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Hidden within the Crinoids,
Lies the Elegant Squat Lobster
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Michael John Thompson 2018
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Summary | |
Allogalathea elegans is a crustacean part of the Allogalathea genus in the Galatheidae family of Squat Lobsters, a taxonomic family of marine lobsters. A. elegans are unique due to the shape of the rostrum, varying body colouration and commensal relation to crinoid species. Despite the species first identified in 1848, little is still known about the species. However, in recent times, the species is now kept in many marine aquariums due to the association with crinoid hosts. This species review collectively reports characteristics of the species from their behaviour to their conservation.
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Physical Description |
External Morphology | |
Colour
The species A. elegans has a highly variable body colour patterns, the most commonly ranging from various uniformly dark colours like red, blackish purple, orange or brown, to patterns such as black with two narrow light stripes, or to alternating longitudinal black and white stripes, to which the number, thickness and tint of these stripes are highly variable as well. Additional colour patterns include a lighter narrow stripe within the middle of each dark stripe. In some of the colour morphs of the species the appendages attached to the body tend to have a noticeable white tip. The variability to the animal’s colouration is due to match the colours of its host however not systemically (Poore, Ahyong and Taylor, 2012). The A. elegans specimen collected demonstrated a colour pattern where it was generally uniformly dark brown with four alternating white and yellow longitudinal stripes across the carapace of the animal. The thickness and length of the stripes varied on the body with the middle two stripes traversing across the whole carapace whilst being slightly thicker than the outer two stripes which covered approximately a quarter of the length of the cephalothorax. Additionally, the specimen had white tips along the leg appendages and the chelipeds tips.
Shape
Like all species from the phylum Arthropoda, A. elegans, is comprised of a chitinous exoskeleton that provides support and structure to the organisms body. Additionally, segmentation is also present in A. elegans with two sections, the Cephalothorax and Abdomen, of which the post-abdomen is located on the underside of the body to aid with locomotion. The A. elegans cephalothorax is shaped like a droplet; this extremity of the body corresponds to the animal’s triangular rostrum, which is position on each side of the pedunculated eyes. Under the rostrum of the species is where the gills are located. The animal’s chelipeds are endowed with pincers and the length exceeds the body length of the organism. Additionally, the last pair of legs is greatly reduced leaving a total of four pairs of limbs unlike most species from the Decapoda order. The whole body and limbs of the organism is covered with small hairs known as setae (Poore, Ahyong and Taylor, 2012). The external morphology characteristics of A. elegans are labelled in Figure 1.
Size
Sexual dimorphism is evident in the species, with A. elegans size being greatly dependant on the sex of the organism. Females of the species are generally larger than the males, however the species don’t generally grow over 2cm (Adams, 1848). The A. elegans specimen collected was approximately 1.2cm in length when stationary, when the claws were included the total body length was 1.7cm as shown in Figure 2.
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| Figure 1 |
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| Figure 2 |
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Similarity to Other Species | |
Within the Allogalathea genus, there are three other similar species to A. elegans, the genus includes the following species in addition; Allogalathea babai, Allogalathea inermis, and Allogalathea longimana.
Allogalathea babai (Baba’s Squat Lobster)
A. babai species body colour is smaller in range only being brown or orange with a distinctive yellow/ white middle longitudinal broad stripe that’s flanked on each side by brown stripes which is less variable than A. elegans. The locality of the species is endemic strictly to Indo-Pacific Ocean region at depths of 10-44m, which overlaps with A. elegans. The size of the species is significantly larger in comparison to A. elegans, with males and females ranging from 2.5-6cm. In addition A. babai is a known associate with the crinoid species Oxycomanthus bennetti, the same host as to A. elegans (Cabezas, Macpherson and Machordom, 2011).
Allogalathea inermis
A. inermis species body colour has three observed patterns, with the carapace and abdomen being uniformly dark, either brown or red, or dark brown with two narrow white stripes, and finally an alternating pattern of black and white longitudinal stripes. Additionally, what separates A. inermis from A. elegans is the absence of spinules on the squamae. A. inermis is distributed generally towards the east side of the Pacific Ocean from Japan to New Caledonia, with a case of the species being observed in Mozambique at depths of 44 to 120m, this overlaps the distribution of A. elegans. In regards to size comparison to A. elegans, A. inermis is slightly smaller in body size in both males and females. Additionally, A. inermis are a known to Oxycomanthus bennetti, similar to A. elegans, and uniquely found on Himerometra robustipinna (Cabezas, Macpherson and Machordom, 2011).
Allogalathea longimana
A. longimana species body colour has a narrow range with alternating stripes from dark brown, white, and to yellow. The middle stripe present on the carapace is always dark brown. Another characteristic that separates A. longimana from A. elegans is the moderately long rostrum, and the arms being significantly longer. A. longimana is known to be distributed along the east Pacific Ocean in areas such as Japan, the Philippines, and Queensland Australia at water depths of 36 to 194 meters. In regards to body size, A. longimana is significantly larger than A. elegans in both males and females. Not much is known specific hosts for A. longimana, however it’s assumed that the crinoid hosts are from the family Comatulidae as the species are widespread across all water depth classes that is similar to A. elegans (Cabezas, Macpherson and Machordom, 2011).
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Ecology |
Commensalism | |
As stated previously the genus Allogalathea are commensal species
towards living in association with crinoids. For the species, A. elegans, the lobster receives
protection and support for its’ feeding. A.
elegans is capable of living outside the host in rock crevices, but however
it greatly reduces the species life expectancy as it is no longer being
shielded by the host from predators. A. elegans
take advantage of the similar diet it shares with its hosts. When coupled with
their hosts, observing the species can be difficult, the A. elegans will expose itself when feeding along the crinoid’s
arms, or it’ll situate the body under the crinoid host along the cirri in order
to hide from predators. A. elegans
live exclusively in association with crinoids from the Comtulida order. The known
crinoid hosts that A. elegans have
been known to be associated with include; Capillaster
multiradiatus, Comaster schlegelii,
Comanthus parvicirrus, Oxycomanthus bennetti, and Stephanometra spicata as shown in Figure 3. (Poore, Ahyong and Taylor, 2012).
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| Figure 3 |
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Life History and Behaviour |
Reproduction Behaviour | |
The preferred method of reproduction displayed
by A. elegans is sexual reproduction.
Although not much is known on the specific reproduction behaviour of A. elegans, the species share similar
mating behaviours with other squat lobsters from the Galatheidae family. Current knowledge suggests that there’s two
mating strategies, ‘Pure Search’ where males search for receptive females, and
the mating interaction is kept brief so the couple separate quickly after the
transfer of sperm. The other strategy is ‘Search & Defend’ where males
guard and protect reproductive females from other competitive males for
extended periods before and after copulation. Each species from the Galatheidae family usually adopts one of
these two strategies for mating (Poore, Ahyong and Taylor, 2012). The presences
of the A. elegans pincers can support
either strategy because the appendages could either be used for clicking in
attracting a mate or defending a female from other males.
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Life History | |
Not much is know about the development stages of
A. elegans after sexual reproduction.
It’s known that in the species, females will attach the fertilised eggs to the legs
and carries them until they’ve hatched. Once the eggs hatch, the larvae are
planktotrophic and undergo through a series of metamorphosis until the larvae
is ready to settle on a suitable host (Fujita, 2010).
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Feeding | |
Taking advantage of a similar diet to their
crinoid hosts, A. elegans is a
planktivore with a diet that consists of planktonic food such as zooplankton
and phytoplankton. The similar diet is possible reasons for the strong
association A. elegans have with
their crinoid hosts, as crinoids tend to position themselves in the most
suitable catchment areas for best access to the food source (Poore, Ahyong and
Taylor, 2012).
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Locomotion | |
A. elegans achieve locomotion through three methods, first is by the use of the chelae, second is the use of the post-abdomen that’s situated under the body, and thirdly is the use of the four pairs of legs. With the first method of locomotion, the species is very active in which causes the organism to create powerful jerks by snapping the chelae quickly together, which allows A. elegans to quickly dart in a backwards motion. Additionally, the second form of locomotion causes violent movements when the post-abdomen segment is bent under the body, this again allows the organism to quickly dart in a backwards motion. However the first two form of locomotion are quite energy expensive and would only be used when moving quickly between sites or to avoid predators. The last method of crawling using the limbs is more of an energy efficient form of locomotion for A. elegans. This form of movement would be ideally seen when there’s no threats to the species present or when crawling through the arms of the crinoid host (Adams, 1848). The locomotion of A. elegans can be seen in the video attached below.
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| The video showcases the different forms of locomotion of A. elegans over different surfaces including two crinoid hosts; Capillaster multiradiatus and Comaster schlegelii. Source: Michael Thompson, 2018. |
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Respiration | |
In some of the A. elegans that reside in deeper waters, the environment can be
quiet anoxic, as well as shallow stagnant waters. Studies have shown that
smaller crustaceans tend to have a higher oxygen content level in the body than
large crustacean species. A. elegans
utilise the gills situated on the underside of the rostrum, and since the
species tend to occupy water depths with a stable oxygen level. If in a
situation where the waters might be anoxic, the species can swim up the water
column to locate a new host in a more stable environment (Burj, 1988).
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Host Preference Selection Experiment | |
Hypothesis
A. elegans are known commensal species that live in association to Capillaster multiradiatus, therefore it’s hypothesised that when placed in an environment with multiple suitable hosts, A. elegans will have a shorter decision processing time and a higher frequency in selecting C. multiradiatus as a host over the other host treatments.
Experimental Design
Experiment 1 – Decision-Making Process;
A. elegans was placed into an individual container alone. There were four separate treatments, one with C. multiradiatus (control), a rock sample (Treatment 1), and two other Crinoid species; Lamprometra palmate (Treatment 3) and Comaster schlegelii (Treatment 4) were all placed in individual containers. During each treatment once A. elegans was placed into the host container, the time it took for A. elegans to settle on the host (if A. elegans spends longer than 5 seconds on host the time stopped recording). The time is an indicator on the decision making process for host selection. This process was repeated at least 5 times (for an average) in each treatment.
Experiment 2 – Host Preference Selection;
Once all four treatments were recorded from Experiment 1, the four hosts were all placed into the corners of one large individual container all separated equally apart from each other. A. elegans was then placed into the large container and left for 30 seconds in order to relax before released. Once released, the time taken to select a host and frequency of occurrence in selecting a host was recorded till it selected a host, this process was replicated 15-20 times. After ever second repetition, the container was turned 90 degrees in order to keep the experiment at random. As shown in Figure 4.
Results
In Figure 5, are the results table from Experiment 1. In Figure 6, are the results table from Experiment 2. Within both tables are the data collected from both experiments.
Discussion
As shown in Figure 7, it illustrates the results from both Experiment 1 and 2. In Experiment 1, it explained the decision making process in A. elegans when approaching a host. The results had shown that when presented with any crinoid host, the decision-making time was significantly shorter than when presented with a rock host. This suggest that A. elegans is able to interpret the type of host in the container and when a crinoid host was present such as in Treatment 2 to 4, the decision making process is under 120 seconds, making it a quick decision process into deciding to settling on a host in comparison to the Treatment 1. Of all the treatments, C. multiradiatus had the quickest decision making process time, this could have been the result of C. multiradiatus being a known host to A. elegans, and the two species already being associated together prior to experimentation. Indicating that memory and learning behaviours could be evident within A. elegans during decision-making. It’s possible that A. elegans is reliant on visual cues presented from the host, however there’s a need for more research into other sensory inputs the host exhibits in order to conclude on the decision-making processes. It’s possible the reason for the quicker decision-making to crinoid hosts is due to a chemical signal. Crinoid host could possibly be producing a chemical as an attractant for organisms like A. elegans. However there is need again for more research on the topic of chemical cues and the reception of these cues by the antennae of A. elegans.
In Experiment 2 on Figure 7, it illustrates the frequency score for host preference selection in A. elegans when presented with four different hosts options at the same time and in Figure 8 it shows the time taken for the decision making process to occur for A. elegans during this process of host selection. The results had shown that when presented with the four treatments at the same time, A. elegans had a higher frequency percentage towards C. schlegelii than any of the other treatments. However, in Figure 8, C. shlegelii had a very variable time score similar to the rock treatment, but unlike C. multiradiatus, the time score is so much less variable indicating the decision making process was more precise when selecting Treatment 2 as a host. Of all the treatment options, Treatment 3 with the crinoid host, L. palmate, wasn’t the most ideal host, for A. elegans. The crinoid L. palmate isn’t a known associate host for A. elegans, this is supported with the treatment with the lowest frequency percentage. This low frequency score when combined with the short time score indicated it would select the L. palmate when in a state of a flight response from danger, despite no induced panic to A. elegans during experimentation. With the rock in Treatment 1, A. elegans had a shorter time in decision-making but also had a high frequency. This suggests that when A. elegans can’t decide on a suitable host, it selects a rock because it can act as a shelter from predators or a last alternative when it is unable to select a crinoid host. Treatment 3 with C. schlegelii was had the highest frequency percentage despite a high variability in the decision-making process time, making C. schlegelii the most suitable host for A. elegans. It’s suspected that A. elegans had variable time scores for this treatment because it needed time to properly assess traits likes possible visual, and chemical cues and size produced by the crinoid host. C. schlegelii is a known host to A. elegans, and when traits like the specimen larger body size (compared to the other crinoids) and colouration matching A. elegans body colour pattern making the crinoid the most ideal host when presented with other options.
Therefore despite C. multiradiatus being a known and familiar host to A. elegans, when presented with other more suitable hosts than C. multiradiatus, such as C. schlegelii. A. elegans will assess visual, chemical and size cues in order to select a suitable host, thus not supporting the initial hypothesis.
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| Figure 4 |
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| Figure 5 |
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| Figure 6 |
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| Figure 7 |
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| Figure 8 |
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Anatomy and Physiology |
Musculature | |
A. elegans share
a similar musculature to all squat lobsters as Decapoda musculature is conservative. The muscle groups can be
classified into four primary groups; main flexor, superficial flexor, extensor
and lateral extensor muscles. Within the
abdomen, the flexor muscles lie ventrally forming the bulk mass of the
abdominal muscle. These muscles are intricately interwoven mass of which
includes transverse, paired longitudinal central muscles, and anterior and
posterior oblique muscle groups. The
combination of the muscles allows A.
elegans to perform a ‘Tail Flip’ action in locomotion. The superficial
flexor muscle is situated beneath the main flexor muscles and connects with the
successive sterna. This muscle group is significantly weaker than the main
flexor muscle group so it’s unlikely to contribute much to abdominal flexion. The
extensor muscles lay over the flexor muscles and are involved with the
extension of the abdominal mass that is accomplished by three series of muscle
strains that aids the movement of the organism. Lastly, the lateral extensor
muscle consists of two muscles from the abdomens somite 1 lateral tergal edge (Poore,
Ahyong and Taylor, 2012). The four groups of muscles when in action help A. elegans to swim through the water
column and perform powerful short bursts of movements to avoid predators.
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Digestive System | |
The digestive system is comprised of multiple
components, the mouth, chitinous labrum, short oesophagus, foregut, reduced
midgut with a pair of heptopancreas, tubular hindgut, and the anus along the
ventral side of the telson. The foregut is comprised of two sections, the
anterior ‘Cardiac Chamber’ and the posterior ‘Pyloric Chamber’. The cardiac chamber
is protected by numerous valves in which helps the forced accumulation of food
material between grinding teeth. The smaller pyloric chamber is comprised of
two compartments separated by two lateral folds (dorsal and ventral). As course
food matter pass through the cardiac chamber, into the dorsal section of the
pyloric chamber, fine food material is dissolved through the ventral section
which acts as a filtration device known as the ‘gland filter’. The structure of
the midgut is very short and lacks chitinous lining, the opening of the
hepatopancreas directly posterior to the pyloric chamber ampulla. The
hepatopanceas in Decapoda functions
include, secretion, absorption, digestive enzymes transportation, and nutrient
storage. The mop-like brown structure is located along the lateral and ventral
cavities in the thorax comprised of numerous blind tubules. This structure is
comprised of the three components, internal microvillous lumen, external
connective tissues, and fine muscle fibres. Lastly, the hindgut structure is a
circular tube that extends posteriorly under the pericardial cavity towards the
posterior end of the reproductive organs, and then enters the abdomen above flexor
muscles until it opens at the telson. The hindgut functions as a duct that
transports any residual faecal pellets (Poore, Ahyong and Taylor, 2012).
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Respiratory System | |
Gas exchange in A. elegans is achieved through the gills. The Galatheidae family of squat lobsters have numerous flat branches/
lamellae known as phyllobranchiate that’s located within the gill chambers on
both sides of the thorax. Unlike most Decapoda,
squat lobsters lack podobranch, and the presence of the arthrobranchs in thoracic
somites 3-7, and pleurobranchs on the thoracic somites 5-8. The epipods are
considered to have respiratory function and in Galatheidae, epipods are located in the maxillipeds 1 and 3,
however absent on 2. The gill structure is comprised of a branchial stem with
two lobes, with rows of palmate lamellae. The afferent branchial vein enters
through the axis of the gill and separated by a thin septum of connective
tissue that connects the efferent branchial vessels. In some Galathelids, it’s suggested that gas
exchange can occur through the movement of the mouthparts, which generates
water currents for respiration (Poore, Ahyong and Taylor, 2012).
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Circulatory System | |
The heart of A.
elegans is dorso-ventrally flattened pear-shaped structure that’s suspended
by connective tissue strands in the pericardial sinus beneath the cardiac
region of the carapace. As blood flows from the sinus to the heart through
three pairs of ostia, it is then pumped out during the systole period to major
arteries, which then distributes the blood to all parts of the body. Blood
enters the afferent branchial vessels in order for gas exchange to occur, the
efferent branchial vessels then transport the newly oxygenated blood towards
the pericardial sinus through the thoracic wall. The flow of the blood occurs
only in a one-way directional flow from the pericardial sinus to the heart
because of the synchronised opening and closing action of the ostia and
arterial valves during systole and diastole (Poore, Ahyong and Taylor, 2012).
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Reproductive System | |
The male reproductive system of Galathelids squat lobsters consists of
symmetrical testes with vasa deferentia that lead to the gonophores on pereopod
5. The testes are situated dorso-laterally to the foregut and ventrally to the
heart within the cephalothorax region. In regards to the female reproductive
system, the ovaries are situated within the same area as the testes and vasa
deferentia. Galathelid sperms are
similar in shape, the sperm consists of a elongated acrosome vesicle that is
capped with an operculum and then penetrated with a perforatorial chamber. The
acrosome inner zone is sectioned into two regions based on differing electron
density, whilst the outer zone occupies the acrosome vesicle posterior
position. Form the cytoplasmic neck region, consists of three microtubular arms
and longitudinal grooves/ septa that are present within the posterior of the
perforatorial chamber (Poore, Ahyong and Taylor, 2012).
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Biogeographic Distribution |
Local Distribution/ Habitat | |
A. elegans
are a commensal species that live in association with various crinoid species.
Thus share similar habitats with their hosts such as tide pools, coral reefs,
and rocky, sandy and muddy substrates, at varying water depths from subtidal
zones to 120m (Poore, Ahyong and Taylor, 2012). A. elegans was first described in the Straits of Sunda which
connects the Java sea to the Indian Ocean, between the Indonesian islands of
Java and Sumatra (Adams, 1848).
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Geographical Distribution | |
With only four species within the Allogalathea genus, the squat lobsters
occupy tropical waters in reef systems to which their crinoid hosts that the
species are associated with occupy. The genus occupies a wide distribution
within the Indo- West Pacific region, in which A. elegans can also be found in. The distribution range for A. elegans is likely wider, including
sites such as Mozambique, Red Sea, Madagascar, Taiwan, Philippines, Indonesia,
Vanuatu, New Caledonia and Chesterfield Islands, with known occurrences, in
areas like South Africa, Sri Lanka, Bay of Bengal, Japan, Western &
South-Western Australia, Queensland, Great Barrier Reef, and Fiji as shown in
Figure 9 (Adams, 1848). The specimen collected was obtained from the Gold Coast
Spit Waterway on April 25th 2018. Using the Atlas of Living
Australia website as a guide, the occurrence record map for A. elegans had shown specimens along the
northern coastline of Australia from Shark Bay, Western Australia, to Crowdy
Head, New South Wales. Records have shown specimens collected along the Gold
Coast seaway, the same area in which the specimen used in this study was
collected from, which further supported the identification of the specimen.
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| Figure 9 |
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Evolution and Systematics |
Evolution | |
Species identification within the Galatheidae family of squat lobsters is
difficult, as many of the morphological traits are considered conservative. The
genus Allogalathea was established in
1969 by Baba in order to include A.
elegans (formerly Galathea elegans).
The Allogalathea is easily
distinguishable from other genus by a morphological trait, the triangular
rostrum of which is extremely elongated, dorsally flattened, and ventrally
carinate, consisting of 5 to 9 lateral teeth, and the lack of a setiferous
striae carapace. However, within the genus, these traits are highly variable.
Based on phylogenetic studies strongly supports the existence of four
mitochondrial clades, A. elegans, A. babai, A. inermis, and A. longimana
as shown by the phylogenetic tree in Figure 10. Therefore characteristics such
as spinulation and length of the chelipeds, spinulation of the limbs, and the
rostrum shape can contribute to classifying the Allogalathea genus (Cabezas, Macpherson and Machordom, 2011).
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| Figure 10 |
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Conservation and Threats |
Conservation | |
A. elegans
isn’t listed as a threatened species under the IUCN, as the species is too
cryptic and difficult in order to determine the population size.
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Threats | |
Despite the lack of population data on A. elegans, the species does face numerous threats to their survival. These threats include overfishing, ocean acidification, and pollution are major threats on species, however the species are highly dependent on their coral reef habitat survival, and a leading cause for possible extinction. Small reef crustaceans are highly dependent in coral reefs as the reef provides protection from predators for the species, additionally in regards to A. elegans, the species is commensal to crinoid species which also occupy reef ecosystems. With a decline in the survival of reef systems, there’s an expected massive loss to not only A. elegans but also a lot of other species of organisms that are dependent on the existence of the coral reef system.
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References | |
1.
Adams, A. (1848). Notes from a journal of research into
the natural history of the countries visited during the voyage of H.M.S.
Samarang ... under the command of Sir Edward Belcher. London: Reeve, Benham
and Reeve.
2.
Burd, B. (1988). Comparative gill characteristics of Munida
quadrispina (Decapoda, Galatheidae) from different habitat oxygen
conditions. Canadian Journal of Zoology, 66(10), pp.2320-2323.
3.
Cabezas, P., Macpherson, E. and Machordom, A. (2011). Allogalathea
(Decapoda: Galatheidae): a monospecific genus of squat lobster?. Zoological
Journal of the Linnean Society, 162(2), pp.245-270.
4.
Fujita, Y. (2010). Larval stages of the crinoid-associated squat
lobster, Allogalathea elegans (Adams & White, 1848) (Decapoda: Anomura:
Galatheidae) described from laboratory-reared material. Crustacean
Research, 39(0), pp.37-53.
5.
Poore, G., Ahyong, S. and Taylor, J. (2012). The Biology
of Squat Lobsters. CRC Press: [s.n.].
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