Marine Unit 1

Riding the Currents

(adapted from

Overview

For millions of years, plants and animals have been making their way to the Hawaiian Islands with the help of air and ocean currents. This unit engages students in exploring how marine life reached the Hawaiian Islands and how ocean currents t helped determine the geographic origin of Hawaii marine species.

Unit Focus Questions

1)What major ocean currents influence the origins of Hawaiian marine species?

2)How did marine life disperse through the Pacific Ocean?

3)How do origin, means of dispersal, and island location influence the species variety and rate of endemism among Hawaiian invertebrates?

Dispersing the Currents: Marine Life on the Move

Coral Reproduction

Corals are sessile animals attached to a particular substrate. They are restricted to areas that receive plenty of sunlight for their symbiotic photosynthetic algae. Hard, stony corals secrete a calcium carbonate skeleton. Because these animals can’t get up and move from one area to another, how did they arrive at one of the most remote island in the world, Hawaii? They arrived here by drifting as larvae, settling down and forming a hard skeleton. As part of their sexual reproductive cycle they produce gametes, i.e., sperm and egg, which take their chances in the water column. They unite to form a zygote. The zygote develops into a planula larva that drifts and eventually settleson the bottom. Some corals are hermaphroditic i.e., they are both male and female. These corals produce egg and sperm packets that get released during spawning, such as with Acropora. Other corals may be gonochoric, i.e., they are either male or female. They release either egg or sperm, such as with Fungia.In addition to these reproductive strategies, some corals are broadcast spawners, which release gametes in the water, while others are brooders, in which the maternal polyp retains the egg in its gut, which is then fertilized by a sperm. The egg develops into a planula larva that is ready for release by the parent. In any case, larva, depending upon the species, may be able to remain in their current state for up to 100 days drifting. This may account for the arrival of corals to Hawaii. Some may have even island hopped, making their way here. However, other corals with shorter larval duration may have rafted, i.e., hitchhike on floating debris, their way to remote locations.

Fig. x Hermaphroditic coral: AcroporaFig. xGonochoric coral: male Fungia spawning

Fig. xxx. Life Cycle of Fungia scutaria (Mushroom Coral). Illustration by Kip Aoki

Spawning Events

Many corals spawn by releasing their gametes in the water at particular times. Some coral species may spawn once a year, some spawn more often. To maximize the chance encounter of egg with sperm corals synchronize their spawning. Environmental cues, such as tidal or lunar cycles, and chemical cues may trigger a spawning event. Rare in occurrence are a mass spawning events, which may take place when numerous coral species release their gametes at the same time. If this happens, there is too much organic matter in the water. The gametes die and are decomposed by bacteria. Bacteria utilize tremendous amounts of oxygen from the water, resulting in anoxic water conditions. If oxygen levels drop many animals such as fish may suffocate, resulting in fish kills.

Spawning Times of Some Hawaiian Corals

Cauliflower coral

Cauliflower or Rose Coral (Pocillopora meandrina): Gonochoric, broadcaster. April -- May, at full moon and/or 2-3 days after, around 7:30 in the morning. Profuse spawning can reduce visibility from 100 ft. to fog in about one hour. This one is a "don't miss"!

Mushroom Coral

Mushroom Coral (Fungia scutaira): Gonochoric broadcaster. June -- Sept. on the first full moon and as much as three nights later, between 5 and 7 p.m...

Rice coral

Rice Coral (Montipora verrucosa): Hermaphroditic, broadcaster. June -- August, 3-5 days after a new moon between 8 and 10 p.m... This spawning may be the easiest to see.

Finger Coral

Finger Coral (Porites compressa): Gonochoric, broadcaster. June- August, 11pm-1pm, one to four nights after a full moon, around midnight. Kind of hard to see, looks like a haze.

Lace coral

Lace Coral (Pocillopora damicornis): Gonochoric, brooder. Year-around, with all phases of the moon. This coral releases larvae throughout the day, but mostly at night.

Fish Reproduction

Reproduction among fishes is highly varied and quite complex. The vast majority of fishes lay eggs. The birth of fully developed young is extremely rare among bony fishes and common only among cartilaginous fishes (e.g., sharks, skates, and rays). Eggs of fishes are typically small (about 1 mm in diameter) and generally take about 1 week to hatch. The eggs hatch into larvae that bear little resemblance to the adults (Fig. 1).

Fig. 1 Flounder. Larvae on left and adult on right.

Fish larvae start off after hatching as tadpole-like creatures with undeveloped eyes and jaws. As it grows the eyes form and the jaw develops. It is a critical time for the fish because its yolk that it uses for nourishment will run out soon (Fig. 2).Most larvae are planktonic, i.e., they drift with the currents. Larvae are adapted to a pelagic life, feeding on other plankton in the water. As they grow larger they consume larger prey. Guided by environmental cues, they may settle on a reef and continue developing to a juvenile and then to an adult.

  1. In the space below, make a drawing that represents each phase of the reproductive cycle of most marine fishes as described in the above passage.
  1. Assume your drawing represents a species of fish native to the Phillipines that dispersed to Hawaii. Label each phase of the reproductive cycle on the map below to indicate where the organisms would be at different stages of its life.
  2. Formulate a hypothesis to explain the difference in the rate of endemism in Hawaiian marine invertebrates and Hawaiian insects. Species “endemic” to Hawaii are found only in Hawaii and nowhere else on earth.

Rate of Endemism

Hawaii marine invertebrates such

as molluscs, sea stars, and brittle stars24%

Hawaiian insects94%

Fig. 2 Map of Pacific Ocean Currents

Endemism among terrestrial Hawaiian fauna, including invertebrates, is often linked to an islands’ isolation from other landmasses and therefore from regular influxes of new organisms and genetic material. In contrast, marine invertebrate receive regular influxes of new organisms and ganetic material arriving on ocean currents.

  1. Do ocean currents favor the dispersal of marine life from the South Pacific to Hawaii? Why or why not?
  2. What part of the world has the greatest concentration of marine species and has acted as the center of dispersal for marine life in the tropical Indian and Pacific oceans, from Africa to Hawaii?

Name the area here and circile it on the Map of the Pacific (Fig. 2).

  1. Name 3 factors that influence wether a coral species from Indonesia would be able to sucessfully colonize Hawaiian waters.

Fig. 3. Sargent Major life cycle


Fig. 4 .Generalized Fish life cycle

For marine species there is a peak in diversity of species in the middle of the NWHI, nearFrench Frigate Shoals. The extreme north has cooler water temperatures that may limitsome coral species and geographic isolation that may limit dispersal and recruitment ofsome species. The middle islands are also closer to Johnston Atoll and other CentralPacific Islands that may serve as stepping stones for recruitment of species from thesouth. In particular, some fishes and acroporid corals appear to exist in the NWHI forthis reason.Abundance of species is good in most places, and historical damage fromrecent human occupation on Kure, Midway, and French Frigate Shoals is now reducedwith the removal of military and Coast Guard facilities. A few endemic marine speciesonly occur in the NWHI. They are: Synchiropus kinmeiensis (a dragonet fish) which hasbeen collected from Maro Reef through Kure Atoll; Scorpaenopsis pluralis (ascorpionfish), known only from the holotype (original described specimen) collected offLaysan; Epigonius devaneyi (a deep water cardinalfish) which has been found from Necker Island to Maro Reef; and Nerita plicata, a shallow water snail that is found onlyin the NWHI. Biological data are gathered by USFWS, NMFS, National Ocean Service,and DAR research programs as well as collaborative research cruises among theseagencies.

Coral Reproduction Questions

Describe broadcast spawning and brooding?

Name 2 Hawaiian corals that are gonochoric and 2 that are hermaphroditic.

When do they spawn?

What are some environmental factors that affect spawning?

What is a larvae and what is the coral larvae called?

How are the larvae dispersed?

Do ocean currents favor the dispersal of marine life from the south Pacific to Hawaii? Why or why not?

What part of the world has the greatest concentration of marine species and has acted as the center for dispersalfor marine life in the tropical Indian and Pacific oceans, from Africa to Hawaii?

Name three factors that influence whether a coral species from Indonesia would be able to successfully colonize Hawaiian waters.