Short science posts | Eluding Ctenophora

Ctenophora, commonly known as comb jellies, are a rather perplexing phylum of beautiful pelagic creatures. Their evolutionary position has been debated for many years as is the origin of their nervous system (some scientists believe they are older than sponges and that sponges lost their nervous system, while others advocate the theory about the nervous system forming independently twice, once in cnidarians and once in ctenophores).
Ctenophora have two nerve nets: subepidermal and less organized subgastrodermal, which recent research identifies as a mesogleal nerve net. Nerve cells from this layer communicate with muscles by synapses and affect the locomotion of the body. The subepidermal net is denser around the mouth, the pharynx, and under the comb rows (comb rows are strips that run the length of the ctenophore body and contain cilia called “ctenes”). Ctenophore neurons can be iso- and multipolar.

They have sensory cells on the whole surface of the body and those correspond to vibrations and thermal and chemical stimuli: more receptors are located around the mouth and pharynx. Ctenophora also have an apical and aboral sensory organ. Such sensory organ consists of a statocyst, a sensor that contains a statolith that balances on four groups of long cilia connected to the comb rows. These organs help the orientation of the ctenophore body.
What’s extremely interesting is that ctenophores use different chemical signalling system than the ones described in the previous posts, mainly because these animals simply lack the neurotransmitters (and genes), such as serotonin, dopamine, noradrenaline, and acetylcholine; glutamate is the only neurotransmitter currently known to be present.

I gathered all this information from different resources, and some are sometimes contradictory or are generalizing conclusions about the whole phylum from the data of only one ctenophora species. This is the best overview I could manage, to show both the similarities and the differences of the ctenophora nervous system, when compared to the Cnidarian system. These lovely animals are not very well researched and I’m sure many wonderful breakthroughs about their anatomy, physiology, and their place in the evolutionary tree are to come.

Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Norekian & Moroz Neural system and receptor diversity in the ctenophore Beroe abyssicola J Comp Neurol. 2019;1–23.
Ctenophores – quick guide
Did the ctenophore nervous system evolve independently?
Aliens in our midst

Short science posts | Hydrozoa

Hydrozoa are the last cnidarian class I’m going to write about. They can exist in two distinct shapes, as hydromedusa and hydropolyp (same as Scyphozoa and Cubozoa). Despite perhaps expecting hydrozoans to be the most advanced in both nervous and sensory systems, they don’t actually have any rophalium. Furthermore, some hydromedusae don’t even have nerve nets. However, they have two nerve rings (outer and inner) on the margins of their bells which are regarded as ganglia by some scientists.

These rings consist of neural pathways which process different sensory inputs (such as light and gravity). Aglantha digitale, a hydrozoan species, has been reported to have as much as 14 distinct neural pathways. A. digitale is also distinct from the other species in the class by having two swimming “modes” – slow (which is a characteristic for all hydrozoan) and escape mode. Transmission through giant ring neurons is responsible for both modes, but the escape mode requires a stronger contraction. The slow swim mode is activated by the input from the pacemaker, which triggers slow calcium spikes. Direct mechanical nerve ring stimulation by tentacles triggers fast sodium spikes. In short, giant ring neurons are capable of generating two different kinds of action potentials.


Gap junctions are also present in (and only in) Hydrozoa, and they transfer electrical signals through the musculature. Furthermore, I would like to emphasize that despite some hydromedusae not having a nerve net, some in fact do, and so do hydropolyps. In polyps, however, some groupings of the neurons could be found around their mouth.

Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Do jellyfish have central nervous systems?
Jellyfish nervous systems

Short science posts | Cubozoa (most advanced cnidarian nerve system)

Cubozoa, or box jellyfish, are another cnidarian class. Their name stems from their distinct cube-like shape. Cubozoa are also distinct from other cnidarian because their venom can be fatal to humans. As with all cnidarians, box jellyfish have two nerve nets and, like Scyphozoa, rophalia. However, box jellyfish also have a distinct nerve ring, as well as more developed eyes that consist of a lens, cornea, pupil, and a layer of retinal cells. Altogether, Cubozoa have 24 eyes, which makes them the most advanced cnidarian class in the sensory aspect.

Rophalia are mutually connected via the mentioned nerve ring. This ring is believed to be an integration center for the swimming, visual, and tentacle systems; it is comprised of oversized neurons, as well as some smaller neurites.
The communication between the nerve net and jellyfish muscles is regulated by chemical synapses.

Most of the information relating to Cubozoa, I already mentioned in the previous post about Scyphozoa, so I only wanted to relay the main differences between the two. These two classes are so similar that, until recently, they were actually considered one class.

Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
The ring nerve of the box jellyfish Tripedalia cystophora
Do jellyfish have central nervous systems?
Jellyfish nervous systems

Short science posts | Scyphozoa – more advanced nervous system

Scyphozoa (true jellyfish) are much more interesting (in a neurobiological way) than previously described corals. One major difference is that Scyphozoa are pelagic animals, which means they are not fixed to the ground. They also have two diffuse nerve nets (subepidermal and subgastrodermal) that consist of bipolar and multipolar neurons – the impulse conduction has been measured at 0,15 m/s. Both nets coordinate the movements of an animal towards the food. Some scientists, however, differentiate one diffuse and one motor nerve net. The motor net is in charge of the activation of muscle contractions after receiving signals from the so-called pacemaker organs (which are in charge of the swimming rhythm). The diffuse net, in this case, is in charge of marginal tentacle contraction and it is also believed it communicates sensory information to jellyfish musculature. Neurons of the motor nerve net are connected by chemical synapses, while neurons of diffuse nerve net are connected by peptidergic synapses that were noted in Anthozoa as well.

Sycphozoa also have much more developed sensory organs than any of the animals previously mentioned. These sensory structures are called rhopalia and they are located on the edges of the jellyfish bell – there are usually four of them (or a number that’s a multiple of four). Rhopalia contain multiple sensory receptors – statocyst (balance receptor), ocelli (light sensitivity), a mechanoreceptor, a chemoreceptor, and aforementioned pacemaker neurons.

I would also like to note here that some authors (I’m referring here to the article “Do jellyfish have central nervous systems?” by R. A. Satterlie) believe this kind of nerve net explanation is rather simplified and that there exist some evidence suggesting that jellyfish have a centralized nervous system, mainly that rophalia are in fact rudimentary ganglia and could be regarded as integrative centers. However, any communications between rhopalia themselves exist only through the nerve nets.

Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Do jellyfish have central nervous systems?
Jellyfish nervous systems

Short science posts | Anthozoa & nerve cells

Anthozoa (corals & anemones) are interesting animals that live exclusively in seas and oceans. Since they are regarded as sessile organisms, their nervous system is not very advanced. In a previous post, I mentioned that all cnidarians have two diffuse neural nets, and that is true for Anthozoans as well. Multipolar nerve net neurons are connected with synapses, and they also possess sensory cells that are particularly numerous around the mouth and on the tentacles.

One paper investigated traveling of electrical waves in a coral nerve network in coral colonies in genus Palythoa. It was experimentally observed that this electrical wave spreads at the constant speed from the site of simulation. Furthermore, peptidergic neurons (the ones using neuropeptides to communicate) were also noted in Anthozoa.

Did I draw this when I was 5 or 25? – A mystery.

Nematostella vectensis, also known as starlet sea anemone, is a species of Anthozoa that is known as a model organism – its genome and development have been carefully studied, including its nervous system. In this species, oral and pharyngeal nerve rings have been reported, as well as  longitudinal tracts of neurites (neurites are usually axons or dendrites). These findings would suggest that some groupings of neural cells exist in at least some Anthozoan species after all. Sensory neurons, interneurons, motorneurons, and neurosecretory‐like gland cells were also reported to exist in N. vectensis.

Note: in case I didn’t mention this before, cnidocytes are often considered neural cells because they display mechanosensory properties and calcium dependent neural‐like properties as well.

Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
The rise of the starlet sea anemone Nematostella vectensis as a model system to investigate development and regeneration
Cnidarians and the evolutionary origin of the nervous system
Model of traveling waves in a coral nerve network

Short science posts | What is a nerve net?

A nerve net is a type of nervous system that consists of many neurons but there is no brain or cephalization. Nerve nets are found in animals with radial symmetry (Cnidaria) and biradial symmetry (Ctenophora). Despite being called a net, there sometimes exist some groupings of neural cells in some Cnidaria classes, which I will write more about during the next couple of weeks.
Cnidaria are specific due to their specialized organelles, cnidocytes, which they utilize to hunt for food or use for securing itself to a surface. Some cnidocytes contain toxins that can paralyze their prey (the burning sensation you may have felt when touching a sea anemone 😉).
As a rule, Cnidaria have two diffuse nerve nets, one in the epidermal layer and a second one in the gastrodermal layer. In between these two layers is the mesoglea, a layer that functions as sort of a skeleton. The epidermal net consists of bipolar and multipolar nerve cells, while the gastrodermal net is made up of only multipolar cells.

Cerianthus membranaecus (known as cylinder anemone or coloured tube anemone)

Cnidarian nerve systems are fascinating but also quite unexplored. What is known is that nerve cells consist of two types of neurons, sensory neurons that respond to stimuli and motor neurons which ultimately trigger a response. Chemical synapses exist and provide the communication between the neurons. Hormones have also been reported in some cnidarians (steroids, neuropeptides) but it is still not known how exactly these signalling molecules work.


In the next couple of weeks, I will write a post about every cnidarian class and also ctenophores, focusing on their nervous and sensory systems. If you have any questions or would like me to focus on something, please let me know!

Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Endocrine-like Signaling in Cnidarians: Current Understanding and Implications for Ecophysiology
Evolution of sensory structures in basal metazoa

Short science posts | Do sponges have a nervous system?

Sponges (phylum Porifera) are sessile multicellular organisms that live predominantly in seas and oceans. They don’t have tissues or organs, and therefore, they don’t actually have a nervous system. However, they do have bipolar and multipolar cells that resemble nerve cells, which are found in the middle, “jelly-like”, layer.
Sequencing of some sponge species showed the presence of many genes associated with neural cells, such as genes that code enzymes for neurotransmitter synthesis and synaptic transmission. It is important to note that these genes have other functions in the organism. It has also been observed that some sponge larvae can respond to outer stimuli and show various “taxis” behaviour – phototaxis (response to light), geotaxis (response to gravity), rheotaxis (response to water current). Phototaxis has been closely studied in species Amphimedon queenslandica (class Demospongiae), a sponge native to Coral Sea.

Aplysina aerophoba, also of class Demospongiae, which can be found in Adriatic Sea.

Potassium channels have been observed in that same species, as well as glutamate, GABA, and NO systems, which have been investigated in Ephydatia muelleri, another species of class Demospongiae. Electrical signalling has been noted in glass sponges (class Hexactinellida). These sponges have bodies comprised of a syncitial tissue and their skeleton is made of silicon dioxide. The scientists were able to measure the action potential (5s long, with 29s refractory period) and deduce this signal relies on potassium and calcium ions.
Some scientists even suggest that sponges used to have a nervous system, but lost it during evolution – they introduced several hypothetical scenarios for this event, proposing that sponges lost their nervous system in order to focus on filtering.

Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Evidence for Glutamate, GABA and NO in Coordinating Behaviour in the Sponge, Ephydatia Muelleri (Demospongiae, Spongillidae)

The GABAergic-like System in the Marine Demosponge Chondrilla Nucula
Where is my mind? How sponges and placozoans may have lost neural cell types
Elements of a ‘nervous system’ in sponges

Short science posts | Nervous system evolution

For the next couple of weeks, I would like to write a bit about the evolution of the nervous system, from early nerve cells to the human nervous system and brain evolution. Alongside nervous I will also focus, to a lesser extent, on sensory systems. These posts will be published on my Instagram account, but I decided to publish them on the blog as well.

Mostly, these posts will be about various animals and the nerve systems they have – nerve nets, nerve cords, complete systems. The main process behind this is called cephalization, and it starts with the groupings of nerve cells and ganglia at one end of the body. After some (long) time, this process led to us having a head with sensory organs and a brain inside it.

But when did all of it start? It is kind of hard to say, for even single-celled organisms, such as bacteria, have voltage-gated channels and genes that support the theory of possible synaptic transmission. These channels are potassium (the oldest), calcium, and, rarely, sodium channels as well. Action potentials have been detected in some algae and diatoms, although their function is mostly unclear. In Chlamydomonas (unicellular green algae) on the other hand, potentials were detected in flagellums, which clearly suggest they play the part in the movement of the algae. Action potentials were also recorded in the cilia of some protists, such as Parmecium.

Of course, the exact evolutionary processes are unknown, and there is a possibility that these organisms acquired the mentioned features later than scientists now assume. It is also possible that some more evolved organisms, such as sponges, subsequently lost some of the features discussed here (more about this in the next week’s post).

Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Bacterial voltage-gated sodium channels (BacNaVs) from the soil, sea, and salt lakes enlighten molecular mechanisms of electrical signaling and pharmacology in the brain and heart
Early evolution of neurons
Deep evolutionary origins of neurobiology
From damage response to action potentials: early evolution of neural and contractile modules in stem eukaryotes