mysid shrimp, Oil & Gas Glossary

The mysid shrimp is the common name for a small marine crustacean, historically catalogued as Mysidopsis bahia and reclassified as Americamysis bahia, that serves as the standard test organism in United States Environmental Protection Agency bioassay protocols used to judge whether oilfield discharges are safe to release into the sea. Mysids are tiny, growing to roughly 8 millimetres, reproduce quickly, and are sensitive to a wide range of contaminants, which makes them ideal sentinels for laboratory toxicity testing. In offshore drilling, the most consequential application is the 96-hour acute toxicity test on the suspended particulate phase of water-based drilling fluid. To run it, a laboratory mixes one part drilling fluid with nine parts clean seawater, lets the slurry settle for one hour, and exposes mysids to dilutions of the unfiltered supernatant for 96 hours. The result is expressed as an LC50, the concentration that kills half the test population. Under United States NPDES permit rules for offshore discharge, a drilling fluid with an LC50 greater than 30,000 parts per million of the suspended particulate phase passes and is deemed non-toxic, while a fluid that kills half the mysids at a lower concentration fails and cannot be discharged. A separate chronic protocol, EPA Method 1007, runs a 7-day survival and growth test on the same species for produced-water and effluent limits, using eight replicates of five organisms in the control and each dilution. This is the regulatory backbone that decides whether spent mud and produced water go overboard or must be hauled to shore. While the mysid LC50 test was developed for United States Gulf of Mexico and federal waters, the same toxicity-testing philosophy governs Canadian offshore activity off the East Coast. Operators on the Grand Banks, in the Flemish Pass, and on the Scotian Shelf work under the Offshore Waste Treatment Guidelines and the Offshore Chemical Selection Guidelines administered jointly by the regulators, the Canada-Newfoundland and Labrador Offshore Petroleum Board and its Nova Scotia counterpart, which require toxicity screening of discharged drilling fluids and chemicals using standardized marine bioassays. The mysid shrimp test, alongside companion tests on other species, is the reason a synthetic-based mud used offshore must demonstrate low aquatic toxicity before a single barrel of cuttings is released, and it links directly to the broader concern of how oilfield chemistry behaves once it enters a marine food web.

Key Takeaways

Standardized sentinel species: Americamysis bahia, formerly Mysidopsis bahia, is a roughly 8 mm estuarine crustacean chosen as the EPA reference organism because it is sensitive, fast-breeding, and easy to culture year-round. Its consistency lets laboratories across jurisdictions produce comparable toxicity numbers from the same standardized test, which is essential for enforceable discharge permits.
The 30,000 ppm threshold: For offshore water-based drilling fluid, the NPDES pass criterion is a 96-hour LC50 greater than 30,000 ppm of the suspended particulate phase. A fluid clearing this bar is classed non-toxic and dischargeable; one with a lower LC50, meaning it kills mysids at weaker concentrations, fails and must be contained and shipped to shore for disposal.
Suspended particulate phase method: The acute test does not expose mysids to neat mud. A 1-to-9 mud-to-seawater mixture settles for one hour, and the unfiltered supernatant, the suspended particulate phase, is diluted and tested. This mimics how fine cuttings and mud solids actually disperse in the water column after discharge rather than the unrealistic worst case of pure mud.
Chronic test for produced water: EPA Method 1007 uses the same species for a 7-day chronic survival-and-growth test with eight replicates of five organisms per dilution, applied to produced water and other effluents. Chronic endpoints catch sub-lethal harm, reduced growth or reproduction, that a 96-hour kill test would miss in long-duration discharges.
Canadian offshore parallel: Off Canada's East Coast the CNLOPB and CNSOPB enforce the Offshore Waste Treatment Guidelines and Offshore Chemical Selection Guidelines, which require marine toxicity screening of discharged fluids and chemicals. Mysid-style bioassays underpin the demonstration that a mud or chemical is low-toxicity before discharge on the Grand Banks or Scotian Shelf is permitted.

Acute Versus Chronic Endpoints

The two mysid protocols answer different questions. The 96-hour acute LC50 measures outright lethality and is the gatekeeper for episodic discharges like drilling-fluid and cuttings releases, where the concern is a short, concentrated exposure as solids disperse from the rig. The 7-day chronic test measures survival plus growth and is suited to continuous discharges such as produced water, where organisms downstream of a platform may sit in a dilute plume for the life of the field. A fluid can pass the acute test yet impair growth at low chronic concentrations, so regulators pair the two to cover both the spill-like and the steady-state exposure pathways that oilfield operations create.

From Lab Result to Discharge Decision

A toxicity number is only useful when tied to an operational rule. When a water-based mud returns a 96-hour LC50 above 30,000 ppm, the operator may discharge cuttings within permit limits, avoiding the cost and deck space of hauling solids to shore. A failing result forces containment, a switch to a less toxic mud formulation, or onshore disposal. Because synthetic-based and oil-based muds generally cannot meet open-water discharge criteria, offshore programs that use them must treat the cuttings, reinject them, or ship them, decisions that hinge in part on the bioassay outcome and that materially affect well cost and logistics.

Fast Facts

The genus once universally called Mysidopsis was split by taxonomists in 1999, moving the EPA test organism into the new genus Americamysis, so older permits and many drilling-fluid specifications still cite Mysidopsis bahia for what laboratories now culture as Americamysis bahia. The animal is small enough that an entire chronic test population, hundreds of individuals across dozens of replicate chambers, occupies a single laboratory bench, yet the kill threshold it establishes governs whether thousands of tonnes of drill cuttings from a deepwater well can legally go over the side.

The mysid test exists to limit bioaccumulation, the build-up of discharged contaminants in marine organisms and up the food chain, by screening out toxic fluids before release. Its primary subject is drilling fluid, whose suspended solids form the test phase, and the related produced water stream that the chronic protocol governs. Together these define the discharge envelope that offshore operators must respect under regulatory waste-treatment guidelines.

Real-World WCSB-Adjacent Scenario: Toxicity Screening on the Grand Banks

An operator drilling a development well on the Grand Banks off Newfoundland plans to discharge cuttings from the water-based-mud sections directly overboard to save deck space and supply-vessel trips. Before spud, the mud program's suspended particulate phase is bioassayed using the standard mysid acute protocol, returning a 96-hour LC50 of roughly 48,000 ppm, comfortably above the non-toxic threshold. With that result in hand and filed under the CNLOPB waste-treatment framework, the operator is cleared to discharge the upper-hole cuttings during drilling.

For the deeper sections, the program switches to a synthetic-based mud that cannot meet open-water toxicity limits. Those cuttings are instead treated and shipped to shore at a cost of roughly CAD 600 per tonne, a planned expense the toxicity testing justified well before the rig arrived on location.