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  Growth
  Only a few studies have been done on growth in west-coast isopods
 
Research study 0
 

photograph of Idotea resecata courtesy Susannah Andersondrawings of flagella of second antennae in differently aged isopods Idotea resecataCrustacean taxonomists commonly use morphological features such as number of segments in various appendages, shape and location of eyes, and so on, to differentiate species.  But what if these features change during growth?  This risks a different interpretation being made depending upon the age of a particular specimen in hand.  This interesting question is addressed by researchers at the Pacific Marine Station1, Dillon Beach, California for Idotea resecata by monitoring change in various features over a range2 in sizes of individuals, including: 1) number of segments in the flagellum of the 2nd antenna, 2) number of segments of the maxilliped palp, and 3) shape of posterior drawings of posterior portions of differently aged Idotea resecatamargin of telson.  Results show in fact that segments in the flagellum of the 2nd antenna increase in number in direct proportion to size until adult status is reached (see drawings on Left).  As for feature #2 above, the there are 4 joints in the maxilliped palp in specimens less than 6.5mm in length, and 5 in larger specimens.  The third feature in the list, the telson, actually changes from convex shape in the embryo, to slightly concave in specimens greater than 6.5mm in length, to the typical concavity appearance in the adult (see drawings lower Right).  This last feature is regarded as the most diagnostic and key feature of the species.  Several other ontological changes are noted.  The authors remark that based on features in use when the paper was written, a young specimen of P. resecata might well be considered new species3 .  On the basis of their results the authors provide a new (and better) list of diagnostic features for I. resecata, too detailed to include here, but perhaps in use by present-day taxonomists.  Menzies & Waidzunas 1948 Biol Bull 95: 107.  Photograph courtesy Susannah Anderson and iNaturalist.

NOTE1  now known as the University of Pacific Marine Station of Biology, Dillon Beach

NOTE2   10 mature embryos 2.2mm in length and 17 juveniles and adults ranging in size from 5-43mm in length

NOTE3   this apparently actually happened in 1926 and led to a new species Idothea rufescens, a mistake that later appears to have been rectified

 
Research study 0.1
 

graph showing growth rates of gribbles Limnoria tripunctata in southern CaliforniaStudies at the Scripps Institution of Oceanography, La Jolla, California provide data on growth of gribbles Limnoria tripunctata in test blocks of Douglas fir.  Gribbles may have more than one brood per year, so that some young emerge from the brood pouch in winter, as opposed to spring and summer.  Based on incubation time for a brood and the fact that a female is able to produce consecutive broods, a female could theoretically produce one brood in winter, 2 each in spring and autumn, and 3 in summer.  However, the extent to which this potential is realised is not known by the authors.  The graphed data show field growth rates for different seasons, as well as for individuals maintained in laboratory culture.  Sexual maturity at 0.5mm is reached by summer individuals about 3 times faster than by winter individuals.  Johnson & Menzies 1956 Biol Bull 110 (1): 54

 
Research study 1
 

graph showing life cycle-growth of isopods Ligia pallasii in the fieldAt Botanical Beach on the west coast of Vancouver Island, young-of-the-year Ligia pallasii appear in spring and early summer.  The earliest of these are from females carrying broods through the winter, while the later ones are from females carrying early springtime broods.  These latter take about 5wk from egg deposition to release.  After a summer’s growth some of the largest females (probably ones from winter broods) may develop a winter brood, but most breed for the first time the following spring when they are about 1yr old. Note that at any given time there are 2 or sometimes 3 generations present in the population. The graph shows the estimated date of release of youngsters from the brood chamber in April 1970. Growth is maximal during spring/summer and more or less quiescent over winter. Note the faster growth of males in comparison with females, most notable in the 1970 generation. This is accompanied by an allometric increase in width of carapace as the males mature into their broad-bodied, photograph of juvenile Ligia pallasii emerging from mother's brood chamber






female-carrying reproductive state. These males are about 30% larger than females and are likely the dominant “fertilisers” of both winter- and spring-brooding females. It is not known how long a male carries a female, nor whether a certain minimum size is required by a male for amplexus, or if these “super”-males out-compete smaller males for access to females.  Average lifespan is 1.5yr for females, and just over 2yr for males that survive past springtime of their second year. Carefoot 1973 Mar Biol 18: 302.

A juvenile Ligia pallasii emerges
from its mother's brood pouch 4X

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