If you read the book "Divine Love Astrology: Revealing Spiritual Truth for Personal Transformation" it briefly mentions the following.
> The symbol for Cancer represents a crab's claws and also symbolizes the yin and yang or the male and the female joined in the fertilized egg.
Cancer I believe has often been viewed as very carrying so maybe this has to do with the opposing opposites of male vs female characteristics but I haven't found a source for this.