Dicyemida

Key takeaways

Dicyemida belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Dicyemida to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Dicyemida from memory before moving on to harder problems.

Reference excerpt

Dicyemida, also known as Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods.

Taxonomy

Classification is controversial. Traditionally, dicyemids have been grouped with the Orthonectida in the phylum Mesozoa and, from 2017, molecular evidence appears to confirm this. However, other molecular phylogenies have placed the dicyemids more closely related to the roundworms. Additional molecular evidence suggests that this phylum is derived from the Lophotrochozoa. The phylum (or class if retained within Mesozoa) contains three families, Conocyemidae, Dicyemidae and Kantharellidae, which have sometimes been further grouped into orders. Authors who treat Dicyemida as an order and separate the family Conocyemidae into a different order (Heterocyemida) prefer 'Rhombozoa' as a more inclusive and unambiguous name to cover all three families.

Anatomy Adult dicyemids range in length from 0.5 to 7 millimetres (0.020 to 0.276 in), and they can be easily viewed through a light microscope. They display eutely, a condition in which each adult individual of a given species has the same number of cells, making cell number a useful identifying character. Dicyemida lack respiratory, circulatory, excretory, digestive, and nervous systems. The organism's structure is simple: a single axial cell is surrounded by a jacket of twenty to thirty ciliated cells. The anterior region of the organism is termed a calotte and functions to attach the parasite to folds on the surface of its host's renal appendages. When more than one species of dicyemida exist within the same host, they have distinctly shaped calottes, which range in shape from conical to disk shaped, or cap shaped. To this day, there has never been a recorded case of two separate species of dicyemida existing in the same host and having exactly the same calotte. Species that share similar or even identical calottes have been found on occasion, but have never been found within the same host. Because of the constant variation in calotte size between species (even within one given host) there is very rarely observable competition between the multiple Dicyemida species for habitat or other resources. Calotte shape determines where a dicyemid can comfortably live. In general, dicyemida with conical shaped calottes fit best within the folds of the kidneys, while those with rounded calottes (disk or cap shaped) are more easily able to attach to the smooth surfaces of the kidneys. This extreme segregation of habitats allows multiple species of dicyemids to comfortably exist within the same host while not still competing for space or resources (by occupying different ecological niches).

Habitat While most dicyemid species have been found to prefer to live within specific cephalopods, no one species is unique in their preferences. In fact, It is also almost unheard of that a host infected with a dicyemid is only infected with one species. This means that if a select cephalopod is found to be infected with one dicyemid species, their body will likely be found to contain organisms with a variety of calotte shapes, which means they are infected with multiple different species. On the occasion that similar (but not identical) calotte shapes happen to be present within one host's body, one species usually ends up dominating the other, indicating that it has adapted more readily to the environment within the host. However, this occurrence is very rare and has only been observed a handful of times. In a study done on octopuses, it was found that Dicyemida that had similarly shaped calottes rarely coexisted in the same individual host, which suggested a strong level of competition for habitat. In Japan, two types of dicyemid parasites, D. misakiense and D. japonicum, have often been discovered living in the same host. In 1938, when the two species were initially discovered, scientists did not classify them as separate species due to their large amount of morphological similarities. In fact, the only difference between the two species that scientists were able to observe was between the shape of their calottes.The idea that D. misakiense and D. japonicum are two different species is still very controversial among scientific groups. Some scientists have speculated that when closely related species of dicyemids coexist in the same region, such as in the case of D. misakiense and D. japonicum, competition for habitat causes them to evolve to develop two distinct calotte shapes.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Dicyemida

Start with the simplest possible case. Write down what Dicyemida claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Dicyemida before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Dicyemida ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Dicyemida

In research: Dicyemida appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Dicyemida in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Dicyemida is common in secondary-school and first-year university syllabi. It links to neighbouring topics Dicyemida, Mesozoa, Parasites of molluscs, so understanding it makes those chapters shorter.
In everyday life: Look for Dicyemida outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Dicyemida in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Dicyemida means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Dicyemida out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Dicyemida in simple terms?

Dicyemida, also known as Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods. Taxonomy Classification is controversial.

Why does Dicyemida matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Dicyemida?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Dicyemida.