Loss of escape responses and giant neurons in the tailflipping circuits of slipper lobsters, Ibacus...

Loss of escape responses and giant neurons in the tailflipping circuits of

slipper lobsters, Ibacus spp. (Decapoda, Palinura, Scyllaridae)

Zen Faulkes*

Department of Zoology, University of Melbourne, Royal Parade, Parkville, Vic. 3010, Australia

Received 30 May 2003; received in revised form 2 December 2003; accepted 12 December 2003

Abstract

In many decapod crustaceans, escape tailflips are triggered by lateral giant (LG) and medial giant (MG) interneurons, which connect to

motor giant (MoG) abdominal flexor neurons. Several decapods have lost some or all of these giant neurons, however. Because escape-

related giant neurons have not been documented in palinurans, I examined tailflipping and abdominal nerve cords for giant neurons in two

scyllarid lobster species, Ibacus peronii and Ibacus alticrenatus. Unlike decapods with giant neurons, Ibacus do not tailflip in response to

sudden taps. Ibacus can perform non-giant tailflipping: the frequency of tailflips during swimming is adjusted by altering the gap between

each individual tailflip. Abdominal nerve cord sections show no LG or MG interneurons. Backfilling nerve 3 of abdominal ganglia revealed

no MoG neurons, and the fast flexor motor neuron population is otherwise identical to that described for crayfish. The loss of giant neurons in

Ibacus represents an independent deletion of these cells compared to other reptantian decapods known to have lost these giant neurons. This

loss is correlated with the normal posture in scyllarids, in which the last two abdominal segments are flexed, and an alternative defensive

strategy, concealment by digging into sand.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Crustaceans; Evolution; Homology; Escape response; Swimming; Scyllarids

1. Introduction

Escaping quickly from a sudden and unexpected threat,

such as the strike of a predator, is so important that many

animals have dedicated, specialized neural systems for

startle responses. Because it is advantageous for a startle

response to be extremely fast, and because neurons’

conduction velocity increases with axon size, the startle

responses of many kinds of animals involve giant

interneurons (e.g. fish: Foreman and Eaton, 1993; squid:

Otis and Gilly, 1990; insects: Boyan and Ball, 1990).

Because of the simplicity of many startle responses, and the

ease of working with giant neurons, startle responses are

among the behaviours best understood at the neural level. In

decapod crustaceans, such startle behaviours involve a

neural circuit containing medial giant (MG) interneurons,

lateral giant (LG) interneurons, and fast flexor motor giant

(MoG) neurons. The MG and LG neurons both trigger an

escape tailflip, which is a single short latency, powerful, and

rapid abdominal flexion. In crayfish, anterior stimuli fire

MG neurons, while posterior stimuli fire LG neurons. The

tailflips generated by these giant interneurons have different

trajectories: directly backwards if MG-initiated, or pitching

the animal’s posterior end up into the water column if LG-

initiated (Wine and Krasne, 1972; Cooke and Macmillan,

1985; Newland and Neil, 1990). The MoG neurons are the

only fast flexor motor neurons that receive direct synaptic

input from MG and LG (Mittenthal and Wine, 1978), and

differences in connections between the MoG neurons and

the giant interneurons explain much of the kinematic

distinctions between MG- and LG-mediated tailflips

(Wine and Krasne, 1972, 1982). The behavioural role and

circuitry of these giant neurons have been intensively

studied in crayfish, making this circuit a powerful model for

investigating synaptic physiology, learning, decision mak-

ing, and so on (reviewed in Edwards et al., 1999).

The neural circuitry for decapod escape also serves as a

model for the study of the evolution of the nervous system

and behaviour (Heitler and Fraser, 1986; Paul, 1989, 1991,

1467-8039/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.asd.2003.12.003

Arthropod Structure & Development 33 (2004) 113–123

www.elsevier.com/locate/asd

* Address: Department of Biology, University of Texas-Pan American,

1201 W. University Drive, 78541-2999 Edinburg, TX, USA. Tel.: þ1-956-

381-2614; fax: þ1-956-381-3657.

E-mail address: [email protected] (Z. Faulkes).

http://www.elsevier.com/locate/asd

2003). Indeed, Krasne and Edwards (2002) observed that

‘reasonable understanding of the nervous system may be

impossible without evolutionary analysis, a most sobering

possibility’ (emphasis added). Decapod crustaceans are

excellent subjects for the study of evolutionary change in

nervous systems, because the basic plan of decapod nervous

systems has proved to be conservative across taxa. Thus,

even small changes in the nervous system across taxa are

conspicuous because the overall similarity in neuron

populations is often overwhelming (Arbas et al., 1991).

This can be true even when neurons are phylogenetically

ancient, as the MG and LG neurons are. The MG and LG

neurons predate the eumalacostracan subclass: MG and LG

neurons are present and trigger escape responses in

Anaspides tasmanidae, a syncarid species whose mor-

phology is thought to resemble that of early malacostracans

(Silvey and Wilson, 1979). Escape-related giant inter-

neurons may even predate the origin of the malacostracan

class. A pair of giant interneurons resembling MG neurons

mediates escape responses in the stomatopod species

Squilla mantis (Heitler et al., 2000). Having a single pair

of giant interneurons is probably the ancestral condition for

stomatopods, rather than being derived from the MG and

LG interneurons of eumalacostracans and syncarids (Heitler

et al., 2000).

Despite the apparent advantages of having dedicated, fast

escape systems, some reptantian decapods have undergone

‘reverse evolution’ (Porter and Crandall, 2003) and lost

some or all of these giant neurons (Fig. 1). Mud shrimp

(Thalassinidae) and hermit crabs (Paguroidea) have lost the

LG neurons (Paul, 1991), but the MoG is retained in all

known species with at least one pair of giant interneurons

(Johnson, 1924; Heitler and Fraser, 1987; Paul, 1989). Squat

lobsters (Galatheodea; Sillar and Heitler, 1985a,b,c; Wilson

and Paul, 1987) and sand crabs (Hippoidea; Paul, 1991) lack

MG and LG neurons, although they still tailflip using non-

giant circuitry. The fate of the MoG varies in species

without giant interneurons: it may be missing entirely

(Wilson and Paul, 1987; Paul, 1989, 1991), or present as an

unspecialised fast flexor neuron, a MoG homologue

(MoGH; Sillar and Heitler, 1985a). True crabs (Brachyura)

can have giant neurons in the nerve cord (Skobe and

Nunnemacher, 1970; Fraser, 1974), but there is no evidence

that those cells are MG or LG homologues (contra Paul,

1989). When the presence of giant neurons is mapped to a

phylogeny of reptantian crustaceans (Fig. 1), the correspon-

dence suggests that the LG neurons have been lost only

once. The MG neurons may have been lost twice, once in

anomurans and once in brachyurans.

Palinura is arguably the most divergent reptantian infra-

order (Fig. 1). By virtue of its basal position in the reptantian

tree, any changes to the escape circuitry in this taxon represent

independent evolutionary events compared to the losses of

giant neurons described above. There is no anatomical

evidence for any of the giant neurons discussed here, although

there is physiological evidence for giant-mediated tailflips

Fig. 1. Phylogeny of reptantian decapod crustaceans (based on Scholtz and Richter, 1995; Crandall et al., 2000) and giant neurons involved in escape

tailflipping (Paul, 1989; this study). The phylogeny of Reptantia is contentious (Schram, 1986, 2001), but evidence from adult morphology (Scholtz and

Richter, 1995; Schram, 2001), larval morphology (Williamson, 1988), sperm morphology (Tudge, 1997), the fossil record (Briggs et al., 1993), and genes

(Vaughn and Traeger, 1976) all support an early divergence of Palinura. The presence of startle-related giant neurons is an ancestral trait for Reptantia: non-

reptantian decapods possess all three (Johnson, 1924), and the giant interneurons are found in non-decapod malacostracans (Silvey and Wilson, 1979). (*) The

fate of the MoG varies in galatheids (see text), but it is not a giant neuron in any known species. Names of infraorders are shown in branches; names of relevant

lower-level taxa are shown above tree. Drawings of animals are not to scale.

Z. Faulkes / Arthropod Structure & Development 33 (2004) 113–123114

in spiny lobsters (Palinuridae; Newland et al., 1992). Of the

two main branches of the Palinura, slipper lobsters

(Scyllaridae) are the most likely to show specializations of

the escape circuitry. Scyllarids are extremely dorso-

ventrally flattened, have flat antennae, and usually hold

their abdomen in a flexed position (Fig. 2). Such unusual

body shapes have been associated with the loss of giant

interneurons (e.g. hermit crabs’ adaptations for living in a

molluscan shell; squat lobsters’ flexed abdomen). Never-

theless, scyllarids do tailflip, using the abdomen for long,

sustained swimming (Jacklyn and Ritz, 1986; Spanier et al.,

1991; Jones and Morgan, 1994). Jacklyn and Ritz (1986)

wrote that Ibacus peronii performed LG-mediated and

‘truncated’ (presumably non-giant) tailflips, but reported no

behavioural evidence for MG-mediated tailflips. The loss of

MG while retaining LG would be a unique configuration of

the decapod escape circuit. There are difficulties in

interpreting the data, however. Jacklyn and Ritz wrote that

I. peronii performed tailflips ‘involving the LG axons… in

which the abdomen was fully extended before flexure; and

truncated tailflips in which the abdomen was only partly

extended before flexure (classified after Wine and Krasne,

1972),’ but it is unclear how tailflips were classified as LG-

mediated based on Wine and Krasne’s text. An initial

abdominal flexion, followed by extension, is characteristic

of both MG- and LG-mediated tailflips; extension, followed

by flexion, is indicative of non-giant mediated tailflips, of

the type used in swimming (Reichert et al., 1981; Reichert

and Wine, 1983).

Here, I describe tailflipping and neurons associated with

it in two species of slipper lobsters, I. peronii (commonly

known as ‘Balmain bugs’ in Australia) and I. alticrenatus.

Hereafter, ‘Ibacus’ will refer to both species.

2. Materials and methods

2.1. Animals

Live I. peronii Leach, 1815 and Ibacus alticrenatus Bate,

1888 of both sexes were bought from a local commercial

seafood supplier (Briarry’s Seafood Connection, Mel-

bourne) and housed in the University of Melbourne’s

seawater system. They were provided with fine sand to dig

in, and fed a diet of squid and shellfish. For unknown

reasons, I. peronii had a much higher survival rate than I.

alticrenatus under these conditions, so some experiments

could only be carried out with I. peronii. Crayfish (Cherax

destructor) were purchased from commercial suppliers and

housed in freshwater tanks in the University of Melbourne.

All housing and experiments complied with the animal care

guidelines of the University of Melbourne and the laws of

Australia.

2.2. Behaviour

To record electromyograms (EMGs), I inserted fine

insulated silver wires (127 mm diameter; Teflon-coated

except for the tip), into small holes, made using a needle, in

the exoskeleton of abdominal segment 2. Each extensor

electrode was positioned slightly lateral to the midline on

the dorsal side of the segment. Each flexor electrode was

placed just lateral to the midline, anchored on the calcified

‘rib’ that crosses the ventral side of the segment. The wires

were cemented in position using cyanoacrylate glue and

wax, and ran to a more rigid ‘saddle’ made of heavier wax

glued to the carapace. The signals were amplified, filtered

(Seewiesen filters), digitised (TL-2 interface analog–digital

board, Axon Instruments, Inc.; 1.5 kHz sampling rate), and

Fig. 2. A: Ibacus peronii, dorsal view. Carapace length ,55 mm. B: Ibacus

alticrenatus, dorsal view. Carapace length ,40 mm. C: Side view of I.

peronii, showing normal posture for both species, with the tailfan curled

underneath the abdomen. D–G: Ibacus peronii walking sequence captured

from video at 5 s intervals. Arrowheads point to a shell in the tank as an

indicator of the animal’s movement. Note that the tailfan remains

underneath the abdomen throughout the walking sequence.

Z. Faulkes / Arthropod Structure & Development 33 (2004) 113–123 115

recorded on a computer using Axotape 1.2 (Axon Instru-

ments, Inc.). Off-line EMGs measurements were made with

AxoScope 8.0 (Axon Instruments, Inc.). I recorded a

minimum of 5 sequences from 8 individuals, for a total of

53 swimming sequences.

Behaviour was videotaped with standard VHS video

equipment (PAL format).

2.3. Histology

The gross anatomy of the central nervous system in

Ibacus is similar to that in the scyllarid Thenus orientalis

(Chacko, 1967). Animals were anaesthetised by chilling on

ice for at least 30 min before they were dissected.

Dissections were performed in chilled seawater. I removed

the abdomen at the thoracic-abdomen joint, trimmed the

protruding ventral flaps of the terga, and then pinned the

abdomen ventral side up in a dish lined with Sylgard (Dow

Corning Corporation). The ventral exoskeleton was

removed to expose the ventral nerve cord. The nerves

connecting the cord to muscles and limbs were severed, and

the entire cord was transferred to a dish containing fresh

seawater. Abdominal fast flexor motor neurons exit via the

dorsal branch of nerve 3 (N3d) in all abdominal ganglia

except the terminal abdominal ganglion, and via N3 of the

most posterior thoracic ganglion. Because the branching

patterns of terminal abdominal ganglion nerves vary

considerably across species (Paul et al., 1985), and

characterizing this variation in Ibacus was not the aim of

this project, the flexor motor neurons of the terminal

abdominal ganglion were not investigated. The N3d nerve in

each of the five anterior abdominal ganglia was backfilled

overnight in a refrigerator using 0.2 M cobalt chloride

(CoCl2) solution. The CoCl2 was precipitated using a few

drops of ammonium sulfide (,10 drops in 10 ml saline),

fixed in 15% formalin in seawater, then dehydrated in a

progressive ethanol series (70, 90, 100% ethanol) and

cleared in methyl salicylate. I made an initial inspection and

a camera lucida drawing of each fill. Many successful fills

were silver intensified (Bacon and Altman, 1977). Fills were

re-hydrated, soaked in Timm’s solution (acacia, citric acid,

sucrose) at 55 8C, and then placed into Timm’s solution

containing 10% silver nitrate until the ganglia turned golden

brown. Fills were again dehydrated, cleared in methyl

salicylate, and re-inspected. At least ten fills were performed

for each of the abdominal ganglia 1–5, and seven fills were

performed for the last thoracic ganglion. To provide a

positive control, these techniques were also used to fill the

fast flexor neurons (including the MoG) in C. destructor.

Nerve cords from I. peronii were fixed for a minimum of

2 h in 15% formalin in seawater, and then stored in 70%

ethanol until they were embedded in wax. Nerve cords were

sliced into 7 mm transverse sections and stained with

hematoxylin and eosin.

2.4. Imaging

I photographed backfills and sections with a 35 mm

camera mounted on an Olympus BX50 microscope. All

photographs and video images were transferred to a digital

format. Slides and film negatives were scanned at 2700 dots

per inch using a Nikon LS-2000 scanner. Images scanned

from film negatives were inverted to a positive image

digitally using Photo-Paint 8 (Corel Corporation; PC

version). Video images were captured using an Apple

Macintosh computer using Apple Video Player 1.7.1

software running on an Apple Power Macintosh 6500/300

computer, then imported into Adobe Photoshop 4.0.1

(Macintosh version). Figures were then assembled into

final proportions using CorelDRAW 8 software (Corel

Corporation; PC version).

3. Results

3.1. Behaviour

Ibacus are often drawn or photographed with the

abdomen fully extended so that the tailfan is visible (e.g.

Fig. 1; Jacklyn and Ritz, 1986; Jones and Morgan, 1994).

This pose is convenient for showing more of the body in a

single image, but it is not the normal posture (Fig. 2).

Typically, the joints between the fifth and sixth abdominal

segments, and the sixth abdominal segment and the telson,

are both flexed at near right angles so that the tailfan is

curled underneath the abdomen. This posture is maintained

even during long walking sequences (Fig. 2D–G). The

exoskeleton of the telson and uropods in Ibacus is quite fine

and not heavily calcified, and appears to be damaged easily.

The rest of the dorsal surface, in contrast, is covered with a

relatively thick and calcified exoskeleton.

I was unable to elicit tailflips with sudden tactile stimuli

(taps), which is the standard method of evoking tailflips in

crayfish (Wine and Krasne, 1972; Edwards et al., 1999).

Individuals did not tailflip when tapped even when the

abdomen was fully extended. The equipment I used to

deliver standard taps successfully evoked giant-mediated

tailflips in crayfish (Cherax destructor) in another study

(Finley, 1999). Typically, the only short-term response to

taps was flattening the eyestalks into the eye cups, although

repeated taps sometimes caused an animal to start walking

away.

Although Ibacus did not respond to taps with tailflips,

swimming by repeated tailflipping could be evoked by

gradual stimuli, particularly attempts to overturn animals.

As described by previous studies (Jacklyn and Ritz, 1986;

Jones, 1988; Spanier et al., 1991), Ibacus are strong

swimmers, which was impressed upon me when one

individual tailflipped out of an aquarium. Swimming in I.

peronii has the same general features as swimming by non-

giant tailflipping in crayfish (Reichert et al., 1981). Most


tailflip sequences recorded by EMGs began with abdominal

extension (Fig. 3A), although a few tailflip sequences

(,15%) began with abdominal flexion (Fig. 3B). I. peronii

changes the frequency of its tailflips by adjusting the

interval between each individual flip, visible as the gap

between the end of a flexor burst and the beginning of the

subsequent extensor burst. This interval is strongly

correlated with the period of each tailflip (Fig. 4). The

correlations between period and extensor or flexor muscle

bursts were statistically significant in some individuals, but

they were always much lower than the correlation between

the flexor/extensor interval and the period (Fig. 4). Extensor

bursts are followed by flexor bursts at short latency. Other

general features of scyllarid swimming are described

elsewhere (Jacklyn and Ritz, 1986; Spanier et al., 1991).

Tailflips occur in at least one other context, namely

digging. Ibacus bury themselves in sand, and often finish the

process of digging into the substrate with a short series of

tailflips.

3.2. Absence of MG and LG interneurons

I. peronii lacks MG and LG interneurons (Fig. 5). In the

abdominal portion of the nerve cord, the fast flexor axons are

easily identifiable because they exit from abdominal cord via

N3, which is the only abdominal nerve that branches from the

nerve cord between ganglia. No axon profiles noticeably larger

than the fast flexor motor neurons (which are among the largest

profiles in these sections) were visible in the dorsal region of

the abdominal nerve cord anywhere along its entire length.

The fast flexor axons were followed anterior towards the

ganglion, searching for any axons crossing over their profiles.

In species with giant interneurons, the non-MoG fast flexor

axons pass between the MG and LG axons; if non-giant

homologues of the MG and LG neurons are present, they

should be located dorsal to the fast flexor axons (Paul, 1991).

That the fast flexor axons are the most dorsal profiles in Ibacus

indicates that the MG and LG neurons are truly absent, rather

than being reduced in size and thus inconspicuous.

I examined sections throughout the thoracic subeoso-

phageal ganglion, where the MG axons are generally largest

(Finley, 1999). Again, no axon profiles in the dorsal region

of the ganglion and adjacent nerve cord were noticeably and

consistently larger than other axons (data not shown).

Although no abdominal nerve cords of I. alticrenatus

were sectioned, no evidence suggests the presence of giant

axons. Even unstained giant axons can be visible in cleared

nerve cords, especially when other cells have been back-

filled (Mittenthal and Wine, 1978; Heitler and Fraser, 1986;

personal observations). Giant axons were never visible in

this manner in I. alticrenatus backfills. Further, I. alticre-

natus lack the MoG neurons associated with the MG and LG

neurons (below).

3.3. Absence of MoG neurons

Ibacus lack a MoG or motor neuron homologue to the

MoG. Backfills of N3d showed that the fast flexor cell

Fig. 3. A, B: EMGs of abdominal extensor (EXT) and flexor (FLX) muscles during I. peronii tailflips. Although swimming normally begins with abdominal

extension (A; see text), it can begin with flexion (B). Flexor activity is visible as cross-talk in the extensor record; extensor bursts marked with bar above

extensor EMG trace. In B, a recording artefact was removed, resulting in a break in the trace (*). Scale bar ¼ 200 ms (shown in B).


bodies of Ibacus are found in the same arrangement as in

crayfish (Fig. 6, Table 1). Note that within any given

abdominal ganglion, the cell bodies of the fast flexor motor

neurons form three distinct clusters and their axons exit

from two nerves, each associated with different ganglia

(Selverston and Remler, 1972; Mittenthal and Wine, 1978;

Sillar and Heitler, 1985a; Wilson and Paul, 1987). The

axons of the FPI (flexor posterior ipsilateral) and FMC

(flexor medial contralateral) neurons exit via the posterior

N3d of the ganglion containing the cell bodies. The FPI cell

bodies are located in the ipsilateral half of the ganglion

relative to the exit point of their axons, whereas those of the

FMC are located in the contralateral half of the ganglion or

near the midline relative to the exit point of their axons. The

axons of the FAC (flexor anterior contralateral) neurons exit

via the N3d of the next anterior ganglion relative to the cell

bodies, which are located in the contralateral hemiganglion

relative to the exit route of their axons.

The FMC cluster contains the MoG in species that have

it. The FMC cluster contains only three cell bodies in Ibacus

(Table 1), compared to four in crayfish (Mittenthal and

Wine, 1978) and Galathea (Sillar and Heitler, 1985a), and

this smaller population is due to the lack of a MoG or MoG

homologue. No fast flexor neurons in I. peronii and I.

alticrenatus meet the morphological criteria for being MoG

neurons. Two main morphological features distinguish the

MoG. First, the MoG axon has a separate, typically more

medial path in the abdominal nerve cord than other fast

flexor motor neurons. All fast flexor axons exit the ganglion

in a single bundle in Ibacus (Fig. 6). Second, the MoG has

no processes within the ganglion, but instead has several

processes in the cord itself, where it makes electrical

synapses with the giant interneurons (Mittenthal and Wine,

1978). No axonal processes are seen in the connective

between ganglia, and all cells have profuse dendritic

branching within the ganglion. Although backfills were

often incomplete, the distinctive morphology of MoG in the

connective would be visible even if the cell body did not fill.

Further, many backfills were of extremely high quality (high

contrast between stained cells and other tissue; no swelling

or blebbing; many fine dendrites filled), which increases the

likelihood of detecting the MoG, if it was present.

Given that a MoG homologue is present in Galathea

(Sillar and Heitler, 1985a), which does not have such a

distinct morphology in the nerve cord, and that the MoG and

Fig. 4. A: Plot of relationship between interval between tailflips (pause

between flexor to extensor EMGs) and tailflip period. Regression formula:

Interval ¼ 2147.86 þ 0.883 £ Period. B: Plot of relationship between

extensor EMG burst duration and tailflip period. Regression formula:

EXT ¼ 82.04 þ 0.058 £ Period. (C) Plot of relationship between flexor

EMG burst duration and tailflip period. Data from 53 swimming sequences

by 8 individuals (minimum 5 swimming sequences per individual).

Regression formula: FLX ¼ 55.23 þ 0.060 £ Period.

Table 1

Number of fast flexor motor neuron cell bodies in Ibacus abdominal ganglia

Ganglion FPI cluster FMC cluster FAC cluster

1 4 3 1

2 4 3 3

3 4 3 3

4 4 3 2

5 3 3 0

6 N/A N/A 0


the flexor inhibitor motor neuron (FI) are located in similar

positions in the ganglion, it was possible that the ‘missing’

FMC cell in Ibacus was FI, and that the MoG was present as

a MoGH. Nonetheless, the anatomy of the most posterior

and lateral cell in the FMC cluster is similar to the FI of

those species it has been identified, with a bilateral dendritic

tree (Selverston and Remler, 1972; Sillar and Heitler,

1985a). In particular, the most posterior–lateral cell in

Ibacus has extensive ipsilateral branches in its dendritic tree

(Fig. 7), which the MoGH in Galathea does not (Sillar and

Heitler, 1985a).

4. Discussion

I. peronii and I. alticrenatus lack startle responses and

the giant neurons that produce them. Given that the major

finding of this study is the lack of the escape circuit, it is

necessary to address the prevalent misconception of ‘anti-

negativism’ (Pasquerello, 1984), which wrongly contends

that it is theoretically more difficult to demonstrate that

something is absent than to show it is present. ‘The

simplicity or difficulty of verification has little to do with

affirmative or negative, but rather with matters of scale,

density, and resemblance’ (Pasquerello, 1984). That is,

entities that are large, numerous, and dissimilar to

neighbouring entities are easily shown to be present or

absent, whichever the case may be. The giant neurons at

issue fall solidly into such a category.

These results contradict a claim for the presence of LG-

mediated flips in I. peronii (Jacklyn and Ritz, 1986).

Perhaps the trajectory often taken by I. peronii when starting

a series of tailflips (i.e. into the water column) was

reminiscent enough of LG-mediated tailflips in crayfish

(which pitch the posterior end of the animal up into the

water column) to cause misidentification. Tailflip trajectory

alone is inadequate to categorize tailflips as giant or non-

giant, because the trajectories of non-giant tailflips are

Fig. 5. Transverse sections of ventral nerve cord connective between second and third abdominal ganglia in I. peronii. The fast flexor axon profiles (FF) are

visible in the dorsal part of the cord. Image was contrast enhanced. Dorsal is towards top of the page. Scale bar ¼ 100 mm.


highly variable (Reichert and Wine, 1983) and may overlap

with giant-mediated tailflips.

Although only adult animals were examined in this

study, it is unlikely that the giant neurons are present in

larvae, then lost in the adults. First, there is no documented

case of the escape response or the associated giant neurons

being lost during development (although the threshold for

tailflips increases with size in lobsters; Lang et al., 1977).

Second, development of a functional escape circuit is

correlated with adult-like body in astacideans. Pelagic

lobster larvae do not perform fast escape tailflips, even

though they have MGs (but not LGs), while immature

crayfish of similar size do perform fast escape tailflips and

have both MGs and LGs (Jackson and Macmillan, 2000).

Both Ibacus species have phyllosoma larvae characterized

by extremely large carapaces and small abdomens (Ritz and

Thomas, 1973; Atkinson and Boustead, 1982), which are ill-

suited to produce fast escape tailflips. The larval stage in

Ibacus is followed by a puerulus stage, which resembles a

small adult (Ritz and Thomas, 1973; Atkinson and

Boustead, 1982). There is no principled reason to suspect

these early stages should have giant neurons when adults do

not.

Although other decapod groups have lost one or both of

the giant interneurons mediating startle responses (Fig. 1),

their loss in scyllarids provides a natural ‘experiment’ that

can be used to ask basic questions about how nervous

systems evolve, and ‘reverse evolution’ (Porter and

Crandall, 2003) in particular. For example, the MoG,

apparently in response to the loss of the giant interneurons,

is absent in several species (Wilson and Paul, 1987; Paul,

1991; this study), but in one species, became a generic flexor

(Sillar and Heitler, 1985a). Presently, it is hard to provide a

reason why only Galathea has retained a MoG homologue

whereas all other species examined have not. Further

comparative and developmental (Jackson and Macmillan,

2000) data would indicate if the fate of the MoG in Galathea

is truly unique, and why. Such data could sharpen

hypotheses of how neural networks respond to deletion of

components.

As representatives from four of the five reptantian

infraorders have lost some or all of the three main giant

neurons for startle responses (MG, LG, and MoG), the loss

of giant neurons might be seen as a trend in the reptantians.

Indeed, it is conceivable that the loss of startle responses

may have driven diversification by allowing taxa to travel

down new evolutionary pathways (Paul et al., 2002; Paul,

2003; Porter and Crandall, 2003). The benthos may provide

more ways for an animal to escape and defend itself than the

water column (the habitat of non-reptantian shrimp and

prawns). Given that startle responses are thought to be

important for predator evasion, the characteristics of such

responses should be correlated with the ecology of species

possessing them. For example, copepods with non-myeli-

nated axons, and slow escape responses, appear restricted to

niches with low predation pressure (e.g. deep ocean),

whereas copepods with myelinated axons, and thus

comparatively rapid escape responses, may be able to

exploit a wider array of habitats (e.g. upper ocean; Lenz

et al., 2000). For decapods, however, knowledge about

predation risk or anti-predator behaviours is patchy for

many species, and is probably inadequate for rigorously

testing hypotheses about correlations between neural

structure and ecology. This is not surprising considering

that the species involved span many families across four

infraorders.

A gross overview of decapod taxa (Fig. 1) nevertheless

suggests two factors that may be correlated with the loss of

startle responses. First, there are alternative anti-predator

Fig. 6. A: Intensified backfill of fast flexor motor neurons in abdominal

ganglion 3 in I. peronii, showing FPI and FMC cell clusters. Note that all

axons group together in connective. Axons projecting posteriorly (past

bottom of frame) are axons of fast flexors found in the FAC cluster of

abdominal ganglion 4. Scale bar ¼ 500 mm. See also Table 1. B: Partial

map of fast flexor motor neurons in crayfish, showing cell body clusters

equivalent to those in A (based on Mittenthal and Wine, 1978). Anterior is

towards top of page. Scale bar ¼ 100 mm.


strategies in taxa lacking giant neurons. Mud shrimp build

burrows (e.g. Nickell and Atkinson, 1995; Bird and Poore,

1999); hermit crabs protect themselves with mollusc shells;

and sand crabs bury themselves in sand (see Fig. 1 and

Introduction). Ibacus spends much of its time buried under

sand (personal observations; see Jones, 1988 regarding

Thenus). Scyllarides latus use shelters (Spanier et al., 1991),

and small Scyllarides tend to cling to their shelter when

disturbed, while large ones tailflip (Spanier et al., 1991; this

trend is the reverse of that seen in clawed lobsters; Lang

et al., 1977). The relationships between these anti-predator

strategies are not simple. For example, crayfish (which have

giant-mediated startle responses) have built burrows for

millions of years (e.g. Babcock et al., 1998; Hasiotis et al.,

1998; Kowalewski et al., 1998) and continue to do so (e.g.

Correia and Ferreira, 1995). Whether the structure or use of

burrows made by crayfish (which have both MGs and LGs)

are quantitatively different than burrows built by taxa that

lack LGs, like thalassinid mud shrimps (Nickell and

Atkinson, 1995) remains to be seen. Second, several species

without startle-related giant interneurons maintain a posture

in which the abdomen is normally flexed (e.g. squat lobsters,

Sillar and Heitler, 1985a,b,c; Wilson and Paul, 1987;

Ibacus, this study). Although postural descriptions are

currently qualitative, the adaptive value of giant-mediated

tailflips should be inversely proportional to the amount of

time the abdomen is held flexed. Because the first movement

in a giant-mediated tailflip is a powerful flexion of the

abdomen (Reichert et al., 1981; Reichert and Wine, 1983),

such a tailflip would produce minimal movement for an

animal in which the abdomen was already flexed when

stimulated. In the case of Ibacus, this would also drive the

abdomen into the substrate, because these animals are often

buried in sand. Based on this argument, any scyllarid in

which the abdomen is normally extended would be a likely

candidate for retaining giant interneurons. These consider-

ations also lead to the prediction that giant neurons are

retained in spiny lobsters (Palinuridae), which have more

morphological and postural similarities to crayfish than

scyllarids do. A survey of other palinuran species could help

define the evolutionary point the giants were lost, and

whether there are palinurans that have lost only some of the

giant neurons (i.e. that are intermediate to Ibacus and the

ancestral condition).

Despite the lack of startle responses, scyllarids are strong

swimmers (Jacklyn and Ritz, 1986; Jones, 1988; Spanier

et al., 1991) and swimming is important to their ecology

(Jones, 1988). Jones (1988) argued that sustained swimming

was an innovation unique to scyllarids. Swimming by

Ibacus shares these features with crayfish swimming: (a)

both are behaviourally flexible; (b) both normally start with

abdominal extension, and; (c) both have the frequency

adjusted by changing the interval between each individual

tailflip. Scyllarid swimming, while remarkable for its

strength, is likely homologous to crayfish swimming and

not an evolutionarily new behaviour. Jones (1988) divided

swimming by the scyllarid Thenus into two qualitatively

different behaviours. ‘Free swimming’ was spontaneous,

with an average velocity of 29 cm/s; ‘escape swimming’

was provoked by external stimuli and had an average speed

of about 1 m/s. These distinctions are functionally useful,

but, from a neuroethological point of view, it seems unlikely

that they represent the workings of two distinct neural

systems. It is more likely that ‘free’ and ‘escape’ swimming

represent the two extremes of the range of output from a

single, flexible motor program, namely non-giant tailflip-

ping. Wilson and Paul (1987) noted that the amount of

abdominal extension varied during non-giant tailflipping in

Munida, and that ‘the extremes are sufficiently different to

give the appearance of two distinct forms of behavior.’

The segmental variation in the number of fast flexor cells

filled (Table 1) is particularly striking for the FAC cluster.

Fig. 7. A, B: Camera lucida drawings of backfills of fast flexor motor neurons in abdominal ganglion 3 in I. peronii. The putative FI (marked with asterisk) has

extensive ipsilateral processes (arrows) relative to the motor neurons of the FPI cluster (on right), which have most of their processes near the midline, where

the axons meet and turn posteriorly to exit the ganglion. Anterior is towards top of page. Scale bar (shown in B) ¼ 200 mm.


In Ibacus and the crayfish species Procambarus clarkii, the

FAC cluster filled from N3d of abdominal ganglia 1–4

contains anywhere from three to zero cells (Table 1;

Mittenthal and Wine, 1978). In some species, the FAC

cluster is completely absent (Wilson and Paul, 1987; Paul,

1989). This variation contrasts sharply to the constancy of

the FPI cluster, which contains four cells in all of the four

anterior abdominal ganglia in all species examined (Wilson

and Paul, 1987). The FMC cluster is similarly constant

within a given species. The number of FMC cells across

species varies only due a loss of the MoG, whose fate

appears closely tied to the presence of giant interneurons.

The only functional hypothesis to explain the variation of

the FAC cells advanced to date is that the number of cells is

reduced due to the reduction or absence of some muscles in

the posterior portion of the abdomen (Mittenthal and Wine,

1978). This notion fails to explain variation across species,

however (Wilson and Paul, 1987), leaving evolutionary

happenstance as the main hypothesis for why the number of

cells in the FAC cluster is more variable than FMC or FPI

clusters (Paul, 1989).

Acknowledgements

This work was completed in the laboratory of Professor

David Macmillan (Department of Zoology, University of

Melbourne); I am deeply thankful to him for his advice and

support. Garry Jolley-Rogers helped to turn video images to

computer files and provided many useful discussions. I also

thank the following staff at the Department of Zoology in

the University of Melbourne for their sterling professional

skills: Bruce Abaloz (histology); John Ahern (animal care);

David Paul (photography and digital imaging). Two

anonymous reviewers provided constructive comments. I

thank Kevin and Karen Faulkes for supplying me with the

tools that allowed me to complete a revision of this paper.

Financial support provided by an NSERC (Canada) post-

doctoral fellowship to ZF and an Australian research

Council Grant to Professor David Macmillan.

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