The Mishna (daf 73 listed 39 primary categories of prohibited labor on Shabbat. On yesterday’s daf we learned that an individual can be liable for performing more than one of these prohibitions in the course of a single activity.
According to the Mishna, among those liable for performing primary categories of labor is one who traps a deer or any other living creature. The liability in this category is discussed on today’s daf:
The Sages taught in a Tosefta: One who traps a ḥilazon and breaks its shell to remove its blood for the dye is liable to bring only one sin-offering. He is not liable for breaking the shell. Rabbi Yehuda says: He is liable to bring two, for performing the prohibited labors of trapping and for threshing, as Rabbi Yehuda would say: The breaking of a ḥilazon is included in the primary category of threshing, as its objective is to extract the matter that he desires from the shell that he does not. The Rabbis said to him: Breaking the shell is not included in the primary category of threshing. Rava said: What is the rationale for the opinion of the Rabbis? They hold: Threshing applies only to produce that grows from the ground. One who extracts other materials from their covering is exempt. The Gemara asks: Even if extracting blood is not considered threshing, let him be liable for taking a life as well. Rabbi Yoḥanan said: This is referring to a case where he broke its shell after it was dead.
There are numerous opinions with regard to the identity of the ḥilazon, from which the sky blue dye is extracted. There is also a dispute as to how to associate the statements in the Talmud and the midrash with a particular species. Most scholars identify the ḥilazon with the Murex trunculus species found on the coast of northern Israel. There is a gland in the ḥilazon that produces a secretion, which is not the blood of the ḥilazon. With the addition of several other ingredients, this secretion can be processed into a dye. Both the quality and quantity of the substance are enhanced by keeping the ḥilazon alive as long as possible. To obtain the liquid, it is necessary to first break the shell of the ḥilazon and then squeeze the secretion from its body.