#rotifers

Meeting a water bear. 🌱

My tabletop roleplaying setting, Mortasheen, will be available to purchase for a limited time beginning on Halloween, 2025. Until then, bonus supplementary monsters will be going up every day of October. DAY 01/31: BRAINSLARK

Click the link for Brainslark's extensive lore, concept/design notes, and gameplay stats constructed by @gutsygills, art and writing by me.

About Mortasheen's Halloween soft-release

The history of the Mortasheen project and game

The wheels on the animal go round and round and I'm rolling up to talk about wheel animals. These tiny critters are pretty bizarre and also very important to their ecosystems. It's always interesting to talk about plankton because they're very cool and important animals that a lot of people don't know anything about.

(Image: a microscope image of a rotifer. It is a small, cylindrical, translucent animal with three notable body segments. On one end is the head, which opens up at the top into two petal-like structures. In the middle is the thick trunk, which is mostly taken up by a large stomach that is colored red from a previous meal. At the end is tapering foot. End ID)

Rotifers, also called wheel animals, are members of the phylum Rotifera. They are tiny animals, most being microscopic. Rotifers can come in a variety of shapes, but are typically cylindrical. The main body plan has three segments: the head, trunk, and foot, which are covered and protected by a tough yet flexible outer covering called the cuticle. The cuticle is secreted by a fibrous layer within the epidermis. This layer can be rigid or flexible. Rotifers with rigid fibrous layers are called loricate and they have fixed, inflexible, box-like shapes. Those with flexible fibrous layers are called illoricate and they have more malleable, worm-like bodies. A trait of rotifers and some other tiny animals is eutely, which means adults of a species have a fixed number of cells. Most rotifers will have around 1000 cells as adults, with variation between species.

(image: a microscope image of a rotifer. This one has a very wide, box-like trunk with backwards-facing spikes to the sides of the foot. End ID)

The head contains the mouth, which is located at the tip of the body. Surrounding the mouth is a structure called the corona, which is the only body part not covered by the cuticle. The cuticle can come in many shapes, from a simple ring of cilia to a funnel, to flaps, and so on. In the class Bdelloidea, which is the most well-studied group of rotifers, the corona appears as paired disk-like segments with some ability to rotate. This is why rotifers as a whole are often called wheel animals. The corona uses cilia to bring food to the mouth. In swimming species, the corona is also used to pull the animal through the water. Once food enters the mouth, it passes into a structure called the mastax. The mastax is a muscular cavity lined with jaw-like structures called trophi. The mastax is used to grind up food into more digestible bits. The trophi are the only hard parts of rotifers and are often the only parts that fossilize. After the mastax, the food passes into an esophagus. Also located in the head is the cerebral ganglion, which is the closest thing rotifers have to the brain. It encircles the mouth just above the mastax and extends nerves down into the body. The head also contains sensory organs including small antennae, eyespots, and sometimes eyes. Connected to the cerebral ganglion is the retrocerebral organ, which consists of a series of sacs and glands that appear to produce and then secrete various chemicals. The function of the retrocerebral organ is unknown. The nervous system contains about 25% of the cells in the body.

(Image: a head-on shot of a corona. It is a heart-shaped structure with two flaps, each lines with cilia. The mouth is a hole in the center. End ID)

The trunk is the largest part of the body and contains most of the organs, including the rest of the digestive system. The trunk is surrounded by a fluid-filled chamber called a psuedocoelom that acts as the base of an endostatic skeleton. Muscles in the trunk push on the pseudocoelom to provide movement. The trunk also houses the reproductive organs. The foot is at the end of the body and end in one to four sticky toes which allow the rotifer to grab onto things. Many species stay permanently in one spot for their whole lives, while others will only grab onto a structure to rest or find a perch to feed from. Some species swim for their entire lives and have lost the foot entirely. In many species, the foot can be retracted into the body to protect it when not in use.

(Image: a rotifer with a very large corona. It takes the form of two flaps lines with cilia, with each flap being larger than the trunk. The foot is also very long and appears to be grabbing onto some debris. End ID)

A group of rotifers that have strayed pretty far from the base body plan are the Syndermatids, who have evolved into parasites. There are two groups of Syndermatids, the Seisonids and the Acanthocephalids. The Seisonids are four known species that live exclusively in the gills of the marine crustacean genus Nebalia and seem to feed on the host's hemolymph (bug blood). They have distinct musculature from other rotifers and likely diverged from them early on. Acanthocephalids are so diverged from other rotifers that they were classified as their own phylum until genetic analysis identified them as rotifers. They are endoparasites who live in the intestines of a host and absorb digested nutrients through their cuticles. They have no mouths or coronas and the mastax has become an external proboscis that is used to cling onto the intestinal lining.

(Image: an Acanthocephalid rotifer. It looks very little like other rotifers, being a worm-like shape with a proboscis lines with sharp hooks. End ID)

Rotifers are mostly freshwater organisms, but there are brackish and marine species as well. They are found worldwide, with many species having a global distribution. They are filter feeders who consume bacteria, algae, and organic detritus. Rotifers perform an important role in their environments by keeping bacteria and algae numbers down, cleaning up detritus, aiding in decomposition, and acting as an important food source for large animals. They live a number of different lifestyles based on species. Some are free-swimming and spend their lives as plankton. Others swim only some of the time and use their feet to cling on to substrate the rest of the time. Some do not swim and move in an inchworm-like fashion. They will grab onto a surface with the foot, bend over and grab the next spot with the corona, then detach the foot and move it to the next spot, repeating until they arrive at their destination. Still others are sessile as adults, grabbing onto a surface with the foot and then never moving again. Many of these will form a solid or gelatinous tube around themselves for protection. There are also 25 known species of colonial rotifers. These will all grab onto the same object and then secrete a glue-like substance that binds them all together. These colonies can be attached to a stationary object or free-floating. Unlike other colonial filter-feeders, colonial rotifers do not share DNA or nutrients. Sometimes, members of a colony will glue their eggs together so the offspring start out with new colony members already there.

(Image: a rotifer colony. It is a group of rotifers with very long feet that all connect in a central mass. The rotifers radiate out from this mass in a circle. End ID)

Rotifer reproduction is notoriously weird. There are three different strategies used by different taxa: sexual, asexual, and alteration of generations. The parasitic roetifers use sexual reproduction. Individuals are dioecious, either male or female, with females being slightly larger. Reproduction is typically internal, with the male either inserting a penis into the female's cloaca or vagina or piercing the body wall to release sperm. In some species, the male will glue the female's vagina shut on the way to ensure she cannot mate with another. Rotifers that reproduce asexually are found in the class Bdelloidea. They are exclusively female and reproduce with parthenogenesis. This effectively makes a clone of the mother. Bdelloid rotifers are capable of sharing genes with other organisms around them, including fungi and bacteria, through a process called horizontal gene transfer. This likely helps them increase genetic diversity without sexual reproduction. The class Monogononta is where things get weird. They spend most of the year reproducing with parthenogenesis, allowing for rapid growth and colonization of new habitats. Eggs produced with parthenogenesis are diploid, meaning they contain two copies of their genome. However, when triggered by climactic conditions (which vary based on species), they will transition to making haploid egg cells. Haploid eggs contain one copy of the genome and can develop either into diploid females or haploid males. If the egg is unfertilized, it will become a male. These males are much smaller than the females and don't have a complete digestive system. They basically just live long enough to mate and then die. If a male mates with a female and fertilizes her haploid eggs, the eggs will then hatch into more diploid females.

(Image: a rotifer swimming with a cluster of small, round eggs attached to her body End ID)

Most rotifer species only reproduce once in their lives. The female will lay eggs that are glued to a nearby surface, often the mother's body, until they hatch. Many species will attempt to attach their eggs to animals like fish and birds, who can then carry the eggs to new habitats. Most species hatch as miniature adults, but some will go through a larval stage first. In sessile species, the newborns will typically be planktonic for a while before settling as they grow. Some species are ovoviviparous, meaning they retain the eggs and let them hatch internally before giving birth to the offspring. Rotifers will grow to adulthood in a few days and have lifespans ranging from days to weeks. However, some can extend that span rapidly. Bdelloid rotifers can enter a state called cryptobiosis where they cease metabolic activity until the right condition return. The most common form of cryptobiosis seen in rotifers is anhydrosis. This is seen in drought conditions and sees the rotifer expelling its internal water and entering a dried-out stage. When water returns to the environment, the rotifer will rehydrate and wake back up. This is used to allow the rotifer to survive dry seasons and repopulate when water returns. They can also survive freezing. In one study, rotifers that had been frozen in arctic permafrost for 24,000 years revived when thawed out.

(Image: one of the 24,000 year old permafrost rotifers. It is very long and slender, with a small corona, making it look like a worm. End ID)

There has been very little research into the conservation needs of rotifers. Most are believed to be abundant in their ecosystems and have a very wide distribution, though there are are species endemic to a certain area. Pollution and habitat loss are likely their main threats. There is a market for rotifers. Desiccated rotifers are sold to be stocked in fish tanks, and aquaculture setups, where they provide food and help improve water quality.

this tiny faced slop eater just learned about printers, and is now printing lots and lots of pictures of his friends and family and anything he likes on the internet!

he doesn't have that much storage space so he'll have to store them in unsafe places like his boss's house and his organs. not like that's going to stop him any time soon though

glorpiness rating: bittywitty

photographed by onotole

The Scientific Research Notes of S. Sunkavally. Years: 1986 - 1990.