One of the world's smallest seahorses faces extinction because of the BP oil spill, conservationists have warned.

The minute creatures, barely 2cm tall, were elusive even before the spill, found only among the seagrass in the shallow waters of the Gulf of Mexico. Now conservationists from the Zoological Society of London's Project Seahorse team are warning populations could fall precipitously because so much of their habitat could have been lost to the spill.

"We have very high levels of concern for this particular species because they have a narrower range," said Heather Masonjones, a seahorse biologist at the University of Tampa.

Although most seahorses are believed to live in shallow water, some also cling to the seagrass mats that float in the open water. During the three months that oil was gushing from BP's well, these mats become collection points for crude. Some of these were set alight in burn fields as BP tried to stop the oil washing on shore. Furthermore, thick clouds of oil in water typically starve seagrass of the light they need to survive, while toxic components of the oil as well as the millions of gallons of chemical dispersants used to break down the spill could also be shrinking suitable habitat for seahorses.

The dwarf seahorses, or Hippocampus zosterae, are particularly ill-suited to escape. They are poor swimmers, making the species extremely vulnerable to a sudden environmental impact such as the BP spill, said Heather Koldewey, the associate director of Project Seahorse. They also mate for life, and produce relatively few offspring, making it more difficult for them to recover from a cataclysmic event.

Masonjones said the experience of earlier oil spills suggested it could take five years for seagrass to make a complete recovery, which represents about three generations of seahorses. It is also unclear how dispersants, which can be hormone disrupters, will affect reproduction cycles, especially on seahorses where males carry the eggs.

Koldewey said it was crucial that BP take steps to help protect the seagrass in the oil spill clean-up in the months ahead to avoid further damage to seahorse populations: "We are urging BP to continue to use booms in the clean-up to isolate the oil slicks. These can be skimmed, left to evaporate, or treated with biological agents like fertilisers, which promote the growth of micro-organisms that biodegrade oil."