I don't see why you couldn't use dry weights for l-w relationships or Fulton's K within your own collection of specimens, but I think it is essential that drying has been standardised across seasons and years. Are these air-dried specimens, or have they been in low temperature ovens, for example? Just bear in mind that humidity can alter dry weights significantly, so dry weights across seasons may be somewhat biased.

I don't see why you couldn't use dry weights for l-w relationships or Fulton's K within your own collection of specimens, but I think it is essential that drying has been standardised across seasons and years. Are these air-dried specimens, or have they been in low temperature ovens, for example? Just bear in mind that humidity can alter dry weights significantly, so dry weights across seasons may be somewhat biased.

Obviously not, Look this parameter index stand with certain factor....

numeric representation of a length-weight relationship.

1) By calling this ratio a condition index or condition factor leads many authors to draw conclusions that one group of fish with higher K was in a better physical condition than another group with a lower K. If we would make the same assumptions for Crab, we'd say that those individuals or groups with the highest BMI had the best condition! ...that means not at all dry weight. 

2) Many authors draw conclusions without understanding that in fish as in other vertebrates, linear length growth which is primarily skeletal growth is a fundamentally different growth process from weight growth, which can be due to protein accretion and increased muscle mass, but also due to fat deposition or gonadal growth. Length and weight growth are regulated by different endocrine systems and may not necessarily happen at the same time. These growth processes can affect the fitness of the fish in many different ways and have different energetic costs.

For give you one reasonable example of this is when anadromous salmonids go though parr-smolt transformation. There's a very significant decrease in K. This does not mean that the smolt is in a worse physical condition than the parr, The K decreases because the fish are growing rapidly in length relative to weight, changing their body shape to a longer, leaner shape. Ultimately, this is advantageous for their transition to the marine environment where they have to swim for long distances in open water.

I will attach a PDF 

Whilst I appreciate what Nilotpol is trying to say, I think his answer might be overcomplicated in that he seems to be arguing against the use of length-weight relationships and Fulton's K entirely, rather than in this specific case. What Nilotpol does highlight very well is the importance of understanding the biology of the species you are studying prior to making inferences about body condition with the aforementioned indices.

Simply put, if you have collected crabs from the same areas over several years and want to know if there have been changes in condition over time or across space, you can use dry weights in a l-w relationship as long as this is specified and methods have been consistently applied across the collecting/measuring period. Have a look at the attached PDF for an example of where a dry weight-length relationship has been used. Google scholar will throw up a few more examples for you also.

Hola Carlos, independiente de las respuestas anteriores que son válidas, me parece que tu pregunta está más orientada a la posibilidad de sacar provecho a los datos que tienen. La verdad es que hay toda una polemica por el uso y sobre todo las conclusiones que se pueden extraer de los llamados índices, pero si las medidas morfométricas y de peso  que tienes han sido a lo largo del tiempo tomadas de la misma manera (estandarizadas), es posible utilizar una serie de herramientas para evaluar como en el tiempo estas han ido variando. Te recomiendo que veas los trabajos publicados por :

1.- Warton D. I., Duursma R. A., Falster D. S. & S. Taskinen. 2012. Application. SMATR3 –An R package for estimation and inference about allometric lines. Methods in Ecology and Evolution. 3: 257-259. doi: 10.1111/j.2041-210X.2011.00153x.

2.- los trabajos de Gary C. Packard.

3.- Otra vía sería analizar la información desde un punto de vista multivariado, existen diversas técnicas que se pueden orientar en base a las hipótesis que tengas sobre la especie en estudio, como lo que presentan Hannes Baur y Christoph Leuenberger en Syst Biol. 2011 Dec; 60(6): 813–825. (Analysis of Ratios in Multivariate Morphometry).

Muchos saludos.

Allometric relationships have a long history with crustaceans. I've always liked this paper but there are many others that address size-weight relationships: Hines, A. H. (1982). Allometric constraints and variables of reproductive effort in brachyuran crabs. Marine Biology, 69(3), 309-320. http://link.springer.com/article/10.1007/BF00397496. If you look at Google Scholar and check the citations for this paper (350+) there will be some other excellent papers to follow up on.

As long as the animals were all processed in the same way, then they will be internally consistent when examining the relationship. Aleksandra Maljković mentioned the low-temp oven. You use low temps to avoid volatilizing the lipids in the tissues. 

The R analysis is nice. You can do these types of regressions in just about any stats package, too.

As Aleksandra said if you have all data standardized there is no problem to calculate the L-W relationship. In this case to estimate the CI could be biased because crabs tissues suffered alteration during the preservation of animals. Some chemicals used for preservation of macrobenthic organism can alter the dry weight (Brey,1986). You must be aware that L-W is not a linear relation (W=aLb) where the coefficient b is the coefficient of relative growth (Wilbur & Owen, 1964; Sparre & Venema, 1997: p. 38). Using these data you can compare (if your preservation method was standardized) the relative growth across spatio-temporal scales by using Analysis of Covariance (Zar, 1999) where space and/or time is the categorical variable, length is the dependent variable and weight is the covariate. Moreover, the exponent b is used in estimates of the secondary production using the mass specific growth rate method (Crisp, 1984). I insist, if your preservation method was the same, I see no problem to calculate the L-W relationship to use this data for population assessment.

The problem with crustaceans, As Jeff well knows, is that they have a step-wise growth. At moult the soft shell is pumped up with liquid to swell the hardening carapace, then the animal grows into the available space prior to the next growth "step". That makes analysis of length/weight somewhat problematic - hence the use of drying to get a standard "flesh weight" for a given carapace length.  The issue then of course is that the flesh weight at a given length gradually increases with time since last moult and if time since moult is not known that makes weight at length very variable. I'm not sure there is a simple solution.

Brian Jones's point about molting is a good one. One could examine this statistically by looking at the residuals and assuming that the larger negative residuals are identifying animals that have recently molted. This assumption can be tested on any subset of animals that you know molted vs. a subset of animals in the intermolt stage. Of course it's not perfect, but it may help. Don't forget that there are inflection points in the relationships with allometric growth. The inflections are due to maturation and pubertal molts. You may need to bin your analysis by maturity state (i.e., juvenile vs. adult) and certainly by sex.

Now...first of which is the moisture content in wet weight ratio, ie as the difference between wet and dry weight of the crab.

The Origin of Fulton’s Condition Factor:

http://folk.uib.no/nfiag/nfiag/reprints/NashETAL2006Fisheries.pdf