The Journal of Experimental Biology

The Journal of Experimental Biology

present. Peptidergic signalling plays a large role in ctenophore and

cnidarian nervous systems (Anctil, 1987; Spencer, 1989), but

sponges could use peptides as signalling molecules even without

nerves. Sponge larvae settle and metamorphose more rapidly in the

presence of GLW-amide peptides (Whalan et al., 2012), so peptides

may be used by sponge larva for locating the right settlement

substrate. Generally biofilms and coralline algae trigger

metamorphosis in invertebrate larvae and the same was found for

Amphimedon queenslandica larvae (Jackson et al., 2002), but

exactly how this works is unknown. Nevertheless, if sponges use

peptidergic signalling, larval cells would be the place to look for

receptors.

In sum, there is currently only physiological and genomic

evidence for amino acid transmitters in coordination of behaviour in

sponges. But without nerves to study and as sponge tissues show

low cross-reactivity to commercial antibodies, there are still few

tools available to study this, as has been found in ctenophores.

Ion channels

Ion channels are responsible for all rapid ionic changes across

membranes. The simplest for cloning and therefore easiest to study

in sponges have been potassium channels. Potassium channels are

responsible for stabilizing membrane potential, and so are indicators

of electrical behaviour. Intriguingly, no voltage-sensitive channel has

yet been identified in sponges, although it hardly seems likely that

they are entirely absent from the group. A K

v

channel was said to be

present in Amphimedon queenslandica (Alié and Manuel, 2010) but

the voltage sensor domain is absent in that sequence. No channels

with a voltage sensor have been found in the transcriptomes of eight

other sponges, so at present we do not know of a K

v

channel from a

sponge. However other K channels have been studied, including

inward rectifying and two pore K channels (K

ir

and K

2P

). The K

ir

channel isolated from Amphimedon shows rapid inactivation, which

indicates that the channel resets the membrane potential quickly –

as though it might respond to depolarization, a hint that electrical

signalling may occur in Amphimedon (Tompkins-MacDonald et al.,

2009). The K

2P

channel shows sensitivity to amino acids and to pH,

but not to temperature, rather like other animal K

2P

channels (Wells

et al., 2012). A temperature- and mechano-sensitive cation channel

has been found in Axinella polypoides, but it is not known to have

a role in directional signalling or coordination of behaviour (Zocchi

et al., 2001).

Perhaps the most intriguing molecular find in terms of ion

channels is that ionotropic glutamate receptor (iGluR)-like

molecules were found in transcriptomes of three out of eight sponge

species: Sycon coactum, Oscarella carmela and Ircinia fasciculata

from Calcarea, Homoscleromorpha and Demospongiae classes,

respectively. These have the Q/R site and the pore motif

SYTANLAAF (Riesgo et al., 2014). Ionotropic receptors imply

there is a need for fast signalling, yet where this happens is not clear

because contractions and indeed responsiveness in demosponges is

not fast. In demosponges, contractions travel at 2–20 μm s

−1

along

epithelia [12.5 μm s

−1

in  Tethya wilhelma (Nickel, 2004) and

0.3–5 μm s

−1

in Ephydatia muelleri (Elliott and Leys, 2007)] except

in the osculum, where a wave of contraction was reported to travel

at 6–122 μm s

−1

in  E. muelleri (Elliott and Leys, 2007) and at

170–350 μm s

−1

in E. fluviatilis (McNair, 1923). Propagation across

a whole animal can take 30 min to 1 h, so a signal cascade via

metabotropic glutamate receptors (mGluRs), which binds glutamate

via a GPCR is expected to be sufficiently rapid for transmitting

signals between cells. The fastest rate of contraction in sponges is

still ten times slower than action potential propagation in plants

(Fig. 4A), so it is unlikely that an electrical signal is involved. If

iGluRs enable a rapid response then one might speculate that it

could be in response to injury – like the pin prick that McNair

(McNair, 1923) used in his studies – and if so, perhaps the primary

response is to release chemical defences, something that has not

been studied. In that case, the ‘slow’ contractions could be a

secondary response, causing the sponge to be smaller and appear

less palatable to a predator.

Glass sponges – electrical signalling

Glass sponges, Hexactinellida, use electrical signalling. Unlike all

other sponges the whole body of a glass sponge forms a single

continuous syncytium (Leys, 1999). Syncytial tissues allow

electrical signals to travel unimpeded by membrane barriers

throughout the whole animal and these cause the feeding current to

stop within seconds of a mechanical or electrical stimulus; the effect

is ‘all or none’ (Leys and Mackie, 1997). Glass sponges are thought

to have diverged from a common ancestor shared with demosponges

during the late Neoproterozoic early Cambrian period (Mehl, 1996;

Antcliffe et al., 2014). We know the development and

morphogenesis of tissues from two species: Farrea occa (Ijima,

1904) and Oopsacas minuta (Boury-Esnault and Vacelet, 1994; Leys

et al., 2006). Glass sponges form cellular embryos, which become

syncytial after the 64-cell stage (6th cleavage) by fusion of

macromeres (Leys et al., 2006). This tissue, the trabecular reticulum,

forms an extremely thin giant multinucleated cell that forms the

outer skin (called the dermal membrane), and all the incurrent

canals, flagellated chambers, excurrent canals and oscula tissues

(Ijima, 1904; Mackie and Singla, 1983; Leys, 1999). All tissues are

cytoplasmically connected and cytoplasm streams throughout the

tissues along giant tracts of microtubules (Fig. 4B) (Leys, 1995).

Arrests of the glass sponge pumping system were first noted by

G. Silver who in the 1970s put thermistor flow meters into the

osculum of Rhabdocalyptus dawsoni in situ at 25 m depth. As divers

approached the sponges and stirred up sediment, the sponges

stopped pumping. Experiments in tanks confirmed this behaviour

and the speed of conduction and ability to travel circuitous paths,

but not to jump between distinct pieces of sponge suggested there

must be an electrical signal (Mackie, 1979; Lawn et al., 1981), but

the thinness (2–10 μm) and elasticity of the trabecular tissue made

it difficult to record from. It was only by developing a novel

preparation of sponge tissue aggregates fused to the body wall that

it was possible to attach suction electrodes and record electrical

signals (Fig. 4C) (Leys and Mackie, 1997).

The characteristics of glass sponge conduction are as follows. (1)

The action potential (AP) is 5 s long and travels at 0.27 cm s

−1

. The

absolute refractory period, the period during which a second AP

cannot be generated, is 29 s. The second of a pair of APs with delays

between 30 s and 150 s have a lower amplitude and slower

conduction velocity, indicating that 150 s is the relative refractory

period (Fig. 4D). The slowness of the AP may be attributed to the

immensely circuitous path that it has to take through the syncytial

strands of the tissues, but it is also considered to reflect a low

density of ion channels in the syncytial tissues. This is substantially

slower than the conduction systems of plants (Fig. 4A).

(2) The action potential is dependent on calcium and potassium

(Leys et al., 1999). Reduction in Na

+

to 25% of normal seawater has

very little effect on the AP – the amplitude is slightly reduced and

delayed (Fig. 4E). In contrast, perfusion with 10 mmol l

−1

Co

2+

,

1 mmol l

−1

Mn

2+

or 24 μmol l

−1

nimodipine – all calcium-channel

blockers – eliminate the AP reversibly. Similarly, perfusion with the

potassium channel blocker TEA (1–5 mmol l

−1

) also blocks the AP

587

REVIEW

The Journal of Experimental Biology (2015) doi:10.1242/jeb.110817

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