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  Moulting, growth, age, & claw morphology
   
 

photograph of shrimp Heptacarpus pictus recently moultedThis topic is split into sections on mud & ghost shrimps (thalassinids) and "true" shrimps (carids).

 

 

 

 

 

 

Caridean shrimp Heptacarpus pictus recently
moulted in a laboratory Petri dish 2X

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  Mud (F. Upogebiidae) & ghost shrimps (F. Calianassidae)
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Research study 1
 

photograph of a preserved mud shrimp Upogebia pugettensis
Mud shrimps Upogebia pugettensis grow to a length of about 13cm and live for 4-5yr.  Feldman et al. 2000 Estuaries 23: 141.



0.6X

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Research study 2
  graph showing size vs. age in mud shrimps Upogebia pugettensis of both sexesgraph showing size vs. age of ghost shrimps Neotrypaea californiensis of both sexes
Studies in Willapa Bay, Washington on mud and ghost shrimps show that annual growth of Upogebia pugettensis (4-5mm Carapace Length per year) is greater than that of Neotrypaea californiensis (2-3mm CL per year).  Males of both species grow significantly faster than females.  Both species live for 4-5yr.  Dumbauld et al. 1996 J Crust Biol 16: 689.
 

graph showing sexual dimorphism of mud shrimps Upogebia pugettensisBoth species are sexually dimorphic.  Growth of chelipeds changes after a certain size is reached.  This occurs at carapace lengths graph showing sexual dimorphism of ghost shrimps Neotrypaea californiensisof about 25mm for Upogebia pugettensis and 10mm for Neotrypaea californiensis (indicated by blue vertical lines on the graphs), presumably corresponding with sexual maturation (also shown on the graphs in orange for the females).  In males of Neotrypaea the claws at this time start to become disproportionately larger, while in females the claws become disproportionately smaller.  In Upogebia it is not the claws of the males that change much (if at all); rather, the female claws become relatively smaller. The authors believe that this reflects the extra investment in reproduction by females as compared photograph of right claw of a male mud shrimp Upogebia pugettensiswith males, as well as a selection for large claw size for sexual aggressiveness and sexual display in males.  Dumbauld et al. 1996 J Crust Biol 16: 689.

NOTE  the authors appear to have calculated 2 regression lines for each sex after separating the data by eye.  This works well for Neotrypaea since the transition points for the two sexes are quite clear, but not for Upogebia where the transition points (if they exist at all) are less obvious, especially for males.  In fact, based on the similarily of slopes, b, for the 2 curves, the justification for splitting the male regression into 2 parts is not at all clear.  Also, the authors choose to use propodus height on the ordinate axis, rather than propodus length.  When looking for allometric changes during growth it may be better to select the longest axis of a structure, because it changes relatively most during growth

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Research study 3
 

The chelae of mud shrimps and ghost shrimps are heterochelous1, with the larger or major2 claw generally being thought to function in sexual display in males, and in territorial defense and protecting the head/carapace in both sexes.  A detailed study on development and allometry of claws in ghost shrimps Neotrypaea californiensis at the Bamfield Marine Sciences Centre, British Columbia reveals a number of interesting facts relating to possible function of the major claw.  First, both sexes exhibit positive allometric growth of chelae, but with major claws representing up to 25% of total live body mass in mature males and about 10% in mature females. Minor claws represent less than 3% of body mass in both sexes.  Second, proportions of right to left major claws do not differ significantly from a 1:1 ratio, although the authors comment that major claws occur more commonly on the right side.  Third, allometry is greater in males than in females in both major and minor claw size.  Fourth, major claws differ significantly in several ways between sexes, mainly in the male claw being proportionally higher relative to length (see photographs). Specifically, major claws in males are 30% higher than in females, have a larger gape, have a more distally hooked dactylus and deeper propodal notch, and exhibit more well-developed teeth than in female major claws.  Finally, mechanical advantage3 is 50% greater in major claws than in minor claws, with no significant difference between the sexes.  The mechanical advantages of the major claws range range around a value of 0.2, which is indicative of selection more for fast movement than for force.  Based mainly on allometric growth of manus height to length, the hooked form of the dactylus, presence of many small regularly shaped teeth lining the claw gape, and a “fast” mechanical advantage, the authors infer that the major claws function for grappling between individuals, possibly in sex- or territory-related competitive interactions.  They add that behavioural observations will be needed to confirm this inference.  Labadie & Palmer 1996 J Zool, Lond, Lond 240: 659.schematic showing claws of male and female mud shrimps Neotrypaea californiensis with various dimensions use in study of claw morphometrics

NOTE1  the more general term “dimorphic” (lit. “two forms”) is often used to indicate differently shaped claws in crustaceans, but heterochelous (lit. “different claws”) is better because it is more specific

NOTE2  the authors use the terms “master/minor” in their paper to designate the different claws, following the lead of some other researchers, but “major/minor” as used here may be more appropriate

NOTE3  expressed here as the ratio of dactylus length to lever length (see drawing)

Major claws of male and female mud shrimps Neotrypaea californiensis
shown at the same scale. An overlay shows the measurements taken for
allometric comparisons. "Manus" is the same as "propodus" or "propodite" used in Research Study 2 above. Mechanical advantage of the closing lever in the claw is
determined by the ratio of input lever length/output lever length (dactylus length)

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Research study 4
 

graph comparing growth and ages of ghost shrimps Neotrypaea californiensis at 3 estuaring locations in Washington and OregonQuantitative extraction of a so-called “age” pigment lipofuscin has allowed much finer discrimination of age-class structure and growth in ghost shrimps Neotrypaea californiensis than available by traditional morphometric measurements.  Use of the technique by Oregon researchers allows 13, rather than the usual 7, age classes to be identified in ghost-shrimp populations in Willapa Bay, Washington and Yaquina Bay, Oregon, relatively clear separation of size and age of shrimps in 3 locations, and age estimates of 10yr or more (see graph).  Bosely & Dumbauld 2011 Mar Ecol Progr Ser 428: 161.

NOTE  lipofuscin is a fluorescent pigment that accumulates over time in various tissues, such as nerves and muscles, in crustaceans.  Its extraction and subsequent quantitative fluorescent-analysis has provided a relatively reliable, easy-to-use, and quick method for age estimation in crabs, amphipods, lobsters, and other crustaceans.  For best application the method requires calibration, done by rearing animals in the laboratory and tracking changes in concentration of lipofuscin over time.  However, in place of this method, the investigators use a young “reference age-class” of shrimps for baseline calibration, place representatives of these in buckets buried at each site, and sample individuals for lipofuscin analysis at 2-3mo intervals over a 10mo total study period

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  "True" or caridean shrimps
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Research study 1
 

There are at least 8 species of west-coast pandalid shrimps, many of which are economically important.  Most undergo a sex change from male to female as they grow.  The largest and most common of these species is Pandalas platyceros, also known as the “spot prawn”, or simply “prawn”. Of all photograph of spot prawn Pandalus platycerosthe pandalids, this species may be the most familiar because as an adult it inhabits shallow water of 3-6m depth and walks on the sea bottom. 

Studies on P. platyceros trawled in the Strait of Georgia, British Columbia show that it has 4 larval stages, stages 1-2 of which occur in deep water and stages 3-4 in shallow water.  Sexual maturation of males occurs in the second autumn at 18mo of age and at about 14-18cm total length. Change of sex to female occurs in the fourth season at around 30mo of age at about 18-graph showing age and sizes of spot prawns Pandalus platyceros in the Strait of Georgia, British Columbia20cm length, although some individuals change much earlier than this.  Females bear about 4000 eggs.  The author provides detailed information on other pandalid species found in the Strait of Georgia.  Butler 1964 J Fish Res Bd Can 21: 1403.

NOTE  so named for the presence of white spots on the 1st and 5th abdominal segments

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Research study 2
 

drawing showing location of statocyst in a shrimpdrawing showing how sand grains are restored in the statocyst of a prawnPost-moult shrimps require not only to harden their newly expanded exoskeletons, but also to renew the functioning of their balance organs or statocysts. Equilibrium/balance in shrimps and prawns is accomplished through sensory feedback from statocysts located at the base of each antennule (see figure on Left). As in other animals, the statocyst consists of a water-filled chamber lined with sensory bristles and containing a small object of high-density, the statolith.  In most crustaceans the statolith is a grain, or several accreted grains, of sand.  The statocyst in crustaceans is an invagination of the cuticle and is open to the outside via a funnel-shaped cana llined with setae (bristles).  An aperture, a slit of about 0.1 x 0.4mm, leads into the chamber of the statocyst. During moulting the cuticular lining of the statocyst is discarded along with the statolith, and the latter has to be replaced within a few hours after each moult.  Before implanting new sand grains after the moult, the prawn has to wait long enough for its limbs to harden sufficiently to support its weight and allow digging. The prawn sifts through the soil with its anterior walking legs and then flicks up the lighter grains towards the region of the statocyst opening (see drawings on Right).  Implantation, that is, actual entry of the grain(s) into the statocyst, is accomplished by retracting the eyestalk repeatedly into its protective cavity.  The eyestalk lies immediately above the statocyst aperture and this motion forces sand grains into the aperture.  Once inside the statocyst cavity the loose grains are cemented together by a sticky secretion.  Haywood & Alexander 1982 Mar Biol 72: 113.

NOTE the description given here for moulting and renewal of the statolith after moulting applies to the genus Penaeus. Although comprising dozens of species distributed widely through the oceans of the world, penaeids appear to be absent from the temperate coast of North America. The information provided, though, should be applicable to most or even all other caridean shrimps inhabiting west-coast shore. The above description applies to Penaeus merguiensis collected near Townsville, Australia

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