(Figure 1. Source https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/argulus)

            Argulus Foliaceus has a unique behavior of laying their eggs in specific habitats based on special conditions that are met. Egg laying is based is generally based on seasonality and water depth (Harrison pg. 1 2006). The eggs are mainly laid in shady areas and within top water 1 meter of the water column. Egg laying patterns are mainly based on season, water temperature, dissolved oxygen, and depth. Even the placement of eggs laid on structures in the water are important in the development of these organisms.

            Generally mating occurs on the host and then the female detaches to lay eggs. Temperatures below 8-10 ℃ cause egg laying to cease. Favorable temperatures for breeding and laying eggs are 18-23 ℃ optimally at 20 ℃ (Kumar 2013). So why is it so important that temperature is taken as a factor of egg laying? In a study done on Argulus eggs were incubated at different temperatures to study the development and hatching of eggs. Eggs that were incubated around 32 ℃ developed and hatched within 10 days. However various lower temperatures tested began to show a trend on the development of the eggs. At 28 ℃ 75% of the eggs hatched and developed into the larvae stage in about 15 days. Eggs incubated at 15℃ show no signs of development (Kumar 2013). Temperature can be affected by depth and seasonality so; fish louse wants their eggs in warmer areas to help the development of their offspring.

The eggs are usually laid on a hard substrate including the underside of rocks, stone, wood, concrete, and even plastic.  Weeds and water plants have also been found as resources to lay eggs on. Fish louse are unlike any other crustacea that lay their eggs in two to four rows between 20 to 300 eggs per string. This can be observed in the picture above. The string is usually .3-.6 mm in length and cemented. It helps the eggs anchor firmly to whatever substrate it laid on (Pasternak 2004).

It’s important to study the behavior and reproduction of these organisms mainly because they can affect the fish market. Infection of fish makes it unappealing for consumers. The seafood market is one of the fastest growing animal foods producing sectors in the world, increasing 6% every year since 2000 (Webster 2006). A. Foliaceus have been known to wipe out entire populations of trout causing problems in trout farming. If we understand where and when the eggs are laid, we can reduce the risk of infection in the fish market. Boat called “Tanks” are required to checked regularly for eggs to prevent unwanted infection of their daily catch. Scientists have conducted experiments using plastic boards placed in favorable conditions of where A. Foliaceus would want to lay their offspring (Harrison pg. 4). By isolating where these organisms’ prey, we can decrease the amount of mortality in fish.

References

Harrison, Aj, et al. “Seasonal and Vertical Patterns of Egg-Laying by the Freshwater Fish Louse Argulus Foliaceus (Crustacea: Branchiura).” Diseases of Aquatic Organisms, vol. 68,

“Hook and Line, and Bob and Sinker: Daniel Webster and the Fisheries Dispute of 1852.” The SHAFR Guide Online, doi:10.1163/2468-1733_shafr_sim040080025.2006, pp. 167–173., doi:10.3354/dao068167.

Pasternak, A., V. Mikheev, E. Valtonen. 2004. Growth and development of Argulus coregoni (Crustacea: Branchiura) on salmonid and cyprinid hosts. Diseases of Aquatic Organisms, 58: 203-207. Accessed March 02, 2011 at http://www.int-res.com/articles/dao2004/58/d058p203.pdf.

Sahoo, Pramoda Kumar, et al. “Egg Laying Strategies and Effect of Temperature on Egg Development of Argulus Siamensis.” Journal of Parasitic Diseases: Official Organ of the Indian Society for Parasitology, Springer India, Oct. 2013, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3793081/.

(Figure 1. Argulus Fish Lice swimming)

Argulus (Branchiura) have a unique set of features that make it successful as an ectoparasite. Often referred to as fish lice for leeching onto ornamental fish like koi fish or goldfish to suck their blood.  These organisms grow to about 6-22 mm long and mature to this size in about 40-100 days depending on temperature. Argulus are flat disc shaped organisms that are usually colored green, amber or translucent (Fig 1). Since they are small and colored similarly to their hosts it can be hard to spot them on fish. They also have small arms that are modified to help it eat and have teethed arms to help it grab onto their hosts.

Argulus feed by injecting fish with venom from its spine, the venom begins digesting flesh and its mouth has special parts to ingest the pre-digested flesh. Argulus are very mobile moving from fish to fish, adults can be seen free swimming in the water volume when examined closely. The fish lice leave open wounds on fish leaving it vulnerable to infections that could be fatal. Below is a picture of Figure 2 which shows prominent features special to Argulus.  

(Figure 2. Drawing of external anatomy)

The figure above displays a fully matured adult with compound eyes on its flattened head with two forms of antennae, large suction cups, thorax and abdomen.  The respiratory areas can be seen on the wings above the appendages including two which have hooks. The abdomen is bilobed which contains the testes. The thorax containing 4 sections has two of the legs fused to the carapace. Each set of appendages have small swimming legs.  It has a preoral stinger that helps pierce the skin of its hosts (Figure 2).

To help the Argulus feed on blood of its victims it uses the special features of its antennae stinger, suction cup and mouth. The antennae have claws that help attach to the hosts, and the suction cups assist in attaching as well. The mouth parts are tubular with jaws that scrape chunks of the skin. The spines and claws on other appendages help the mouth too. The mandibles are like tooth hooks which are incorporated with the proboscis. The proboscis is a hooked spiny feature in its parasitic lifestyle.

 (Figure 3.)

With its spiny hooked stinger, the Argulus can inject toxins into its host and then feed on blood. Most of the time they will hook on the external surface or under the gills. When feeding the spine like stylet is injected to release toxins, however it is not clear that this toxin breaks down the skin (Figure 3). Some toxins have also been known to kill eels and release pathogens. Usually sites that have been fed on are engorged and hemorrhaged wounds. This leads to secondary infection from viruses, fungi, bacteria, and even other parasites. Usually the toxins give hosts erratic swimming patterns, stunted growth, as well as weakness overall.

Argulus have distinct structures in mating as well. Males have third and forth limbs that transfer sperm to its partner while mating. They produce eggs .3-.6 mm long covered in a gelatinous capsule. Generally, 20 to 300 eggs are laid at a time.The eggs are fastened onto a rock by the female for about 12-30 days and once the larvae hatch, they swim to find a host.

 Males are also generally smaller than females, both molt when they are sexually mature. Sexes can be determined by looking at abdomens very closely.  

This has been a general overview of the anatomy of Argulus (Branchiura). Below is a brief video showing the general anatomy of this interesting little parasite.

https://www.youtube.com/watch?v=OH6nTDii_as&t=56s

References

“Branchiura.” Branchiura – an Overview | ScienceDirect Topics, https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/branchiura.

Encyclopedia.com, Encyclopedia.com, 2019, https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/fish-lice-branchiura.

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