Perhaps that is not the question as the same can be said of shallow water isopods.

How were the samples collected?

I wanted to suggest you speak to Niel Bruce, but I see he has already responded! You could also try contacting George Wilson:

Dr George Wilson

Australian Museum

6 College Street

Sydney, NSW 2010

Australia

Email: [email protected]

2 principal causes for female-biased sex ratios in arthropods: infection with reproductive parasites and high mortality of males due to e.g., predation

It might depend also on reproductive strategy (at least at terrestrial isopods). Is it a semelparous species or an iteroparous one? If iteroparous, is it univoltine (reproducing once in a year) or bi-multivoltine? E.g. at Porcellio ficulneus in the Mediterranean the sexual rate is 1 male : 8 females. It reproduces once in a year. It might depend also on timing: one male can fertilize several females. After fertilization there is no need for the males (in case of semelparity, univoltine iteroparity....). There may be a high male activity (male rate) during the time of mate searching, afterwards male may hide or die... Females are able to store sperm in the case of several terrestrial isopod. And yes, as Elena wrote, e.g. at terrestrial isopods Wolbachia infection also changes sexual rate. :)

In deep see, I guess, environmental conditions don't play a role?!

We have this biased ratio also in deep-sea benthic copepods. Very few males, and for some families males even have reduced mouthparts, so that they dont feed as adult. My interpretation: In an habitat where food is scarce, males should not compete with females for food. So they are short living and more motile. Their task is to find quickly a female for copulation and then die. So, maybe, the impression of few males is just that they live shorter (so the chance to find them is lower), not that the ratio of hatching males and females is biased.

pedro

This result could be explained by the presence of parasites. It has been shown in amphipod that some parasites like Microsporidia (e.g. Nosema sp, Dictyocoela sp.) are present in the gonadal tissue of females and could cause the feminization of male juveniles during the juvenile developement. You can find some information in publications of Haine et al., Rigaud et al., or Gismondi et al. , (we have worked mainly with Gammarus sp.)

What is your sampling method?

It is possible that males do not present the same distribution or behavior than females, which could introduce bias depending on sampling method.

Thanks so far for the helpful answers! A short summary for the present collection of knowledge:

- My observation is true not only for deep-sea isopods but also occurs in shallow-water isopods as well as totally different taxa, e.g. many other arthropods.

- Sex ration could be biased either primarily (genetically or due to parasites) or secondarily

- Secondary shifts in sex ratios could be due to higher mortality or/and lower life expectancy in males

- the observed sex ratio could be biased by male pre-mating dispersal (hence sometimes we find single males in a sample that has no conspecific females)

I would like to add to my first question:

Is there any evidence for secondary masculinization of adult females (protogyny) in isopods? I have made only very few observations where an obviously female asellote isopod (with oostegites) had a developing first pleopod. The rarity of such observations may suggest that these were some kinds of genetic error rather than a commonly occurring phenomenon.

I would like to respond to some questions:

The samples I was talking about were collected across all oceans in abyssal depth. The gear used predominantly were several types of epibenthic sledge.

I am not talking about one specific species but as far as I can say at least the whole superfamily Janiroidea, maybe this observation is true for most deep-sea taxa (see Pedro's answer).

Unfortunately, I cannot say much about the reproductive strategies these isopods have since most were collected only once in a particular region. Nevertheless, due to the population structure that I frequently observe independently of the season of the year (roughly expressed as: many ovigerous female, as many non-ovigerous females, some juvenile males and females, some mancae, no or few copulatory males) I would like to believe that these critters reproduce across all seasons (though slightly summer biased due to preferred smooth sea conditions during expedition). There is some evidence that females may go through multiple reproductive cycles for instance in Macrostylidae but I am not certain about the males. At least I so far found only one size class.

@ Pedro: In Macrostylidae and other isopods that I had a glimpse at, copulatory males have well-developed mouthparts and oftentimes a full tummy. But I agree that ecological reasons such as limited food availability may lead to sex-dependent niche partitioning and morphological sexual dimorphisms. Nevertheless, considering the limited food availability and patchy distribution of the species: shouldn't the males hang out with the females to make sure they find them first upon becoming receptive? Would be interesting to study the degree of inbreeding in deep-sea isopod populations!

The answers you've been getting are spot on. There are a number of explanations for changes in sex ratio in crustaceans. Several pathogens (microsporidia, wohlbachia) can skew the sex ratio, sex-selective mortality is a possibility (known in a few crustaceans), and evolutionary adaptations to mate finding in harsh environments could be invoked as well. The latter reminds me of similar adaptations to parasitism. There is also the possibility of sex change (http://www.nature.com/nature/journal/v388/n6640/abs/388373a0.html), and this is probably one of the first things I'd look for.

I see the same pattern with tanaids in shallow water. In this case, sex change from female to male results in 'terminal' transformation for males, since they no longer feed. Additionally, it appears that parthenogenesis is part of the reproductive strategy. This may explain your pattern. The sex change initiation may relate to environmental shifts necessitating sexual reproduction (seasonal food availability, salinity shifts (i.e., in estuaries)) or possibly 'lack of males' perceived by a female.