Life History and Bioacoustics of the Plainfin Midshipman (Porichthys notatus)
Plainfin Midshpman are found along the west coast of the United States from Sitka, Alaska to Magdalena Bay, Mexico, (as shown in yellow) (Walker & Rosenblatt, 1988).
Plainfin midshipman nest in rocky substrate of calm bays. An example of this is the near shore nesting grounds of midshipman at Seal Rock near Brinnon, WA. Photo by Paul Forlano.
The plainfin midshipman fish is a marine teleost fish that belongs to the family Batrachoididae, which includes the toadfishes and midshipman. Plainfin midshipman are found along the west coast of the United States from Sitka, Alaska to Magdalena Bay, Mexico (Walker & Rosenblatt, 1988). These fish were given the common name "midshipman", because the pattern of their bioluminescent photophores (white dots on the body) resembles the button pattern on a naval midshipman uniform. The plainfin midshipman spends most of the year in the ocean deep-water benthos except for late Spring and early Summer when they seasonally migrate into the shallow rocky intertidal zone to spawn (Bass, 1996).
Interestingly, there are three reproductive morphs in this species: females and two types of males (type I and type II). During the breeding season, type I males excavate nests in the rocky substrate of intertidal zone and begin to court females at night with an advertisement call or "hum" (Brantley & Bass, 1994).
Plainfin midshipman have photophores on their bodies, which resemble the button pattern on naval midshipman uniforms earning their nickname the midshipman. Photos by Margaret Marchaterre.
Plainfin midshipman have three types of reproductive morphs that include females (second from right) and two types of males: type I (largest fish in center) and type II (small fish on the left and far right). Photo by Margaret Marchaterre.
Cartoon of a type I male (fish in nest) producing a hum to attract a female (fish to the right) for spawning. Fertilized eggs from previous matings can be seen near the top of the male's nest. Artwork by NSF.
Females find nesting type I males by localizing the chorus of hums created by advertising males (Sisneros, 2009). For this reason, they exhibit a robust phonotaxis behavior to the male advertisement calls and to pure tones that mimic the fundamental frequency of the hum (McKibben & Bass, 1998). Once a female has found a mate, she deposits her eggs in the nest while the courting males fertilize them with his sperm (Brantley & Bass, 1994). After mating the female returns to the deep offshore waters while the type I male remains in the nest to care for the fertilized eggs (Brantley & Bass, 1994). During egg incubation, the male will continually fan and brush the eggs to keep them free of detritus while also fending off potential predators (Sisneros, 2009). While caring for their young, the males will continue to hum and attract more females until their nest is filled with fertilized eggs (Brantley & Bass, 1994).
Type II males do not build nests, produce hums, or care for their young (Brantley & Bass, 1994). Instead, these males sneak into nests while type I males and females are spawning and try to steal fertilizations (Brantley & Bass, 1994). The type II males are able to do this, because their small size can mimics that of a female (Remage-Healey & Bass, 2007). Beyond difference in appearance, the type II males have other morphological differences when compared to type I males. Type II males have a substantially smaller ratio between their sonic muscle and body mass, which explains their inability to produce hum, growls or grunt trains (Brantley & Bass, 1994). Furthermore, type II males have an incredibly high testis to body mass ratio or gonadosomatic index (GSI) (Bass, 1996) in which the gonad weight can be up 20% of their body weight.
Different successful matings can be observed by the various stages of embryonic development. The more pigmented embryos are further along in development, while the pale ones are not. Photo by Margaret Marchaterre.
Plainfin Midshipman Vocal Signals:
Sample of the advertisement call or hum. Hums are used by nesting type I males to attract females during the breeding season. Recording courtesy of the Bass Lab.
Sample of a single grunt. Grunts are vocal signals that are produced in an agonistic context. All three reproductive morphs can produce grunts. Recording courtesy of the Bass Lab.
Sample of a growl. Growls are vocal signals are produced only by type I males. These signals are often produced during the defense of a nest site. Recording courtesy of the Bass Lab.
The plainfin midshipman have a relatively simple repertoire of vocalizations that include the advertisement call or hum, grunts and growls. The hum is produced by type I males as they rapidly contract their sonic muscles that are attached to their swim bladders (Brantley & Bass, 1994). The hum is a multi-harmonic signal that contains a fundamental frequency that can range between 90 to 100 Hz (depending on temperature) along with multiple harmonics up to 1000 Hz (Brantley & Bass, 1994).
In addition to the hum, the midshipman can produce two other types of vocal signals: grunts and growls. Grunts are an agonistic short-duration broad-band signal produced by all sexes (Brantley & Bass, 1994). However, only type I males are capable of producing trains of grunts, which consist of successive grunts with dominant frequencies that range from 97 to 110 Hz (Sisneros, 2009). The type I male is also capable of producing a growl. This call is initiated by a grunt but immediately followed with a multi-harmonic component that has a fundamental frequency that range between 59 and 116 Hz (Sisneros, 2009). The growl is mostly heard at the beginning of the breeding season while males are establishing nesting sites (Sisneros, 2009).
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