"Dear frog and fish, or newt and shark,
you needn’t worry when it’s dark;
you’ll escape or dine just fine,
svenning with your lateral line."
(Platt et al., 1989)
There are several types of lateral line organs, including superficial neuromasts that are found externally, canal neuromasts that are found within canals, spiracular organs, and vesicles of Savi. Neuromasts are organs made of receptive "hair" cells (these are not actual hairs like in mammals) that are covered by the cupula, a flexible semi-solid (jelly-like to be scientific) substance that can be displaced with the movement of water around it. The "hairs" are organized from tallest to shortest (like a staircase). The cupula will be displaced to a degree that is proportional to the strength of the stimulus (i.e. stronger flow will create more shear), causing the sensory "hairs" to also shear or deflect. This can produce either depolarization and increased neurotransmitter release and signal transduction (shear toward longest hair) or hyperpolarization and decreased release and transduction (shear toward shortest hair). These signals are then sent off to the brain to be interpreted.
Spiracular organs are associated with the spiracle, a circular opening found in most cartilaginous fish and some primitive bony fish like the coelacanth. Its function is fairly well-constrained to be for mechanoreception. Perhaps the most interesting thing is that the spiracular organ has been homologized with the paratympanic organ (associated with pressure and altitude detection) of the middle ear in amniotes like birds (e.g., von Bartheld & Giannessi, 2011; O'Neill et al., 2012). The vesicles of Savi are found only in a few genera of rays belonging to the Torpediniformes, which are commonly referred to as "electric rays" or "torpedo rays" (e.g., Nickel & Fuchs, 1974; Shibuya et al., 2010). As an aside, the Romans used to use live electric rays for medicinal purposes to try and cure things like headaches by putting the ray on top of someone's head (Winter, 1976)! The vesicles of Savi are concentrated on the underside of the snout and around the margins of the large electric organs that give these fish their name. There seems to be some debate over their function, mostly about whether they are primarily mechanoreceptors similar to other lateral line organs or whether they play an important role in the electric organs' function.
Anyone interested in the deep history of electric fishes' relationship with humans should check out this paper!
The lateral line in temnospondyls
The lateral line occurs in many, but hardly most, temnospondyls. It is formed by typically deep and conspicuous grooves running along the ornamented surfaces of bones, disrupting the various patterns of ornamentation. Whether these are better referred to as 'grooves' or as 'canals' is a semantics argument that I'm not really interested in having; canals architecturally are not enclosed, but often times the term is implied as such in biological systems. 'Sulcus' is another term that's more along the line of 'groove.' Terminology for identifying the canals is typically pretty standard; as in modern vertebrates, the infraorbital canal runs beneath (in tall skulls) or lateral (in flat skulls) to the orbit, while the supraorbital canal runs above or medial to the orbit. The postorbital canal (sometimes called the temporal canal because it occurs on the temporal region [cheek]) extends from the posterolateral corners of the skull up towards the eye, then curves behind it and loops back down toward the back of the skull.
Generally speaking, the lateral line arrangement is pretty consistent across the skulls temnospondyls and across aquatic early tetrapods. For example, the supraorbital canal cuts across many of the same elements (e.g., pre- and postfrontals, frontals) in many temnospondyls. The arrangement can look quite different between taxa, but this is often because there is a tight correlation between the grooves and the elements that they cut across; in other words, noticeable changes to proportions of elements will lead to correlated changes in the grooves. The infra- and supraorbital grooves are very long in taxa with long snouts, for example. There is some disparity in whether the postorbital groove is a single groove or formed by at least two distinct grooves (usually divided into the temporal [going toward the midline] and the jugal [going toward the edge of the skull] grooves in that case). The curvature of the grooves is also pretty consistent as a result; the term 'lyra / lyre' that was sometimes used to describe the grooves comes from the U-shaped inflection of mainly the supraorbital groove between the nose and the orbit (but the infraorbital groove can also be inflected, and the postorbital groove is a 'U' in its entirety). The name is after the U-shaped instrument of Greek antiquity (not the constellation Lyra, which does derive etymologically from the same origin). There can also be an occipital groove (green below) that is short and mostly transverse (side-to-side) in orientation along the back of the skull.
The lateral line in lissamphibians
All three groups of lissamphibians, anurans (toads + frogs), caudates (salamanders + newts), and caecilians have lateral line systems with neuromasts, as in other vertebrates (Fritzsch, 1989). Compared to temnospondyls, lissamphibians have shifted toward neuromasts embedded in the epidermis rather than in the bone; when exactly this occurred is unknown. Caudates and caecilians also have ampullary electroreceptors. The developmental relationship of neuromasts and ampullary organs remains poorly resolved in amphibians. There are typically three lines (dorsal, medial, ventral) on the trunk of the body, but there are the usual exceptions (the aquatic salamander Siren has four) and reductions (the tailed frog Ascaphus has two). The presence of four lines in some lungfish suggests that the primitive condition is four lines, with subsequent reductions in tetrapods (possibly due to pedomorphosis in lissamphibians). There are typically four lines on the head, all supplied by the trigeminal nerve (cranial nerve V). These are the supraorbital line (above / medial to the eye), the infraorbital eye (below / lateral to the eye), the jugular / angular / oral line (on the jaw), and the postorbital / gular line (behind the eye). This terminology is not always consistently used, as there is sometimes further division of the lines. The supraorbital and the infraorbital lines are commonly used terms in temnospondyls and are considered homologous to those in fish (and are sometimes used to attempt to infer homologies of skull bones) (e.g., Moodie, 1908).
During metamorphosis, the lateral line system and the ampullary organs are frequently lost (e.g., Wahnschaffe et al., 1987). However, many salamanders only cover the lateral line system with the epidermis during metamorphosis; this retention is similar to what lungfish do during aestivation to wait out the dry season. There is some evidence to suggest that the retention is also associated with the aquatic breeding habits of many caudates and anurans. Caudates shed their skin when they enter the water to breed, exposing the neuromasts (e.g., Fritzsch & Wahnschaffe, 1983), and many semi-aquatic anurans retain some of the non-tail neuromasts, though to differing degrees that probably reflects their degree of terrestriality (e.g., Fritzsch et al., 1987). Second-order neurons associated with the neuromasts are typically lost, although there is a less common phenomenon where the neuromasts are degraded but the neurons remain.
About the blog
A blog on all things temnospondyl written by someone who spends too much time thinking about them. Covers all aspects of temnospondyl paleobiology and ongoing research (not just mine).