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Oil companies tanked after OPEC announced that it would not be cutting its production rates.

The energy sector as a whole took a hit on Friday, as even oil's competition saw losses following West Texas Intermediate's drop below $70. In fact, the S&P Energy Sector plunged into bear market territory—off about 21 percent from its record high in June—with a more than 6 percent drop on the day.

The last time the sector traded lower than 6 percent was Aug. 8, 2011, following the S&P downgrade of the U.S. credit rating.

Drillers all fell in Friday trading: Seadrill, Transocean, Hercules Offshore, Ensco and Nabors Industries dropped more than 8 percent. Transocean fell over 29 percent in the past month, and Seadrill saw its price drop more than 37 percent in that time.

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Companies that sell retail gasoline also saw losses, as Exxon Mobil, Chevron and BP all traded more than 4 percent down.

Oilfield services companies, like Schlumberger, Baker Hughes, and Halliburton also saw heavy losses on the day.

Energy XXI, an exploration and production company focused on the Gulf of Mexico, was one of the energy sector's biggest percentage loser on Friday: It's stock traded down more than 36 percent on Friday.

Shares in Goodrich Petroleum, an exploration and production firm focused on crude oil and natural gas in the Tuscaloosa Marine Shale, fell around 34 percent.

But even as beneficiaries of the oil business fell on Friday, so did its competitors.

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Solar stocks—including First Solar, Solarcity, and Sunpower—traded lower on fears that cheap oil will mean weaker adoption for alternative energy solutions.

And while most transports saw major stock gains on lower oil prices, one group is actually suffering. Major railroad firms Union Pacific, CSX and Norfolk Southern were all down more than 3 percent.

One explanation for the drop in railroads is that a significant portion of these companies' shipping tonnage comes from petrochemicals. But investors may also be worried that they'll see a slowdown in the need to ship construction and drilling equipment tied to the country's energy boom.