Dr. Geoffrey Smith
NWFSC Biology Instructor
June typically signals the full onset of summer here in Florida, and a common occurrence in many of our nearshore waters during the summers is the arrival of mats of Sargassum. Sargassum is a genus of several species of brown algae found globally in tropical to temperate ocean waters. Some species grow attached to the seafloor while other form large floating mats. These are the species most people are familiar with. These floating species grow year round in open ocean water but tend to peak in abundance in the spring through fall. Changes in current and wind patterns during these times, especially in the summer, can bring these mats close to shore where they can actually be washed ashore. In recent years, warming waters and increased nutrient availability have contributed to large than normal blooms of this algae leading to large massed being washed ashore in the Caribbean and along some of Florida’s beaches. In rare cases, this has led to some issues with smothering of certain shoreline ecosystems. It can also be unsightly to some beachgoers and may produce a foul odor once the algae starts to decompose, but Sargasssum is not all bad. Numerous species of fish and invertebrate species are dependent on these floating mats for part or all of their lives. Important fishery species such as amberjack species Seriola sp. and Gray Triggerfish Balistes capriscus often utilize the Sargasssum mats as juvenile nursery areas. Tripletail Lobotes surnamensis, Dolphin Corypahaena hibburus, and a number of other species utilize the Sargassum mats as juvenile nursery areas and adult feeding areas due to the number of small fish attracted to the mats. There are also a whole host of fish and invertebrate species that have no commercial or recreational value that have evolved to spend their whole lives in the Sargassum, such as the Sargassum Swimming Crab Portunus sayi, Slender Sargassum Shrimp Latreutes fucorum, and Sargassumfish Histrio histrio. The latter will be the focus for this post. These species, have developed color patterns that mimic the Sargassum exceeding well and providing excellent camouflage to both avoid predation and hunt prey (Figure 1).
The Sargassumfish’s distribution, globally in temperate and tropical waters, mirrors the distribution of the floating Sargassum species, as this fish is almost always found living in association with floating Saragassum (Figure 2). On occasion, individuals are found on nearshore reefs, in seagrass beds, and among mangrove roots after Sargassum mats have been washed into nearshore and inshore waters. Sargassumfish can reach approximately 8 inches in length and are various shades of yellow to brown with varying spots based on the Sargassum they are residing in. The Sargassumfish is a member of the Family Antennariidae, one of a number of fish families that are collectively known as anglerfish. The anglerfish, including the Sargassumfish, have a modified first dorsal fin spine, illicum, that is used as a “fishing rod” tipped with a “lure”, the esca (Figure 3). This spine can be independently moved to twitch the lure and attract prey, such as small fish and shrimp. Many of the deepwater anglerfish have a unique means of reproduction because it is so rare to find a mate in the deep ocean waters. The males have essentially become parasitic and attach to a female upon encounter. If the male, which is incapable of feeding on its own, will die if it does not attach to a female in a certain amount of time. The female then has an available source of sperm each year without necessarily having to find a mate. Most of the shallow water anglerfish, including the Sargassumfish, do not reproduce in this manner as the more nutrient rich surface waters allow for greater populations of fish in general, so no specialized reproductive system has been developed as is the case with the deepwater angerlfish. Another unique adaption of many of the anglerfish, especially among the Family Antennariidae, including the Sargassumfish, are modified pectoral and pelvic fins (paired side and lower fins). These fins have been modified to act like an arm and hand. The fin has a bendable “elbow” and the rays of the fin act like moveable “fingers” that allow them to grasp and crawl through the Sargassum to ambush their prey (Figure 4).
Figure 1: From left to right – Sargassum Swimming Crab, Slender Sargassum Shrimp, and Sargassumfish. Photos by Seabird McKeon (https://ocean.si.edu/ocean-life/fish/world-adrift-life-sargassum-slideshow)
Figure 2: Sargassumfish distribution in the central western Atlantic ocean. Image from: https://biogeodb.stri.si.edu/caribbean/en/thefishes/species/3184
Figure 3: Typical Sargassumfish coloration with the illium and esca, “fishing rod and lure”, denoted. Illustration by Val Kells © 2011.
Figure 4: Modified pectoral fin of a Sargassumfish being used to hold onto a piece of Sargassum. Image from: https://bios.asu.edu/currents/sargasso-sea-lesson-featured-in-journal-of-marine-education/
