tissues are degenerate except for the bursae- or sperm storage sac. It develops early. The Conuli is deposited early at this time through regular copulation with a penis that lacks stylets or adhesive structures. The prebursa stores the conuli until the rest of the reproductive system regenerates.

It is believed that these cycles of sterility and sexual maturation may be a means of eliminating sperm products from old copulations.

Of course all of this information is based on electron microscopy, ultrastructural studies, and other morphological studies since no one has ever seen a gnathostomulid copulate.

The evolutionary affinities of gnathostomulids with flatworms have been debated for years. Here is a synopsis of the debate regarding all the major points of contention:

1) Segmented pseudocoelomate

2) Cuticular plates divide segments

3) One pair of protonephridia

4) No circulatory or respiratory structures

5) Cuticle that is molted periodically

6) Complete gut

7) Separate sexes

SHOW OVERHEAD KINORHYNCS

Kinorhyncs (formerly known as Echinoderida) represent about 150 species of small (less than a millimeter) marine interstitial animals. Superficially, they resemble copepods in many regards.

1) They are segmented pseudocoelomates. There are 13 segments in adult kinorhyncs. 1 makes up the head, 1 the neck, and 11 in the trunk. Segments are often referred to as zonites. The head is retractable into the trunk.

2) Cuticular plates divide segments, so it appears to exhibit metamerism, based on serial repetition of the nervous system, and muscles.

3) One pair of protonephridia are present, but no respiratory or circulatory structures

4) The cuticle is shed periodically during development, although unlike in nematodes, the number of molts is variable

5) Complete gut, with specialization

6) Separate sexes.

These are interstitial burrowers too.

1) Pseudocoelomate

2) Regionally specialized throughgut

3) Pharynx modified as mastax

4) Anterior Ciliated corona

5) Posterior toes and adhesive glands

6) Cuticle present

7) Protonephridia, no repiratory or circulatory structures

8) Separate sexes or parthenogenetic

9) Cleavage spiral and determinate

Rotifers are composed of about 1800 species. They are ecologically important because they make up a substantial portion of the zooplankton, especially in freshwater systems. The name rotifer means "wheel bearer" They are also known as the Rotatoria by specialists.

They were first observed in the 1600’s by early inventors of the microscope such as Antony van Leeuwenhoek.

1) They are pseudocoelomate, but due to their small size (3mm or less), the pseudocoel is packed with organs.

2) Regionally specialized throughgut. This includes a mastax, which is a modifed pharynx and trophi which are jaws within the mastax used for mascerating food.

3)They also have a corona that they use for both locomotion and sweeping food into the mouth.

4) There are feet and toes, with adhesive glands for adhering to surfaces. Some will filter feed while clinging to a substratum.

5) There is a distinct cuticle that may have spines or knobs on it. The cuticle may also be annulated to increase flexibility. In many species the posterior portion of the animal, the Foot and Toes, may be retracted into the body cavity.

The cuticle exhibits eutely, in that the number of nuclei present is constant across a species. (usually between 900 and 1000 )

6) Protonephridia are present, but no circulatory or respiratory system. Typical of invertebrates of this size range.

8) There are separate sexes when there are males.

Reproduction is interesting among rotifers and may contribute to their success and number in ephemeral habitats such as small ponds.

Fertilization is internal, some males use hypodermic insemination, others, like gnathostomulids, have penes that are adhesive and attach to the body wall of a female-which may drag the male around for some time.

The Lansing effect has been found in rotifers (King, 1983), that is that the offspring of old parents tend to have shorter lifespans than the offspring of younger rotifers.

This effect was thought to be the result of some genetically controlled aging factor.

Instead, it is more likely to be a mechanism of alternation between sexual and parthenogenetic reproductive means.

SHOW OVERHEAD OF SUMMER AND AUTUMN CYCLES.

Rotifers may be "old" that were produced in the fall-and remained a dormant zygote (cryptobiotic) for the entire winter. These "old" rotifers tend to produce short-lived rotifers that reproduce quickly. Usually old rotifers produce females only through parthenogenesis and these rapidly reproduce as well through parthenogenesis, giving rise to only females during the summer. This is a way to saturate a new resource with your genes before competition becomes a factor.

This period of parthenogenetic reproduction is known as amictic (because the females were produced by mitosis instead of meiosis).

Male sperm is produced by mitosis. These sperm fertilize the haploid ova-the resulting eggs are extremely resistant to dessication and low temperatures-when they hatch out in the spring-they return to the parthenogenetic mode of reproduction.

This cyclic mode of reproduction serves as an interesting system to study the competitive advantages of sex in natural populations. Apparently, when populations are expanding rapidly and there are unlimited resources, it is better to only produce females, since males can’t make more males. If there is intense competition for food, apparently, sexually produced rotifers fare better. This is an example of r and K selection within the same species (remember that from sophmore biology?)

Terrestrial rotifers are associated with water films on lichen, moss and soil. They may dessicate are remain in a cryptobiotic state for years before rehydration occurs.

Lets discuss rotifer behavior for a bit--

Rotifer sensory structures-rotifers have incredible numbers of sensory receptors for an animal it’s size. Unfortunately, most of what we know about rotifer senses comes from ultrastructural studies rather than functional studies.

Photoreceptors-behavioral studies have found that rotifers have eyes that can respond to direction, quantity, quality, and duration of light.

No apparent recognition of movement or form however. They will scoot right by a food item even if hungry unless they touch it.

SHOW ROTIFER EYES (PAGE 99 ROTIFER BIOLOGY)

Chemoreceptors-both distance chemoreception and touch reception.

Mechanoreceptors-form, texture, and vibrations

Feeding

Rotifers have a number of feeding modes that relate to their movement patterns.

Cilia in the corona sweep food into the mouth.

Other rotifers with equally large corona, use the cilia for locomotion and drag the body behind.

Still others form use a crawling behavior on the substratum and pluck of food items as they encounter them.

Rotifers may use any of these feeding and locomotory modes exclusively or in combination.

SHOW OVERHEAD PAGE 309 RUPPERT AND BARNES

Interestingly, many rotifers exhibit what are known as developmental polymorphism. This is also seen in crustacea, insects, and some protozoa.

It is the expression of alternative morphotypes under different ecological conditions by organisms of a given genetic constitution.

The different castes found in social insects is probably the best example-different kinds of workers.

The flexible developmental pathways are triggered by environmental cues.

Cyclic patterns of developmental polymorphisms are known as cyclomorphosis and is common in some species of rotifer.

An example is Brachionus calyciflorus which may be induced to produce spines when exposed to substances produced by the predatory rotifer Asplanchna. These spines prevent Brachionus from being eaten-but do not normally appear in Asplanchna-free environments.

Rotifer life cycles

Amictic cycle during the summer (2n) eggs produce only females. Harsh conditions-dramatic change in pH, salt content, or high population density causes the production of mictic eggs that are haploid. If they are not fertilized (no males around), they turn into males. If fertilized, they become heavy dessication-resistant dormant eggs that sink to the bottom.

Parthenogenetically produced females may double their population size every two days.

Females can produce one or the other type of egg--but not both at the same time (one type only). Environmental conditions prevalent during oocyte development determines egg type.

Cleavage spiral and determinate

1) Pseudocoelomate?

2) Body divided into four parts (head, neck, thorax, and abdomen). The first 3 can be retracted into the fourth.

3) Abdomen housed in cuticular lorica

4) Mouth on a cone with stylets

5) Complete gut

6) No circulatory or respiratory system

7) One pair of protonephridia

8) Separate sexes with Higgins larva

Loriciferans are the newest animal phylum. They were discovered in 1983-so there is little information about any aspect of their biology at this point other than species descriptions. Originally it was named based on a single individual.

There have been 8 described so far. I was unsatisfied with the scant literature on these animals so I e-mailed Reinhardt Kristensen in Denmark to get more information. This is the guy that originally described the first phylum member.

I tried to get him to send me copies of his publications-and even a slide specimen if he could swing it.

He said that he isn’t sending specimens until he gets his described-so that people don’t publish on specimens he collected. He isn’t really sure how many species, genera, families, orders, or classes of loriciferans he has. He did say that he has at least 80 or 90 different morphotypes that are certainly different species or weird intermediate larval stages and the diversity is astounding. He said people will be surprised as the body shapes of some of these things. One of them has a head that can extend at least 4 times the distance of the rest of the body and is probably used in pulling the animal through sandy sediment.

He said he would try to send me one after he gets some more papers published because he has quite a few specimens of some of the more common types.

SHOW OVERHEAD OF LORICIFERAN AND BODY FORM

The name Loricifera means "corset-bearer" which refers to a well-developed lorica. They live interstitially between sand grains and are less than a millimeter long. The lorica has a series of spiney scalids and plates covering most of the body.

It has a retractable introvert, much like a priapulid or a kinorhync.

It also has numerous spines that probably anchor the animal between sand grains.

The gonads take up a considerable portion of the pseudocoel (if that’s what it is?) and fertilization is probably internal.

The larval form is called a Higgins larva. It is named after Robert Higgins who found an adult Loriciferan in 1974 but thought it was a priapulid larva.

The larva is similar to the adult but it has a few toes used for attachment to the substratum, much like how a rotifer does and probably has adhesive glands too.