The reason I am asking this question is because the Arizal writes (Shaar Maamare Rashbi pg. 120- new version) writes that the Techelet comes from the Kineret. If so, according to the Arizal (based on msh210's answer) the Murex trunculus could not be the real Techelet.
The source for the Chilazon being in the Kineret is the Zohar, II, 48b, on parashat Terumah:
- "And blue" (Shemot 25:3): Rabbi Yitzchak said: Blue [techelet] is from that fish that is in the Sea Genosar, WHICH IS THE SEA OF GALILEE, which is in the portion of Zvulon. This color is needed for the work of the tabernacle to show this color, AS IT IS WRITTEN BEFORE US.
The sea of Galilee is the same as the Kineret, which is the same as the Sea of Genosar. So it is not originally the Arizal who says this but the Zohar. This is apparently somewhat problematic, in making Zevulun's portion that of the Kineret; there does not seem to be another source for this assertion.
If this Zohar is from Rashbi, and this Rabbi Yitzchak was a contemporary of Rashbi, this would be testimony from the time that techelet was still around.
The sea of Galilee is a freshwater lake.
But Nebuzaradan the captain of the guard left of the poorest of the land to be vinedressers [kormim] and husbandmen [yogbim].3 'Kornim:' R. Joseph learnt: This means balsamum gatherers from the En Gedi to Ramah. Yogbim: These are those which catch hilazon4 from the promontory of Tyre as far as Haifa.5
They still had techelet in the time of Rav Yosef.
And the Rambam (Hilchot Tzitzit 2:2) claims it is found in the Dead Sea, which is saltwater. (Though apparently some claim this is a scribal error for Mediterranean Sea.)
ב כֵּיצַד צוֹבְעִין תְּכֵלֶת שֶׁלַּצִּיצִית: לוֹקְחִין הַצֶּמֶר, וְשׁוֹרִין אוֹתוֹ בְּסִיד; וְאַחַר כָּךְ מְכַבְּסִין אוֹתוֹ, עַד שֶׁיִּהְיֶה נָקִי; וּמַרְתִּיחִין אוֹתוֹ בְּאַהֲלָא וְכַיּוֹצֶא בּוֹ, כְּדֶרֶךְ שֶׁהַצַּבָּעִין עוֹשִׂין כְּדֵי שֶׁיִּקְלֹט אֶת הָעַיִן. וְאַחַר כָּךְ מְבִיאִין דַּם חִלָּזוֹן, וְהוּא דָּג שֶׁדּוֹמֶה עֵינוֹ לְעֵין הַיָּם, וְדָמוּ שָׁחוֹר כַּדְּיוֹ, וּבְיָם הַמֶּלַח הוּא מָצוּי. וְנוֹתְנִין אֶת הַדָּם לְיוּרָה, וְנוֹתְנִין עִמּוֹ סַמָּמְנִין כְּמוֹ הַקִּימוּנְיָא וְכַיּוֹצֶא בָּהּ, כְּדֶרֶךְ שֶׁהַצַּבָּעִין עוֹשִׂים; וּמַרְתִּיחִין אוֹתוֹ, וְנוֹתְנִין בּוֹ הַצֶּמֶר, עַד שֶׁיֵּעָשֶׂה כְּעֵין הָרָקִיעַ. וְזוֹ הִיא צוּרַת הַתְּכֵלֶת שֶׁלַּצִּיצִית.
"and it is found in the Yam HaMelach".
As some point out:
Needless to say, there is no species that lives in all three habitats.
So, as msh210 points out in his answer, the murex is not found in the Kineret. It is found in Mediterranean, just as the gemara states.
Should this be a problem with identifying the murex with the chilazon? No, because Rabbinic literature is rife with mutually exclusive descriptions of the chilazon (and other species). (E.g. the Rambam says the blood of chilazon is black, while Rashi says it is blue.) If one were to insist on literal and simultaneous fulfillment of all of these mutually exclusive requirements, there would be no species that would fit.
One can explain away the Zohar by saying that this was a late insertion (see Chasam Sofer) or claiming it has deep mystical meaning rather than literal meaning. One can explain away the Rambam as a taut sofer. One can perhaps explain away the gemara by asserting that they were referring to a different type of species falling under the general term chilazon, snail.
To add a bit, based on the clarified question, yes, the Arizal brings it down and seems to treat it as literal. Thus, in Shaar Maamarei Rashbi, page 120, we read:
ודע, דהאי נונא, מטטרו׳ן, דיוקנא דצדיק חי עלמין , דאיהו הוא דאפיק האי גוון תכלא. ונמצא שהוא בבריאה , בשית דרגין לכרסיא. וממנו היצירה שהוא עץ הדעת טוב ורע , כי כמוך לו הקליפות, והיא מתכסה בתכלת זה, הבא לה על ידי מטטרו׳ן, דיוקנא דיוסף הצדיק , כדכתיב והוא נער. וזמ"ש , דיוסף חפי על אימיה, שלא יסתכל בה עשו, םטרא אחרא בישא ונתכוונו למה שכתבנו. ותכלת זה נמצא בדג שבים כנרת, והוא בחלק זבולון, שהוא ירכא שמאלא, הוד, ובה היה אוחז דוד, והיה הכנור מנגן מאליו. ולפיכך היה דוד אדמוני , בסוד ההוד שהוא דין, וממנו התעוררות לזווג בחצי הלילה, שהיה קם דוד. בסוד שמאלו תחת לראשי. (א) ועל ידו, הקב״ה שט בי״ח אלף עלמין, ח)כי על ידו מתעורר צדיק חי העולמים . ולפיכך היה דוד חי וקיים, כי על ידי ההוד מתעורר היסוד, שהוא חי העולמים.
It seems to me that it is a straightforward citation of the information brought forth in the Zohar. And he gives is mystical meaning, that Kineret corresponds to the Kinor of David; and the portion of Zevulun corresponds to the ירכא שמאלא. I agree that this can be read as a statement of the metzius, followed by a mystical explanation of the significance of that metzius. Yet I think someone who is motivated could well read the entire thing as a mystical interpretation of the cited Zohar.
Personally, I don't see the need to appeal to mystical explanations. The Zohar (plausibly) contradicts the gemara. And the Arizal did not actually see the chilazon. He is interpreting the Zohar. So, even if, according to the Arizal, murex could not be the real techelet, that does not mean that we must pasken like the Arizal and the Zohar.
There's a new booklet being distributed (not very widely) by someone in Bnei Brak. In the front material has a very long letter by Rav Moshe Sternbuch shlita, which argues very strongly against bnei Torah wearing Ptil Tekhelet. Some of the reasons given sound a bit surprising to me (i.e. were beyond my limited understanding); one of them was this Zohar about techeles being from the Kinneret. Rav Sternbuch learns that the Gemara is referring to some sort of dye used for other purposes, not for tzitzit. I'm working from memory because I can't remember where I put my copy of the booklet (word for the wise: that shows you something about my memory). The booklet came out toward the beginning of 5773 and the letter was dated about a year earlier.