Adult Luciaphorus perniciosus mites measure approximately 100 micrometers long. The mite is a pest of mushrooms, and its reproductive habits could enable its populations to increase as much as threefold per day in warm temperatures. (Photo credit: Prapassorn Bussaman, Ph.D.)

By Meredith Swett Walker

Parenthood can be rough for many species, but imagine if reproducing meant having your abdomen swell grotesquely to house dozens of offspring. While inside you, these squirmy offspring reach their full adult size and sexual maturity. In fact, they even mate—with each other—while in your bloated belly, then burst forth ready to repeat the cycle. You, however, will never meet your grandchildren; the rupture of your massive balloon-abdomen has killed you.

Meredith Swett Walker

This sounds like something out of Greek mythology, reminiscent of the tale of the goddess Athena bursting fully grown and clad in armor from her father Zeus’s forehead—only with more offspring and more incest. But it is a real, though rare, form of reproduction called “physogastry reproduction.”

The term physogastry usually refers to the extreme enlargement of the abdomen in egg-producing queen ants and termites. These queen insects essentially become “egg factories.” However, mother mites of the species Luciaphorus perniciosus essentially become “adult factories.” The species exhibits the form of physogastry reproduction described above; its offspring grow to full size and sexual maturity inside their mother’s “physogastric hysterosoma.”

In new research published today in the Journal of Economic Entomology, Prapassorn Bussaman, Ph.D., of Mahasarakham University, along with colleagues from institutions in Thailand, Turkey, Taiwan, and the Czech Republic, describe physogastry reproduction in Lu. perniciosus and its implications for understanding population growth. Aside from being a bit of bizarre biology trivia, why is the reproductive strategy of Lu. perniciosus important? Because this mite is an economically devastating pest of cultivated mushrooms including shiitake, wood ear, and enoki mushrooms. (The mites are commonly known in the mushroom trade as “fish egg mites.”) Understanding Lu. perniciosus’s reproductive strategy helps determine how fast its populations can grow, and understanding population growth is crucial for controlling this pest.

Luciaphorus perniciosus mites can infest cultivated mushrooms including popular varieties like shiitake. (Public domain photo)

To describe a population statistically, ecologists often use a “life table,” essentially a mathematical tool that defines variables including probability of survival at different ages, age at which reproduction starts, and how many offspring an organism has each time it reproduces. Life tables were originally developed for humans by the life insurance industry to determine life expectancy.

Traditional life tables work well for species whose life cycles are determined by their chronological age, be they raccoons, eagles, or oak trees. But many arthropods, like insects and mites, have life cycles that are better defined by different stages such as egg, larva, nymph, and adult. These species don’t fit well in traditional life tables, and Lu. perniciosus’s bizarre physogastry reproduction complicates matters even more.

In this study, Bussaman and colleagues collected data on Lu. perniciosus’s survival and reproduction at three different temperatures. (Temperature affects how fast mites and many insects develop.) They used this data in an age-stage, two-sex life table and devised a new method to analyze it that accommodates physogastry reproduction. The results indicate that Lu. perniciosus populations are capable of a startling growth rate, increasing up to threefold per day at warm temperatures like 35 degrees Celsius. As an example, the researchers say a population founded by just 10 adult female mites could reach a stable age-stage distribution and begin growing exponentially in only 18 days.

A female Luciaphorus perniciosus mite’s abdomen swells with offspring, which grow to adult size and even mate (with each other) inside her abdomen, before the mother’s abdomen ultimately ruptures, releasing the offspring and killing the mother in the process. In this image, the mite’s mature offspring are visible inside her enlarged abdomen. (Photo credit: Prapassorn Bussaman, Ph.D.)

Many questions remain unanswered about physogastry reproduction in Lu. perniciosus. For instance, the researchers observed highly female-biased sex ratios in their study populations—about 90 percent of the mites were female. Does mating in the confined space of their mother’s abdomen allow a small number of males to successfully find and fertilize a very large number of females? A population consisting primarily of very fertile females can increase rapidly, but mating while still inside the mother’s abdomen necessitates that mites mate with their siblings. So, are these populations highly inbred?

A female Luciaphorus perniciosus mite (viewed through a microscope) ruptures, releasing her offspring. (Video credit: Prapassorn Bussaman, Ph.D.)

What is known is that Lu. perniciosus is a highly destructive pest; infestations of mushrooms can reduce yield by up to 80 percent when severe. This tiny mite can also spread quickly. It may be transferred by contaminated tools, moved by air currents, or even hitch rides on other insects—a phenomena called “phoresy.” The researchers hope that their construction of a life table for Lu. perniciosus will eventually help farmers and pest managers trying to control the mite decide the ideal time to employ chemical or biological controls. For now, the researchers recommend frequent inspection of the crop: “At this moment, we can only suggest that the mushroom growers check the mushroom bags every five to seven days, depending on the temperature, and destroy infested bags. This suggestion is based on the predicted population growth calculated from life table data.”

Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.