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Student Project

Biology of the Portunid crab Thalamita prymna with special attention to antennule structure and function.

Derek Richardson 2018


Thalamita prymna is a brachyuran crab (true crab) of the family portunidae, the swimming crabs.  The genus Thalamita is an extremely successful and diverse clade, with at least 60 species found in Australia and 93 described worldwide.  T. prymna specimens' coloration and size vary greatly within the species, and they are visually similar to several other members of the genus, so careful inspection is required to confirm this species’ identification.
Like the majority of Portunid crabs, T. prymna is a predator.  It is known to prey on various species of mollusc, including economically important juvenile abalone, using it’s strong cheliped (claws).  

Physical Description

Identification of Adult and Subadult

Starting with an unidentified brachyuran (true) crab, look at its last pair of legs. If the last two segments are flattened (reminiscent of a paddle), it is almost certainly a portunid swimming crab and a possible member of the genus Thalamita.

Genus Level: Thalamita

- The taxanomic state of Thalamita is in flux, and is both morphologically and taxonomically diverse. As such, confirming a potential diagnosis of Thalamita is difficult without going to the species level.  However, Thalamita have some features incommon that may help narrow down the search. 

1. Carapace (back of shell, viewed from the top) is at least1.5 times as wide as it is long, and is of roughly sub hexagonal or subtrapezoidal shape.

2. Carapace is widest at the back-most anterolateralspine/tooth. 

3. The chelipeds (claw arms) are longer than the legs usedfor walking and/or swimming, and the claws themselves have spines on the menusor wrist.

"Prymna group" or clade Thranita

The clade T. Prymna is in, sometimes referred to as the“prymna group” or recently as it’s own genus, Thranita, has additional features for identification:

1. Six frontal lobes of about equal size

2. Four or five anterolateral teeth.  If there is a fourth one, it is small orrudimentary.

Species Level: T. prymna

If the above criteria are met, diagnosis for T. prymna is:

1. Long, sharp spines on chelipeds with black (or very dark)tips.

2. Basal antennal joint has 3-5 distinct spines

Note:there is some contention over this feature, with some claiming it to be diagnostic, others claiming their specimens have 2 fused spines. 

Fig.1 Hexagonal carapaces of Thalamita sp. viewed from above (T. prymna right middle).
Fig.2 Spines, lobes, ridges and other terminology used in identification above


Perils of Predictability

From the earliest known swimming crabs in the fossil record to the extant members of Portunidae (including Thalamita prymna), the hunting habits of intelligent piscine and cephalopod predators have played a strong role in the evolution of escapes.  

As one would expect when facing an insurmountable foe, escape attempts are most likely to be successful in the direction directly away from the predator (within ~20

Life History and Behaviour

Love doesn't have to be Hard

An early focus on commercially important portunid crabs indicated the entire TAXON consisted of soft-shelled mating crabs.  Females of this mating-strategy can only mate in the window of time immediately following a moult.  The relatively small window of time and vulnerability of females during moulting has created a strong evolutionary pressure for the characteristic mating habits of portunids.  Soft-shelled mating almost invariably involves large males, female guarding behaviour, and other traits that select for successful fertilization by the healthiest male possible.  The only males that will pass on their genetic material are ones that can successfully grow to the size and strength necessary to protect a female from predation or rival males until she is ready to moult, and the male must be able to survive without feeding during the interim.

However, it has been since discovered that some portunids exhibit hard-shelled mating, including T. prymna and other members of Thalamita.  This substantial increase in the window available for mating greatly improves the likelihood of successful fertilization, at the cost of potentially reducing mate quality.   This strategy therefore is only likely to evolve if a species’ size, habitat, population density, predation pressure, or some other factor limit chances of a successful mating encounter.  One would naturally expect this strategy in small or cryptic portunids such as members of Thalamita.  In addition, the larger window of potential mating scenarios allows for the evolution of varying mating strategies in hard-shelled maters with differing life habits.

Norman et al. (1997) found evidence for hard-shelled mating in T. prymna both in the field and in the laboratory.    Over 20 months of field collection/observation just of Banda Marine Laboratory in Tokyo, Japan found no pre- or post-copulatory guarding of females by male T. prymna.  Two pairs were collected “in-the-act,” both females being in their hard-shelled intermoult stage.  In the lab, the only successful fertilization events took place with hard-shelled intermoult females and failed in the soft- or paper-shelled stages.  In addition, after a successful mating, females were rarely willing to mate again.  An immature female was reared to maturity, and after one mating encounter during her intermoult phase she produced fertile eggs.  This led the researchers to conclude that hard-shelled mating is the primary strategy of fertilization in T. prymna. Successful copulation required 34.5 +/- 2.4 minutes.

Antennular Flicking

Barring a particularly stimulating stimulus, T. prymna spends much of it’s time quite still.  If observed at any degree of closeness, there is one source of continuous motion. Described first by myself, at the time very-much lacking in crustacean anatomy know-how, as karate-chop thingies, these are the antennules.  This specialized primary pair of antenna is characteristic of all adult decapods.  For a description of their structure and function in greater detail, see ANATOMY AND PHYSIOLOGY.
If likened to an arm, the constant, asynchronous flicking is ranges from full-arm karate-chops in rapid succession, each a different direction, to occasional flicks of the wrist from a relatively static orientation.   Manipulation of T. prymna will demonstrate three states of antennular action.  First, any visually perceived threat will induce rapid retraction of antennules, where they are folded and held flat in a groove below the frontal margin.  Second, shortly after disturbance the antennules re-emerge and commence moderately rapid flicking.  Third, once T. prymna adjusts to whatever environment it’s in, the flicking will decrease in size of flicks and frequency.  Changes to the rate of antennule flicking also corresponded to non-visual stimulus. 

T. prymna flicking antennules

Retracted when threatened

Low-frequency scanning

Detecting prey increases rate

A crab was situated in a rectangular container with a circular dish place in such a way that movement and sight were restricted, but water could flow between both ends of the container.  After antennular flicking slowed to a assumed constant rate, prey (in the form of a live or crushed mussel) in the opposite compartment of the container to the crab.  Prey items were introduced and removed for five-minute intervals, with the activity of the antennules captured by video recording.  As disturbance had shown a clear effect on action of antennules, water was not changed between prey introductions.  Rather, it was hoped the intervals between exposure to prey items would allow for adjustment to whatever chemosensory information remained in the water.
Data were recorded after the fact by timing key-strokes in a keylogger with events in experimental footage.  Separate keys were assigned to flicks of a selected antennule, video stop/start, and experimental manipulation of prey items.  This produced a data file of key-strokes separated by pauses, which was fed through a python script to output a series of flick-durations vs total time elapsed, a list of times for prey introduction/removal, and a series of antennular flicks per 10 second interval.
The examined T. prymna displayed strong indications that its antennule flicking frequency can be affected my non-visual stimulus, specifically the detection of prey items.  Relatively high flick frequency was typical for a short time after a disturbance but dropped back to a low rate when prey was removed.  Introduction of crushed mussel caused a very high spike in frequency and dropped off gradually.  There was one point where flicking flowed, corresponding with the crab in question repeatedly cleaning his antennules and attempting to reach around the visual barrier with his cheliped.
In contrast, the introduction of live mussel both times caused a spike-drop-spike pattern.  One possible hypothesis is that the spike is caused by disturbance (crab sees my hand) and chemosensory detection of a new object (closed mussel).  The crab acclimatizes, dropping its flick-rate back to resting level, but after about two minutes the mussel opens.  This sends a new and/or more powerful signal to the crab, bringing the rate up to about two flicks per second per antennule.  Testing this would be straight-forward as long as opening of the mussel was easily detected.  
Increased antennular flicking is characteristic behaviour of hunting portunids.  Blue crabs (Callinectes sapidus) exhibited this behaviour immediately prior to digging up buried clams.   

Crab Graph!

Anatomy and Physiology


Much of the success of the crustaceans, and phylum arthropoda in general, is due to the specialization of similar segmented body-parts.  The decapod antennule is a fantastic example of this specialization for the purposes of chemo-sensation.  This primary pair of antenna is the only part of a brachyuran crab with aestetascs,or “hairs that can smell” (olfactory sensillae).  It is with these organs that the animal can initiate and direct foraging behaviour by detection and characterization of scent trails. 

Through the antennular flick, T. prymna is capable of“sniffing” the water.  Each flick takes a discrete sample of the water.  During rapid down-stroke of the flick, the high speed forces water between the aesthetasc hairs.  This is followed by a slower up-stroke, where water is held between the hairs.   This type of sampling allows for fine-scale tracking of scent trails and makes speed and frequency of the flicks very important to their function.

The structure, length, andflexibility of these hairs is ideally suited for detecting scent trails inwater, and crabs adapted for life on land have significantly altered structure,but still variation on the same central design.


Biogeographic Distribution


T. prymna
is found in the Red Sea and throughout the tropical indo-pacific. In Australia, it is found on both the eastern and western coasts, with documented occurrences in QLD, NSW, NA, and WA. Given the intra-specific variability and similarity to several members of Thalamita, documented observances may only roughly reflect actual distribution.

Evolution and Systematics

  The earliest confirmed brachyuran crab in the fossil record is from the early jurassic.  A crustacean carapace from the Carboniferous may be from an early or proto- brachyuran, but cannot be confirmed as such.  However, the diversity of the true crabs exploded in the late Jurassic and early Cretaceous,likely due to pressure from bony fishes.  Recent review of the superfamily portunoidea has exposed the genus Thalamita as potentially paraphyletic.  T. Prymna was found to fit into a clade with several of the larger members of Thalamita, and Evans (2018) proposes their placement in a new genus: Thranita. The species effected include T. Spinimana, crenata, spinicarpa, danae, foresti, rubridens, prymna, coeruleipes, and pseudopelsarti.  The study did not include representatives from all species of Thalamita, and thus there may be additional members of this gen. nov.

Conservation and Threats

T. prymna has a wide distribution, can take advantage of a broad range of potential microhabitats, and is not targeted by any commercial fisheries.  Though there have been no world-wide population estimates for the species, it’s status is likely to be of least concern.


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