A while back I made reference to the process of cephalization. This was honestly my off the cuff attempt at an argument with the tools I had available. Either I have an over-active imagination which is determined to pattern match everything, or there are similar constraints between internal decision making systems of biological organisms and human organizations. Naturally this is not going to be a one to one mapping. If we can extract certain patterns, as many others have done, from the biological origins of judgment, we might learn something about our own strange systems.
Honestly I have too much to read before I will have any semblance of an answer. So rather than sit quietly and think to myself I have decided to subject my readers once again to thinking out loud. This week I have some commentary on a paper about sponges.
Origin of the neuro-sensory system: new and expected insights from sponges
Renard E, Vacelet J, Gazave E, Lapebie P, Borchiellini C, Ereskovsky AV (2009) Origin of the neuro-sensory system: new and expected insights from sponges. Integr Zool 4(3):294–308
When I look at a sponge I basically wonder how the thing is alive. It doesn’t really do anything, does it? I mean (land) plants at the very least have distinguishable organs. Yet a sponge is alive.
The capacity of all cells to respond to stimuli implies the conduction of information at least over short distances. In multicellular organisms, more complex systems of integration and coordination of activities are necessary. In most animals, the processing of information is performed by a nervous system. Among the most basal taxa, sponges are nerveless so that it is traditionally assumed that the integrated neuro-sensory system originated only once in Eumetazoa, a hypothesis not in agreement with some recent phylogenomic studies. The aim of this review is to show that recent data on sponges might provide clues for understanding the origin of this complex system. First, sponges are able to react to external stimuli, and some of them display spontaneous movement activities. These coordinated behaviors involve nervous system-like mechanisms, such as action potentials and/or neurotransmitters. Second, genomic analyses show that sponges possess genes orthologous to those involved in the patterning or functioning of the neuro-sensory system in Eumetazoa. Finally, some of these genes are expressed in specific cells (flask cells, choanocytes). Together with ultrastructural data, this gives rise to challenging hypotheses concerning cell types that might play neuro-sensory-like roles in sponges.
I will side step the phylogenomic studies issue for this post. I’m not qualified to answer whether neurosensory organs originated with Eumentazoa (the orthodox opinion) or with some earlier ancestor (a new hypothesis based on phylogenomic studies). What I am interested in is the part about what sponges can tell us about “the origins of complex systems.” First let’s get a better idea of what a sponge is.
“Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues and organs, and have no body symmetry. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum. Many sponges have internal skeletons of spongin and/or spicules ofcalcium carbonate or silicon dioxide. All sponges are sessile aquatic animals.” –La Wik
|Cells in each layer bound together||No, except that Homoscleromorpha have basement membranes.|
|Number of cells in middle “jelly” layer||Many|
|Cells in outer layers can move inwards and change functions||Yes|
Fairly simple creatures…
All living organisms are able to respond to some stimuli. This implies the existence of electrical or chemical mechanisms for conducting information at least over short distances at intracellular level.
Together with the acquisition of multicellularity, signal transduction over longer distances as well as intercellular communication mechanisms are required to ensure efficient coordination, movement or behavior of the whole organism. This has been well documented for both plants and animals where both chemical pathways and electrical signal transmissions are involved (Brenner et al. 2006).
So despite appearing basically inert both sponges and plants can respond to stimuli, and communicate between cells in order to facilitate coordination. Remember however, that sponges have no organs: that means no neurons.
Since Aristotle (384–322 BC), it has been observed that sponge adults are capable of reacting. Responses to various stimuli were observed: mechanical (e.g. injury), electrical and chemical stimuli, changes of light, temperature, oxygen, salt concentration, presence of sediment (for review: Jones 1962; Leys & Meech 2006; Elliott & Leys 2007).
So how exactly to sponges react?
Responses might affect the aquiferous system (opening/closure of oscula (exhalant pores) and ostia (inhalant pores), current velocity, flagellar activity of choanocytes), as well as more or less localized tissue contractions (Simpson 1984; Leys & Meech 2006; Pfannkuchen et al. 2008).
Well they don’t do much, but they do something.
Intrinsic rhythmic contractions have been well documented in Tethya (Demospongiae), resulting in contraction of the body volume up to 70% within 20 min in Tethya wilhelma Sarà et al., 2001 (Lieberkühn 1859; Schmidt 1866; Reiswig 1971; Sarà & Manara 1991; Nickel 2001, 2004, 2006; Nickel & Brummer 2003)
While 70% contraction of the body is impressive taking 20 minutes is a bit slow.
Even more unexpectedly for sessile animals, a few species are capable of crawling along a substratum (Bond & Harris 1988; Pansini & Pronzato 1990; Nickel 2006), albeit rather slowly: 1–4 mm per day for Chondrilla nucula Schmidt, 1862 (Bond & Harris 1988); and 4 mm per day for T. wilhelma (Nickel 2006). Experiments show that locomotion is modulated by environmental factors such as the nature of the substrata or the light intensity (Pronzato 2004; Nickel 2006) and that T. wilhelma is capable of changing direction almost instantaneously (Nickel 2006). Once again, this coordinated behavior, even if exceptional insponges, implies efficient integrated perception–conduction mechanisms.
So some species of sponge can move at brisk pace of 1-4 mm per day. Alright so the author has demonstrated that sponges can both perceive, and react to stimuli in a coordinated fashion. They aren’t going to win any races, if they are mobile at all.
In the absence of neurons, various hypotheses were proposed to try to explain the experimental observations (signal propagation from a few mm to 0.3 [cm/s] observed in sponges versus several hundred [cm/s] often observed in neuronal conduction; Leys & Mackie 1997; Leys et al. 1999; Elliott & Leys 2007)
For perspective, one of the faster neural conductors isType Ia sensory fiber. Ia neural conduction velocity 120 m/s (that is meters not cm or mm). So the signal propagation for the fastest nerves is 400,000 times faster than the fastest signal propagation in sponges. There might be some measurement error or exceptions but at the very least neurons are orders of magnitude faster than whatever mechanisms sponges use.
This might be explained by the fact that all attempts to record electrical signals in sponges have so far failed except in this hexactinellid. In R. dawsoni, action potentials have been recorded (Lawn et al. 1981; Leys & Mackie 1997; Leys et al. 1999) through the trabecular syncytium. The conduction velocity of 0.27 [cm/s] is slow compared with conduction in nerves, whereas the absolute and relative refractory periods (29 and 150 seconds, respectively) are very long. This electrical conduction is temperature sensitive
Again a refractory period of 29 seconds is ridiculously slow. This means that a sponge would at minimum be able to react to stimuli in a coordinated fashion (via electrical signals) once every 29 seconds. This is assuming, of course, other sponges utilize this mechanism for intercellular communication. The actual mechanism for coordination of cell activity is unknown. This paper examines many of the suggested mechanisms but once again doesn’t come up with a clear answer.
Apart from the controversial bipolar cells described by Pavans de Ceccatty (1966) in the mesohyl of Tethya, no cells with obvious ultrastructural features reminiscent of eumetazoan neuro-sensory cells have been reported. Therefore, adult sponges are considered to be devoid of specialized conduction cells.
Again they haven’t found anything that serves as a specialized cell for intercellular communication. So despite being able coordinate intercellular movements, the coordination mechanism is apparently not specialized cells.
For conduction mechanisms as well as other aspects of sponges, it has become more and more obvious that these animals are not as simple as generally described in zoological textbooks. The absence of neurons and obviously identified sensory cells does not indicate the absence of an efficient perception-conduction system enabling adaptive responses to environmental changes. On the basis of the data surveyed in this review, it should be obvious that sponges are not devoid of sensory cells, and use cellular, chemical and/or electrical signals to coordinate their activities, even if we have still got a long way from identifying all the cells and understanding the whole processes involved.
We hope that this review may serve to convince the reader that despite their lack of identified neuroid cells, sponges are promising models for understanding the origin of the neuro-sensory system in the animal lineage.
What have we learned from the humble sponge? That non-specialized cells are relatively inefficient at propagating signals and that though nervous systems are unnecessary for coordination of multicellular reactions, they speed up the process quite a bit.