I'm happy to report that despite everything, I was finally able to find the time to finish this illustration and break out of my 20-illustration slump. I actually have another one ready from last year, but I want to save that one for later; in fact, I hope to explain it in a future post. And I apologize for any discomfort or strangeness you may have felt.
In this post, I present the illustrations I've created of Hatena arenicola N.Okamoto & Inouye 2006. The illustrations are free to use under CC BY-SA 4.0, non-commercial, attribution required (DOTkamina 2026).
It belongs to the family Katablepharidaceae, order Katablepharidales, class Katablepharidophyceae, which is part of the subphylum Rollomonadia. This means that Hatena arenicola is related to cryptomonad algae (class Cryptophyceae) and goniomonads (class Goniomonadophyceae), which are also within Rollomonadia. Aside from that, it's the same story I've explained in other posts: Rollomonadia belongs to the phylum Cryptista, clade Pancryptista, and CAM clade. The latter includes Archaeplastida as the sister group to Pancryptista, and encompasses the plants and algae related to its ancestors. Therefore, Hatena arenicola is a distant relative of plants. It's also worth mentioning that another way to classify it is to include the phylum Cryptista as part of the subkingdom Hacrobia, kingdom Chromista, and superkingdom Corticata, where the other kingdom is Plantae. But anyway, the conclusions regarding relationships are almost the same.
I consulted a single source for the morphological and descriptive information of Hatena arenicola, as well as the primary source for the illustrations: "Hatena arenicola gen. et sp. nov., a Katablepharid Undergoing Probable Plastid Acquisition" by Noriko Okamoto and Isao Inouye (2006).
What's curious about this organism is that it seems to have been left "halfway" in the process of acquiring chloroplasts. To give you some context, it's believed that modern plants evolved from eukaryotes that acquired photosynthetic bacteria (primary endosymbiosis, including the ancestors of Viridiplantae, Glaucophyta, and Rhodophyta) or algae (that is, eukaryotes that already had a photosynthetic bacterium integrated as a chloroplast). This latter case is known as secondary endosymbiosis, and includes Euglenophyta and Chlorarachniophyta (which acquired green algae), as well as Heterokontophyta, Haptophyta, Cryptophyta, Dinophyta, and Apicomplexa (which acquired red algae).
There is some evidence of organisms that have undergone secondary endosymbiosis, but very little about how this process occurred. And that's what makes Hatena arenicola stand out, as it appears to be an organism that hasn't yet fully integrated an alga as an organelle. Instead, it's "halfway" integrated; it has a life cycle where it can live freely, and it captures a free-living alga (of the genus Nephroselmis), which it doesn't digest (as it would with other algae), but rather integrates as another part of its body, in a state of endosymbiosis.
That's why it's best to present the following image as the front page:
The illustration essentially depicts how an individual of Hatena arenicola, lacking a symbiotic alga, possesses a feeding apparatus (predatory phase). It uses this apparatus to ingest the symbiotic alga, which then integrates with it, giving rise to the symbiotic state of Hatena arenicola. Due to the process of acquiring the symbiont, the feeding apparatus disappears. When it needs to divide, the nucleus (originally located posteriorly) shifts to the anterior region. The organism divides, with one daughter cell retaining the symbiont and the other not. This daughter cell without a symbiont will develop a feeding apparatus and continue its predatory phase until it acquires another symbiotic alga, Nephroselmis. The daughter cell that inherited the symbiont can continue its life in the "plant phase," living off the photosynthesis provided by the symbiont, and can divide again, generating one daughter cell with a symbiont and another without. As an additional note, I consulted "Phylogeny and ultrastructure of Nephroselmis and Pseudoscourfieldia (Chlorophyta), including the description of Nephroselmis anterostigmatica sp. nov. and a proposal for the Nephroselmidales ord. nov." by Nakayama et al. (2007) for information on the ventral and dorsal locations of the Nephroselmis symbiont.
That said, it is clear that Hatena arenicola has at least two main states: with a symbiont and without a symbiont. Let's begin with the state with the symbiont stage. Hatena arenicola does not possess a feeding apparatus in this stage. Aside from that, it does have everything else: a furrow from which the flagella emerge (the exact area where they emerge is known as the "flagellar insertion zone"). The flagella are derived from basal bodies, which I haven't shown here. There are two types of ejectisomes: type I (large, arranged in two rows near the flagellar insertion zone), and type II (smaller, distributed in numerous rows throughout the cell, except in the area surrounding the symbiont's eyespot).
In addition to the ejectisomes, it also has a nucleus located in the middle posterior region of the cell (when the cell is not ready to divide), with electron-dense chromatin that is always condensed (i.e., in a heterochromatic state). I've represented this heterochromatin as darker clumps in the nucleus; I think it's more visible in the version without the symbiont, which I'll discuss later. In addition to the nucleus, there is also a single "Golgi body" between the nucleus and the flagellar apparatus (basal bodies + flagella). In Figure 6D, you can see some "lines" within the Golgi body, and that's how I've chosen to represent it as well. Next to the Golgi body,
I've depicted a lysosome, which is evidence of the predatory lifestyle of Hatena arenicola. It's assumed that this organism lives by preying on other algae until it finds a Nephroselmis symbiont. With other algae, it simply digests them completely, and their scales may remain within the lysosomes. I don't know how many lysosomes there are, but I've only depicted one, containing Pyramimonas scales. In the context of these illustrations, and especially for this one of the symbiotic state, it's assumed that in this specific case, Pyramimonas scales still remain in the lysosome, even after acquiring the Nephroselmis symbiont. Incidentally, when it engulfs Nephroselmis, the latter also undergoes a reduction in its structures, which are digested, leaving remnants such as the (more or less) star-shaped scales of Nephroselmis, which also remain inside the lysosome.
There are many mitochondrial profiles throughout the cell, and the authors think they could be sections or pieces of a single large reticulated mitochondrion, so I've represented it that way. The endoplasmic reticulum is distributed loosely throughout the cell. The article refers to "rough endoplasmic reticulum" that "extends beneath the cell surface," which implies the existence of a smooth endoplasmic reticulum. In other illustrations, I've represented both arrangements of endoplasmic reticulum, but frankly, in this illustration, I was just too lazy (besides, the image was going to become even more oversaturated), so I've left it simply as "endoplasmic reticulum."
And now, the symbiont. Earlier I mentioned that Hatena arenicola literally engulfs the symbiont Nephroselmis. How do the authors know that it actually becomes a symbiont and isn't merely a hijacking of structures and eventual death of the ingested organism? Well, because the symbiont Nephroselmis literally grows inside Hatena arenicola. It's true that it loses several structures, such as flagella, endoplasmic reticulum, and the scales on its cell surface, but in return, it undergoes modifications in its eyespot, its chloroplast grows to almost occupy most of the space in the Hatena arenicola cell, and it develops more pyrenoids, because originally the symbiont only has one.
This is a response to a symbiotic adaptation in which it must obtain energy from the sun through photosynthesis, but no longer solely for itself, but also for the host (Hatena arenicola). Hence, it develops more chloroplasts, eyespots, and pyrenoids: to generate more energy for both organisms. In exchange, Hatena arenicola loses its feeding apparatus, as it now obtains the energy it needs to live from the photosynthesis of its symbiont. What does the symbiont gain from this? Well, I suppose it gains protection from Hatena arenicola, since it is "covered" by the host.
I should mention that the symbiont's cytoplasm is preserved, although it is called "vestigial cytoplasm." It's not very noticeable in the illustration, but I've drawn it there. Even under a microscope, it's not very visible because most of the symbiont's cytoplasmic space is occupied by the chloroplast itself, which in the article is simply called a "plastid." The life cycle diagram only shows the chloroplast, but keep in mind that in reality, it's not just the chloroplast that exists; it's actually located within the vestigial cytoplasm.
The symbiont's pyrenoids have some invaginations of the chloroplast's thylakoids. I've represented these invaginations as slight convex curves. I haven't shown the thylakoids themselves. The pyrenoids are surrounded by starch sheath, as is common for those who have read the protist posts to date. The symbiont also retains the reticulated mitochondrion, with flat, often degraded cristae (which is why I've represented the cristae as small elliptical spots distributed along the symbiont's mitochondria). The symbiont's nucleus is also preserved, located face-to-face with the nucleus of Hatena arenicola. There are also sacs that resemble those of a Golgi apparatus, but it is thought to be in an inactive or degraded form since it has no associated vesicles. The eyespot, of course, located where the feeding apparatus would be, is conspicuous and made of a single-layered sheet of osmiophilic granules. The eyespot is located beneath the chloroplast membrane.
In the illustration of the symbiont state, I have depicted the symbiont's chloroplast in its most massive form, occupying almost the entire cell space. In the illustration of the non-symbiont stage, the Hatena arenicola structures that do not belong to the symbiont are more clearly visible. Additionally, the feeding apparatus is present, which is actually a microtubular network made of two parts: transverse tubular rings (shown here in pink), and longitudinal microtubules arrayed in a single layer (shown here in yellow).
Obviously, these are not the actual colors; I represented them this way to make them stand out against the blue background. Within the microtubular skeleton of the feeding apparatus, there are several electron-opaque granules, some large and elongated (light gray), and others smaller, granular, and pigmented (i.e., darker). I've represented both types, but I don't think I've managed to distinguish between them well, and they become barely visible with the colors I used for the feeding apparatus. Oh well, I never passed color theory.
As always, remember that the colors used in these illustrations are more for educational purposes than to accurately represent reality. My ribs are hurting, so I'll stop writing here.
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